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ОглавлениеChapter 2 Macronutrients and Nutrition Therapy for Diabetes
Marion J. Franz, MS, RD, CDE
Macronutrient Distribution in the Nutrition Prescription
Carbohydrates and Diabetes Nutrition Therapy
Protein and Diabetes Nutrition Therapy
Dietary Fat and Diabetes Nutrition Therapy
Summary of Recommendations for Macronutrients
Highlights Macronutrients and Nutrition Therapy for Diabetes
• No ideal distribution of macronutrients—carbohydrate, protein, or fat—for a diabetes nutrition prescription has been identified. Instead, as for all Americans, a healthy eating pattern is recommended. For guidelines on healthy eating patterns, the Dietary Reference Intakes or the Dietary Guidelines for Americans, 2010, are helpful resources.
• Available evidence does not show an adverse effect on insulin sensitivity from carbohydrate intake; a higher carbohydrate intake may instead improve insulin resistance.
• Three 1-year studies in individuals with type 2 diabetes comparing higher-carbohydrate diets to lower-carbohydrate, low-fat, or high–monounsaturated fat diets found no differences in A1C, LDL cholesterol, or triglyceride levels; blood pressure; or weight at study end. Therefore, total energy intake and a healthy eating pattern should take precedence over the distribution of macronutrients.
• Sucrose-containing foods can be substituted for other carbohydrates in the food/meal plan. However, as for the general public, excessive intake of sugars should be avoided in a healthy eating pattern. Whenever possible, nutrient-dense foods containing whole grains and fiber should be selected.
• In people with type 2 diabetes, ingestion of protein does not increase postprandial glucose or lipid responses but does cause an acute insulin response. This response does not result in a long-term effect on insulin levels.
• Although it is commonly stated that fats slow absorption and delay peak glycemic responses, evidence as to the magnitude of this effect is difficult to find.
• Chronic intakes of higher total and saturated fats are associated with an increase in insulin resistance, and consumption of saturated and trans fatty acids is associated with an adverse effect on lipid/lipoprotein profile and increased risk of cardiovascular disease. It is recommended that saturated fats be replaced with unsaturated fats and intake of trans fat be minimized. Ingestion of omega-3 fatty acids from fish is recommended.
Macronutrients and Nutrition Therapy for Diabetes
Nutrition therapy implemented appropriately contributes to important and essential outcomes in the management of diabetes. However, just as there is no one medical therapy appropriate for all individuals with type 1 or type 2 diabetes, there is no one prescription for nutrition therapy appropriate for all people with diabetes. It is clear, as outlined in Chapter 1, that a variety of nutrition interventions lead to positive outcomes. What is essential is that health care professionals select nutrition therapy interventions that will lead to positive outcomes in the patients they are counseling. Agreement from diabetes patients as to their willingness and ability to implement nutrition interventions is equally essential.
Primary goals of nutrition therapy for diabetes are to improve glycemic, lipid, and blood pressure control, thus contributing to reduced risk for potential long-term complications of diabetes and heart disease, and to improve the quality of life for individuals with diabetes. How best to achieve these goals has been, and remains, controversial. Because all fields of medicine have moved toward evidence-based recommendations, so has the field of nutrition therapy and diabetes. In 2008, the Academy of Nutrition and Dietetics (Acad Nutr Diet; formerly the American Dietetic Association) published evidence-based nutrition practice guidelines (EBNPG) for adults with type 1 and type 2 diabetes in the Acad Nutr Diet Evidence Analysis Library (Acad Nutr Diet 2008). Subsequently, a review of the research leading to the EBNPG, a summary of the research published after the completion of the Evidence Analysis Library (through 1 September 2009), and evidence-based diabetes nutrition recommendations were published (Franz 2010). In 2008, the American Diabetes Association (ADA) also published a position statement titled “Nutrition Recommendations and Interventions for Diabetes” (ADA 2008). These recommendations are integrated into their annual standards of medical care in diabetes and updated as new evidence becomes available (ADA 2012).
This chapter reviews and summarizes previously published macronutrient (carbohydrate, protein, and fat) evidence, incorporates research published after 1 September 2009, and summarizes key recommendations related to the role of macronutrients in nutrition therapy for diabetes. A literature search was conducted using PubMed MEDLINE, and additional articles were identified from reference lists. Search criteria included the following: carbohydrate, protein, fat, research in human subjects with diabetes, English language articles, and publication after completion of the literature search for the Acad Nutr Diet EBNPG review. The initial search produced 58 articles, of which 42 were excluded because titles or abstracts did not meet inclusion criteria. Sixteen articles were retrieved for more detailed evaluation. Six of these articles are included and four were added from the review of reference lists, making a total of 10 studies (8 clinical trials and 2 observational studies) that met inclusion criteria. These studies are summarized in Table 2.1. Evidence published before September 2009 is included in the tables in “The Evidence for Medical Nutrition Therapy for Type 1 and Type 2 Diabetes in Adults” (Franz 2010) and in the Acad Nutr Diet Evidence Analysis Library (http://www.adaevidencelibrary.com). Evidence from the Report of the Dietary Guidelines Advisory Committee (DGAC) on the Dietary Guidelines for Americans, 2010, is also referenced and is publicly available at http://www.cnpp.usda.gov/dgas2010-dgacreport.htm and at http://www.nutritionevidencelibrary.gov.
