Читать книгу Clinical Obesity in Adults and Children - Группа авторов - Страница 24

Given how sudden the development of obesity has been on a population basis, one has to ask what happened in the early 1980s to make such a difference to the prevalence of obesity in our environment? One clear societal change is the introduction of computers for a myriad of tasks in the early 1980s but in particular a complete transformation of many people’s office hours so now they spend their time sitting while working on their computers for hours on end. Even in such physically active jobs as construction or repairs, the introduction of a myriad of mechanical aids has also drastically reduced the need for sheer physical effort. The major development of the internet has also brought multiple opportunities for home entertainment and thus sedentary leisure time and reduced the need to leave home for entertainment. Life at home has also been transformed by new approaches to food preparation with microwaves, dishwashers, washing machines, etc. becoming a routine addition to kitchen hardware. It is regularly noted that with supermarkets providing ready‐to‐cook meals, the preparation time for meals has reduced from 2 to 3 hours to as little as 20 minutes.

Given all these changes, it is clear that the demand for energy expenditure has dropped dramatically on a secular basis since the early 1980s. Estimating the extent of this change in energetic terms is difficult in the absence of detailed sequential D₂O¹⁸ data on energy expenditure. However, calculations can be made using the latest analyses of energy requirements by WHO [55] based on the original James and Schofield methodology [56]. Thus if a population of young men in their twenties weighing 70 kg was previously moderately active, i.e. with a physical activity level (PAL) of around 1.76 and a similar group are now sedentary (PAL 1.53), then men of this age would have reduced their average energy requirements by 400 kcal/day. Similarly, young women weighing 57 kg and moving from moderate to sedentary activity with PAL changes similar to men would reduce expenditure by 305 kcal/day (see examples in Table 5.1 of the UNU/WHO/FAO 2001 energy requirements report [55]). This would require a substantial and sustained reduction in food intake to match the lower energy expenditure. Originally when assessing UK household intakes in different decades, James estimated that from the 1950s to modern times there might on average have been an average reduction of energy demand for physical activity of about 700 kcal/day on the basis of a marked reduction in the proportion of adults engaged in very active jobs where PALs were 2.0 or more. The overall effects are illustrated in Figure 1.4.

Obtaining independent analyses of secular changes in energy expenditure is not easy, and if one simply relies on questionnaires, then sometimes total physical activity does not seem to have changed over time, e.g. in Finland [58]; but in China, there has been a marked decrease in physical activity [59], and the overall conclusions are that there has been a substantial decline in physical activity demands [60]. Secular studies in Norwegian children studied with accelerometer readings in 2005 and then again up to 2012 showed there was a consistent reduction in more intense physical activity even within this short time interval. In addition, there was an age‐related decline in physical expenditure as children became adolescents [61].

Figure 1.4 An illustration of the principal factors leading to the obesity epidemic. It is assumed that in the 1950s and 1960s, the average intake of the adult male population when weight stable was about 3000 kcal/day when the population was engaged in a lot of physical activity. Then over the next 40–50 years, significant industrial developments occurred, limiting the need for physical activity and in effect reducing the energy needs by perhaps 700 kcal/day on average, as shown by the continuous line falling from 3000 kcal/day. This means that intake should have fallen by the same 700 kcal/day. However, in many European countries, intake did not fall sufficiently, so positive energy balance led to weight gain. This weight gain involved an increase in lean as well as fat tissue, so the total maintenance energy needs of the concomitant lean tissue rose, and there was also an increase in the weight‐bearing cost of physical activity. The overall increase in energy maintenance costs amounts to about 239 kcal/day for each10 kg weight gain, as estimated by Hall et al. [57]. However, in the United States, it has been estimated that food intake has risen, which probably applies to many populations in the Middle East, thereby explaining their marked current obesity rates. In European countries and elsewhere, the physiological drive to limit intake at lower expenditure levels was unsuccessful because the food industry developed techniques to enhance food and drink purchases (see below).

Now, as well as the secular decline in physical activity, there is a concomitant reduction in activity as people age. This was shown