Table 2.1 Studies on Macronutrients (Carbohydrate, Protein, and Fat) Published After September 2009
MACRONUTRIENT DISTRIBUTION IN THE NUTRITION PRESCRIPTION
The ADA 2008 nutrition position statement concluded that it is unlikely that there is an optimal mix of macronutrients for the diabetic diet (ADA 2008). For guidance on macronutrient distribution, the Institute of Medicine’s dietary reference intakes (DRIs) for a healthy eating pattern for adults may be helpful (Institute of Medicine 2002). The DRI acceptable macronutrient distribution ranges for carbohydrate, fat, and protein are 45–65, 20–35, and 10–35% of total energy, respectively. The position statement also notes that regardless of the macronutrient distribution, total energy intake must be appropriate for weight management. The mix of macronutrients is adjusted to meet metabolic goals and individual preferences of the person with diabetes (ADA 2012).
A 2012 ADA systematic review of the literature regarding macronutrients, food groups, and eating patterns in the management of diabetes concluded that several different macronutrient distributions may lead to improvement in glycemic and/or cardiovascular disease (CVD) risk factors and that many different approaches to medical nutrition therapy and eating patterns effectively improve glycemic control and reduce cardiovascular risk among individuals with diabetes (Wheeler 2012).
The Acad Nutr Diet EBNPG reviewed a total of 18 studies using differing percentages of carbohydrate, fat, or protein and also concluded that research does not support an ideal percentage of energy from macronutrients in the food/meal plan for people with diabetes. It is recommended that registered dietitians encourage consumption of macronutrients on the basis of DRIs (Acad Nutr Diet 2008; Franz 2010).
The Dietary Guidelines for Americans, 2010 can be used to identify a healthy eating pattern that is not a rigid prescription, but rather includes options that can accommodate cultural, ethnic, traditional, and personal preferences as well as food costs and availability. Although healthy eating patterns that meet nutrient needs over time at an appropriate calorie level can be diverse, some key elements exist: an abundance of vegetables and fruits, an emphasis on whole grains, moderate amounts and a variety of protein foods, limited amounts of foods high in added sugars, and more oils than solid fats. Research is available on beneficial health outcomes from examples of healthy eating patterns, such as the Dietary Approaches to Stop Hypertension (DASH), a Mediterranean-style eating pattern, and a vegetarian eating pattern (U.S. Department of Agriculture and U.S. Department of Health and Human Services 2010).
CARBOHYDRATES AND DIABETES NUTRITION THERAPY
Carbohydrates consist of sugars, starches, and fibers. These are the preferred names for carbohydrate categories rather than simple or complex carbohydrates, since they are based on chemical composition (DGAC 2010). Along with the acceptable macronutrient distribution ranges, the DRIs set a recommended dietary allowance (RDA) for carbohydrates of at least 130 g/day for adults and children (Institute of Medicine 2002). This RDA is based on the estimated average requirement for carbohydrate ingestion that will provide the brain with adequate glucose without additional glucose from protein or triglycerides stored in the fat cells (100 g/day) and a coefficient of variation of 15% based on the variation in brain glucose utilization. The RDA is equal to the estimated average requirement plus twice the coefficient of variation to cover the needs of 97–98% of individuals. Therefore, the RDA for carbohydrate is at least 130% of the estimated average requirement, or at least 130 g/day of carbohydrate. The ADA notes the following: “Although brain fuel needs can be met on lower-carbohydrate diets, long-term metabolic effects of very-low-carbohydrate diets are unclear, and such diets eliminate many foods that are important sources of energy, fiber, vitamins, and minerals and that are important in dietary palatability” (ADA 2012).
Definitions of carbohydrate intake have not been well defined. A high-carbohydrate intake is often defined as a carbohydrate intake ≥55% of total energy. A low-carbohydrate intake may be defined as <25% of total energy or <130 g/day. A very-low-carbohydrate ketogenic diet is defined as <20 g/day. However, differing definitions for carbohydrate intake are used. For example, a meta-analysis used 9–45% of total energy as carbohydrate as a definition of low-carbohydrate intake (Kirk 2008). As a result of this definition, there was an overlap of carbohydrate intake in the low- and high-carbohydrate groups (carbohydrate in the high-carbohydrate group ranged from 40 to 70%). Of interest is a Mediterranean-style eating pattern in subjects with type 2 diabetes that was considered to be low carbohydrate, with <50% of daily calories from carbohydrate (actual intake ~44%) (Esposito 2009), whereas for most individuals with type 2 diabetes, this intake of carbohydrate would be considered a moderate-carbohydrate intake.
It is important to note that most individuals with diabetes do not eat a low- or high-carbohydrate diet but rather report a moderate intake; studies reported an intake of ~46% in individuals with type 1 diabetes (Delahanty 2009) and ~44% in individuals with type 2 diabetes (Vitolins 2009). Furthermore, it appears difficult for people with type 2 diabetes to eat a high-carbohydrate diet. In the U.K. Prospective Diabetes Study, despite receiving individual education from dietitians on the recommended carbohydrate intake of 50–55%, patients reported a carbohydrate intake of 43% energy intake, which was similar to the general public (Eeley 1996).
Carbohydrate and Insulin Resistance
If consuming a high-carbohydrate intake contributed to insulin resistance, carbohydrate intake should be reduced, especially in people at risk for or with type 2 diabetes. Although evidence is limited, available evidence does not report an adverse effect on insulin sensitivity from carbohydrate intake; instead, carbohydrate may improve insulin sensitivity. A review compared short-term intervention studies with higher (>50% of total energy) versus lower carbohydrate intake in subjects with and without diabetes (McClenaghan 2005). Of 11 studies in subjects without diabetes, 7 reported an increase in insulin sensitivity from the higher-carbohydrate diet and 5 reported no differences. Of eight studies in subjects with diabetes, five reported improvement in insulin sensitivity from the higher-carbohydrate diet and three reported no difference. The author concluded that higher-carbohydrate diets do not adversely affect insulin sensitivity and may offer some benefits. Longer-term clinical trials and epidemiological studies in people without diabetes have also reported no adverse effects on insulin sensitivity from higher-carbohydrate diets (Ard 2004; Bessesen 2001; Howard 2006).
However, examining the effect of carbohydrate on insulin action is difficult because any change in one component of the diet is accompanied by changes in other components of the diet. Therefore, as carbohydrate intake is increased, fat is generally decreased, and vice versa. Chronic consumption of foods high in fat, especially saturated fats, as will be reviewed later, is reported to increase insulin resistance. Therefore, it is unknown if the benefit on insulin sensitivity is due to the higher carbohydrate intake or the lower fat intake.
Carbohydrates and Glycemia
The balance between digestible carbohydrate and available insulin is a major determinant of postprandial glucose levels. However, other intrinsic and extrinsic variables also influence the effect of carbohydrate on glucose levels. Continuous glucose monitoring systems can be used to better understand the postprandial effects of carbohydrate. For example, in people with type 2 diabetes, a lunch meal containing double the carbohydrate content did not double the glycemic response (Powers 2010). Under debate is what amount of carbohydrate intake best facilitates glycemic control in people with diabetes.
Type 1 Diabetes
In people receiving intensive treatment in the Diabetes Control and Complications Trial, a lower carbohydrate (37%) intake and higher total (45%) and saturated (17%) fat intakes were associated with worse glycemic control at year 5 compared to a higher carbohydrate (56%) intake (A1C values of 7.5 versus 7.0%, respectively), independent of exercise and BMI (Delahanty 2009). The authors suggest that the carbohydrate content is less critical than the total and saturated fat content, to which it is usually inversely related. They note that high-fat meals have been shown to interfere with indexes of insulin signaling, which results in a transient increase in insulin resistance (Savage 2007), and that a lower-fat diet reduces basal free fatty acid concentrations and improves peripheral insulin sensitivity in type 1 diabetes (Rosenfalck 2006).
To determine the effects on CVD risk factors in people with type 1 diabetes, a eucaloric diet higher in carbohydrate and lower in fat was compared to a diet lower in carbohydrate and higher in monounsaturated fatty acids (MUFAs). After 6 months, there were no significant differences between groups other than decreased plasminogen activator inhibitor 1 and weight gain in the lower-carbohydrate/MUFA group. This result suggests that if individuals choose to lower carbohydrate intake, these calories should be replaced with unsaturated fats rather than saturated fats and special attention should be paid to total energy intake (Strychar 2009).
Type 2 Diabetes
A cross-sectional study of American Indians with type 2 diabetes in the Strong Health Study assessed dietary intake in 1,284 participants. Lower intake of carbohydrate (<35–40% of energy) and higher intakes of total (>25–30% of energy) and saturated fats (>13% of energy) were associated with poorer glycemic control. A lower fiber intake and a higher protein intake were marginally associated with poor glycemic control (Xu 2007).
Clinical trials in individuals with diabetes have compared lower carbohydrate intakes and higher total fat and saturated fat intakes to higher carbohydrate intakes and lower total fat and saturated fat intakes. A meta-analysis of 19 short-term studies (10 days to 6 weeks) with 306 individuals with type 2 diabetes compared lower-carbohydrate, higher-fat diets (40%/40%) to higher-carbohydrate, lower-fat diets (58%/24%) and found no significant differences between diets in the reduction in A1C and total and LDL cholesterol. The higher-carbohydrate diets did increase triglyceride levels and decrease HDL cholesterol. However, the higher-carbohydrate diet did not elevate triglycerides when energy restriction was prescribed. Therefore, total energy intake is a factor when determining the effect of carbohydrate on triglyceride levels. Studies in which glucose-lowering medications were changed and that included an increase in fiber and whole grains were excluded from the meta-analysis because such diets are high in fiber, which in itself has beneficial effects on glycemia and lipemia, regardless of the carbohydrate-to-fat ratio (Kodama 2009). In general, total and LDL cholesterol change more favorably in individuals assigned to low-fat/higher-carbohydrate diets, whereas, HDL cholesterol and triglyceride values change more favorably in individuals randomized to low-carbohydrate diets (Nordmann 2006).
Since publication of the meta-analysis, three 1-year studies in people with type 2 diabetes comparing higher-carbohydrate diets to lower-carbohydrate, low-fat, or high-MUFA diets have been published and reported no differences in A1C, weight loss, LDL cholesterol, triglycerides, or blood pressure (Brehm 2009; Davis 2009; Wolever 2008). Vegetarian and vegan diets are high in carbohydrate. A vegetarian diet (52% energy from carbohydrate) was compared to a diet high in MUFAs, with reported beneficial effects from the vegetarian diet on lipids (total cholesterol, LDL cholesterol, postprandial triglycerides), glucose, and insulin levels (De Natale 2009). A 22-week low-fat vegan diet (75% carbohydrate) compared to a control diet (60–75% carbohydrate and MUFAs) showed greater improvements in A1C levels (~1% point reduction), body weight, and lipids (total and LDL cholesterol, triglycerides) from the vegan diet in secondary analysis. The intent-to-treat analyses, however, showed no significant differences between groups (Barnard 2006).
In summary, in observational studies in people with type 1 and type 2 diabetes, higher-carbohydrate diets compared to diets higher in total fat and saturated fat are associated with lower A1C levels. However, in clinical trials, both high- and low-carbohydrate diets lead to similar improvements in A1C and body weight. It appears likely that the total energy intake of the eating pattern outweighs the distribution of carbohydrates. High-carbohydrate diets, which are generally low in fat, tend to have beneficial effects on total and LDL cholesterol, whereas low-carbohydrate diets tend to have beneficial effects on triglycerides and HDL cholesterol. Because of beneficial and/or similarities in outcomes, it would seem prudent to recommend an eating pattern with moderate amounts of carbohydrate (which is how many people with diabetes already eat) and that includes fruits, vegetables, whole grains, and low-fat dairy foods—all carbohydrate sources—in appropriate amounts and portion sizes.
Types of Carbohydrate
Sucrose. After reviewing 15 studies in which sucrose was substituted for isocaloric amounts of starch, the Acad Nutr Diet EBNPG concluded the following: “If persons with diabetes choose to eat foods containing sucrose, the sucrose-containing foods can be substituted for other carbohydrate foods. Sucrose intakes of 10% to 35% of total energy do not have a negative effect on glycemic or lipid level responses when substituted for isocaloric amounts of starch” (Acad Nutr Diet 2008; Franz 2010). The ADA also concluded, “sucrose-containing foods can be substituted for other carbohydrates in the meal plan, or, if added to the food/meal plan, covered with insulin or other glucose-lowering medications” (ADA 2008). However, as with the general public, care should be taken to avoid excess energy intake, and excessive intake of sugars should be avoided in a healthier eating pattern. The DGAC recommends a maximal intake level of ≤25% of total energy from added sugars, based on research showing that people with intakes of added sugars at or above this level are more likely to have poorer intakes of important essential nutrients (DGAC 2010). For a daily energy intake of ~2,000 kcal, this would be about 10 teaspoons of added sugars; however, average intake for all individuals in the U.S. is ~22 teaspoons per day. (One 12-ounce can of cola contains ~8 teaspoons of added sugar, for ~130 kcal.) In general, it is recommended that most women should eat or drink no more than 100 kcal/day from added sugars and most men no more than 150 kcal/day (Johnson 2011).
There is a natural liking of sweet tastes and, in that regard, people with diabetes are similar to people without diabetes. Unfortunately, people with diabetes are often made to feel guilty if they choose foods that contain added sugars. Knowing the total carbohydrate content, including sugars, of foods can assist people with diabetes to make appropriate food choices that they will enjoy while maintaining glycemic control.
High-fructose corn syrup. High-fructose corn syrup is composed of either 42 or 55% fructose and is similar in composition to table sugar (sucrose). Therefore, the recommendations discussed above related to sucrose also apply to high-fructose corn syrup. High-fructose corn syrup does not differ uniquely from sucrose and other nutritive sweeteners in metabolic effects (glucose, insulin, and triglycerides), subjective effects (hunger, satiety, and energy intake at subsequent meals), and adverse effects such as weight gain (Acad Nutr Diet 2012). It is the sweetener commonly used by the beverage industry.
Fiber and whole grains. Foods containing fiber and whole grains are also recommended. After reviewing 15 studies reporting on the effect of fiber intake on glycemic and lipid outcomes in individuals with diabetes, the Acad Nutr Diet EBNPG concluded the following: “While diets containing 44 to 50 g fiber daily are reported to improve glycemia in persons with diabetes, more usual intakes (up to 24 g/day) have not shown beneficial effects on glycemia. Recommendations for fiber intake for people with diabetes are similar to the recommendations for the general public (DRI: 14 g/1,000 kcal)” (Acad Nutr Diet 2008; Franz 2010). However, the guidelines do recommend including foods containing 25–30 g fiber per day, with special emphasis on soluble fiber sources (7–13 g) because of their beneficial effect on lipids.
The ADA also recommends that people with diabetes choose a variety of fiber-containing foods such as legumes, fiber-rich cereals (≥5 g fiber/serving), fruits, vegetables, and whole-grain products because they provide vitamins, minerals, and other substances important for good health. The first priority is to achieve fiber-intake goals set for the general population of 14 g/1,000 kcal (ADA 2008). Interestingly, the DGAC notes that it is difficult to meet dietary fiber recommendations with a low carbohydrate intake (DGAC 2010).
However, consumption of whole-grain foods is likely to be of equal importance in reducing CVD risk as fiber. Whole-grain foods contain fiber, vitamins, minerals, phenolic compounds, phytoestrogens, and other unmeasured constituents, which have been shown to lower serum lipids and blood pressure, improve glucose and insulin metabolism and endothelial function, and alleviate oxidative stress and inflammation in the general population (He 2010). In a prospective study of 7,822 women with type 2 diabetes, intakes of whole grain, cereal fiber, and bran were inversely associated with all-cause and CVD mortality during a 26-year follow-up (He 2010). Bran intake had the strongest association, and germ intake, which was also evaluated, was not associated with all-cause or CVD mortality.
Glycemic index/glycemic load. The glycemic index (GI) measures the relative area under the glucose curve of 50 g digestible carbohydrate compared with 50 g of a standard food, either glucose or bread. The GI index does not measure how rapidly blood glucose levels increase after eating different types of carbohydrate-containing foods, which implies that a high-GI food peaks quickly and a low-GI food peaks later. In a review of studies comparing different types of low- and high-GI foods and glucose, in people without diabetes, glucose peaks occurred consistently at ~30 minutes, regardless of whether the food was categorized as low-, medium-, or high-GI, with a modest difference in glucose peak values between high- and low-GI foods (Brand-Miller 2009). In contrast to what is often stated, low-GI foods did not produce a slower rise in blood glucose, nor did they produce an extended, sustained glucose response.
The estimated glycemic load of foods, meals, and eating patterns is calculated by multiplying the GI by the amount of carbohydrate in each food and then totaling the values for all foods in a meal or eating pattern. The glycemic load is used most often in research studies, especially in epidemiological studies, but because of the calculations needed, it is not likely a practical approach for individuals to use for planning meals or prandial insulin doses.
After reviewing 15 studies reporting on the relationship between the GI values of foods/diets and metabolic outcomes, the Acad Nutr Diet EBNPG concluded the following: “Studies comparing high- versus low-GI diets report mixed effects on A1C levels. These studies are complicated by differing definitions of high-GI or low-GI diets or quartiles, as well as possible confounding dietary factors” (Acad Nutr Diet 2008; Franz 2010). The guidelines note that definitions of low- versus high-GI diets range from 38 to 77% for low-GI diets and from 63 to 98% for high-GI diets. Other problems include the variability of GI responses from carbohydrate-containing foods within and among individuals. As with carbohydrate, most individuals with diabetes appear to consume a moderate-GI diet, and it is unknown whether reducing the usual GI by a few units will result in improved glycemic control. Of the 15 studies reviewed, 12 are of short duration (<3 months) with a limited number of subjects. Only three studies were of 1-year duration. After 1 year, one study reported no difference in actual GI between the low-GI and control groups, and two studies reported no differences in A1C between the low-GI and control groups.
The ADA also noted the conflicting evidence of randomized clinical trials of low- versus high-GI diets and also expressed concern about the variability in responses to specific carbohydrate-containing foods. ADA also noted that most individuals already consume a moderate-GI diet; however, for individuals consuming a high-GI diet, consuming a low-GI diet may result in a modest benefit in postprandial hyperglycemia (ADA 2008).
Nonnutritive sweeteners and sugar alcohols. Five nonnutritive sweeteners are approved by the U.S. Food and Drug Administration (FDA) as food additives: aspartame, saccharine, acesulfame K, neotame, and sucralose; one other—stevia—is approved as Generally Recognized As Safe (GRAS). The FDA also sets a sweetener Acceptable Daily Intake (ADI), which is the level a person can safely consume on average every day over a lifetime without risk. The ADI is typically 1/100th of the amount of the nonnutritive sweeteners shown to be safe in animal studies. The ADA notes that before being allowed on the market, all nonnutritive sweeteners undergo rigorous scrutiny and are shown to be safe when consumed by the public, including people with diabetes and women during pregnancy (ADA 2008). The Acad Nutr Diet EBNPG note that although nonnutritive sweeteners independently do not effect changes in glycemic responses, some of the products sweetened with nonnutritive sweeteners contain energy and carbohydrate from other foods, and these foods need to be taken into consideration (Acad Nutr Diet 2008; Franz 2010).
Reduced-calorie sweeteners approved by the FDA include sugar alcohols (polyols) such as erythritol, isomalt, lactitol, maltitol, mannitol, sorbitol, xylitol, tagatose, and hydrogenated starch hydrolysates. They produce lower postprandial glucose responses than sucrose or glucose and contain on average about 2 calories/g. There is no evidence that the amount of sugar alcohols likely to be consumed will reduce glycemia, energy intake, or weight. Although safe to use, they may cause diarrhea, especially in children (ADA 2008).
Carbohydrate Summary
Carbohydrates eaten and available insulin are the primary determinants of postprandial glucose levels. Foods containing carbohydrate—fruits, vegetables, whole grains, legumes, and low-fat dairy foods—are important components of a healthy eating pattern. For people with diabetes, these foods should be included in appropriate amounts and portion sizes in their food/meal plan. In addition, nutrient-dense foods are recommended. Nutrient-dense foods are foods and beverages that have not been “diluted” with the addition of added solid fats and added sugars. Monitoring carbohydrates, whether by carbohydrate counting, choices, or experience-based estimation, remains a key strategy in achieving glycemic control (ADA 2012). Although some individuals may note improvements in postprandial glucose responses with the use of the GI/glycemic load, the concept of the GI/glycemic load adds an additional level of complexity to nutrition therapy recommendations and is perhaps best used for fine-tuning postprandial responses after first focusing on total carbohydrate.
The Acad Nutr Diet EBNPG recommend the following: “In persons receiving either medical nutrition therapy alone, glucose-lowering medications, or fixed insulin doses, meal and snack carbohydrate should be consistently distributed throughout the day on a day-to-day basis.” Nutrition therapy for people with type 2 diabetes is discussed in Chapter 6. The Acad Nutr Diet EBNPG also recommend the following: “In persons with type 1 (or type 2) diabetes who adjust their mealtime insulin doses or who are on insulin pump therapy, insulin doses should be adjusted to match carbohydrate intake (insulin-to-carbohydrate ratios). This can be accomplished by comprehensive nutrition education and counseling on interpretation of blood glucose patterns, nutrition-related medication adjustment, and collaboration with the health care team” (Acad Nutr Diet 2008; Franz 2010). Nutrition therapy for people using insulin therapy is discussed in Chapters 5, 7, and 21.
PROTEIN AND DIABETES NUTRITION THERAPY
In people with type 1 or type 2 diabetes with normal renal function, both the Acad Nutr Diet EBNPG and the ADA currently have not found adequate evidence to support recommending a change in the usual protein intake of 15–20% of total daily energy intake (Acad Nutr Diet 2008; ADA 2008). Exceptions for change in protein intake are in individuals who consume excessive amounts of protein foods high in saturated fatty acids, in people who have a protein intake less than the RDA of 0.8 g good-quality protein/kg body weight/day (on average ~10% of energy intake), or in patients with diabetic nephropathy.
In people with type 2 diabetes, ingestion of protein results in acute insulin and glucagon responses with minimal, if any, postprandial glucose or lipid responses (Acad Nutr Diet 2008; Papakonstantinou 2010a). Studies lasting 5–12 weeks comparing high-protein diets to lower-protein diets showed no differences in longer-term insulin response despite the acute insulin response.
Studies in people with type 1 diabetes and protein intake are limited. In a study in which a standard lunch (450 kcal) was compared with a protein-added (+200 kcal) lunch, the early glucose response was similar, but the late glucose response (2–5 h) was slightly increased and required 3–4 units of additional insulin. However, the total insulin requirement over the 5 h was not increased (Peters 1993). Large amounts of protein appear to have the potential to modestly increase postprandial glucose levels and may require additional small amounts of prandial insulin. If protein is lowered, insulin doses may also have to be decreased. Perhaps the best assumption is that prandial bolus insulin doses cover the meal carbohydrate needs for insulin and the protein needs for insulin are covered by basal insulin doses. Generally, an individual’s protein intake is fairly consistent, and the need for extra insulin only becomes an issue when excessive protein is included in meals. Evidence does not support recommendations that suggest protein slows absorption of carbohydrate, contributes to a sustained elevation of glucose levels, or is helpful in the treatment of hypoglycemia (Franz 2002). Because protein does not increase circulating blood glucose levels and, in people with type 2 diabetes, increases insulin levels, it should not be used to treat acute hypoglycemia or to prevent overnight hypoglycemia (e.g., by adding protein to bedtime snacks) (ADA 2008).
Recent research has focused on higher-protein diets and lower-carbohydrate diets for beneficial effects on glycemia and CVD risk factors. In a small crossover study, a high-protein, low-fat (30% protein, 50% carbohydrate, 20% fat) diet was compared to a low-protein, high-fat (15% protein, 50% carbohydrate, 35% fat) diet, each for 4 weeks. Both diets had beneficial effects on weight loss, fasting glucose, and total and LDL cholesterol, with no differences in postprandial glucose and insulin responses. However, the high-protein, low-fat diet improved both triglyceride levels and blood pressure (Papakonstantinou 2010b). In two small 5-week and 10-week studies of men with untreated diabetes, weight-maintaining diets containing 30% protein, 30% carbohydrate, and 40% fat decreased glycated hemoglobin (% GHb) by 13% at 5 weeks and 25% at 10 weeks with no changes in insulin, glucagon, and blood pressure and without the addition of glucose-lowering medications (Gannon 2010).
Studies thus far on high-protein diets for people with type 2 diabetes have been of short duration and with small numbers of subjects with diabetes. Although beneficial outcomes have been reported, studies on higher protein intakes are usually conducted in research centers or food is provided to subjects, and the ability of individuals to increase protein intake long term is unknown (Brinkworth 2004).
DIETARY FAT AND DIABETES NUTRITION THERAPY
Dietary fats are said to slow glucose absorption and delay the peak glycemic response after consuming carbohydrate foods. However, evidence to support this statement is difficult to find. In an early study in subjects with type 2 diabetes, 5, 15, 30, or 50 g fat (butter) were added to 50 g carbohydrate (potato), resulting in a mean glucose area response that was similar after ingestion of the potato with or without the differing amounts of butter (Gentilcore 2006). In another study, 50 g potato alone or with 100 g butter or 80 g olive oil were compared, and the addition of both fats had no effect on glucose or insulin postprandial responses (Thomsen 2003). In subjects with type 1 diabetes, the addition of 200 kcal (22 g fat) to a standard meal also did not affect the glucose response or insulin requirements (Peters 1993). Therefore, in acute studies, with a limited number of subjects, the addition of fat to meals appears to have minimal effects on postprandial glucose.
Epidemiological data and controlled clinical trials have reported that long-term higher levels of total fat intake results in greater whole-body insulin resistance. However, obesity may complicate the relationship (Lovejoy 2002). The data further support an adverse effect of saturated fatty acids on insulin sensitivity. The DGAC reviewed the evidence for the effect of saturated fatty acid intake on type 2 diabetes or increased risk of CVD and concluded that intake of saturated fatty acids increases total and LDL cholesterol, increases risk of CVD, and increases markers of insulin resistance and risk of type 2 diabetes. The committee concluded from 12 studies published since 2000 and reviewed in the nutrition evidence library that a 5% energy decrease in saturated fatty acids, replaced by MUFAs or polyunsaturated fatty acids, decreases risk of CVD and type 2 diabetes in healthy adults and improves insulin responsiveness in insulin-resistant individuals and individuals with type 2 diabetes (DGAC 2010).
The risk of CVD associated with trans fatty acids is due to their positive association with LDL cholesterol and the reverse association with HDL cholesterol, the effect on inflammatory processes, and their interference with fat metabolism. The majority of trans fatty acids come from hydrogenation of unsaturated fats industrially, but ~1–2% (<2% of total energy intake) is found naturally in the gastrointestinal tracts of ruminant animals, ending up in meats and dairy products. The DGAC concluded that avoiding industrial trans fats is important, but small amounts of ruminant trans fats in the diet is acceptable (DGAC 2010).
The DGAC also reviewed the evidence for the effect of dietary cholesterol. From a review of 16 studies published since 1991, the committee concluded that consumption of one egg per day is not associated with risk of CVD or stroke in healthy adults, although consumption of more than seven eggs per week has been associated with increased risk. They note, however, that in three methodologically strong prospective cohort studies, in individuals with type 2 diabetes, egg consumption (one egg/day) does have negative effects on serum lipids and lipoproteins and does increase risk of CVD. Therefore, it is recommended that dietary cholesterol be limited to <200 mg/day for people with type 2 diabetes (DGAC 2010). Conflicting evidence comes from a study in 65 adults with type 2 diabetes or impaired glucose tolerance comparing a hypoenergetic high-protein, high-cholesterol diet (two eggs/day) to a high-protein, low-cholesterol diet (100 g lean animal protein). At 12 weeks, weight loss was similar and LDL cholesterol was unchanged. All the subjects experienced a reduction in total cholesterol, A1C, and blood pressure (Pearce 2011).
Consumption of n-3 fatty acids (omega-3 fatty acids) from fish or from supplements has been shown to reduce adverse CVD outcomes in persons with and without diabetes. A Cochrane Systematic Review and a second systematic review and meta-analysis concluded that omega-3 supplementation in persons with type 2 diabetes lowers triglyceride levels, but may raise LDL cholesterol and have no effect on glycemic control or fasting insulin (Hartweg 2008; Hartweg 2009) (see Chapter 13).
Endothelial dysfunction precedes the onset of atherosclerosis and the occurrence of CVD risk. The correction of fasting endothelial dysfunction with n-3 fatty acids is reported, but in people with type 2 diabetes, postprandial vascular dysfunction is also of concern. Supplementation with 2 g n-3 fatty acids or olive oil in people with type 2 diabetes revealed that n-3 fatty acids also improved postprandial vascular function (Stirban 2010). The DGAC concluded that two servings of fatty seafood per week (4-oz servings) providing an average of 250 mg/day of n-3 fatty acids decreases risk of CVD (DGAC 2010). The ADA also recommends two or more servings of fatty fish per week (with the exception of commercially fried fish filets) (ADA 2008).
The Acad Nutr Diet EBNPG reviewed a total of 43 studies related to the prevention and treatment of CVD in people with diabetes. It is recommended that cardioprotective nutrition interventions be implemented in the initial series of nutrition therapy encounters, since both glycemic control and cardioprotective nutrition interventions improve the lipid profile, reduce CVD risk, and improve CVD outcomes (Acad Nutr Diet 2008; Franz 2010). Nutrition interventions include reduction in saturated and trans fatty acids and dietary cholesterol and interventions to improve blood pressure. Chapter 13 reviews nutrition therapy for lipid disorders.
SUMMARY OF RECOMMENDATIONS FOR MACRONUTRIENTS
Since no ideal percentages of macronutrients—carbohydrate, protein, and fat—appear to exist, it would seem prudent to base the nutrition prescription for individuals with diabetes on an appropriate energy intake and a healthy eating pattern. Individuals with both type 1 and type 2 diabetes report eating a moderate carbohydrate eating pattern (~45–50% of total kcal), which would appear to be of less importance than total energy intake. However, nutrition interventions must always be based on changes that the individual with diabetes is willing and able to make. Even small changes in food/nutrient intake can result in beneficial outcomes.
Although total carbohydrate intake appears to determine glycemic responses more than the type of carbohydrate (i.e., starch vs. sugar or high- vs. low-GI foods) and because of similarities and conflicting metabolic outcomes from differing amounts of macronutrients, attention to a healthy eating pattern appears more appropriate. A healthy eating pattern includes the following: a caloric intake that attains and maintains a healthy weight for adults and appropriate weight gain in children and adolescents; foods from all food groups in nutrient-dense forms and in recommended amounts; replacement of solid fats with oils when possible; reduced intake of added sugars, refined grains (replaced with whole grains), and sodium; and if consumed, moderate alcohol intake (DGAC 2010). Intake of vegetables and fruits, whole grains, fat-free or low-fat milk and milk products, and seafood should be increased and foods containing saturated fat and trans fatty acid decreased.
For all people with diabetes, monitoring total carbohydrate intake remains a key strategy for achieving glycemic control. Individuals on nutrition therapy alone, glucose-lowering medications, or fixed insulin doses appear to do better when carbohydrate intake is kept consistent on a day-to-day basis. Medical therapy then needs to be adjusted appropriately to cover the carbohydrate. Individuals self-adjusting their insulin doses can use insulin-to-carbohydrate ratios and correction factors (Chapter 5) to meet their glucose goals.
There is no conclusive evidence that changing usual protein intake in people with diabetes would be beneficial. The effect of protein on glycemia depends on the state of insulinization and the degree of glycemic control. In people with well-controlled diabetes, consistent amounts of protein will have minimal acute effects on glucose or insulin.
The ADA recommends limiting saturated fat intake to <7% of total energy intake, minimizing intake of trans fat, and limiting dietary cholesterol to <200 mg/day (ADA 2008). They note that reducing saturated fatty acids may also reduce HDL cholesterol, but importantly, the ratio of LDL cholesterol to HDL cholesterol is not adversely affected. Major sources of saturated fatty acids in the American diet include regular cheese, pizza, grain-based desserts, dairy-based desserts, chicken and chicken mixed dishes, and sausage, franks, bacon, and ribs (DGAC 2010). Saturated fatty acids can be replaced with foods containing monounsaturated and polyunsaturated fatty acids. Solid fats can be replaced with vegetable oils such as canola, olive, safflower, soybean, corn, or cottonseed oils. Synthetic trans fatty acids are found in partially hydrogenated oils used in some margarines, snack foods, and prepared desserts and should be avoided. Natural trans fatty acids are present in meat, milk, and milk products, and eliminating them is not recommended.
In summary, 1) focus nutrition interventions on nutrition therapy strategies shown to improve metabolic outcomes—glycemia, lipids, blood pressure—and quality of life, prioritizing goals for each individual with diabetes; 2) negotiate with individuals on lifestyle changes they are willing and able to make; and, perhaps the best advice, 3) instruct patients on appropriate portion sizes of foods shown to have health benefits.
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Marion J. Franz, MS, RD, CDE, is a Nutrition/Health Consultant at Nutrition Concepts by Franz, Inc., Minneapolis, MN.
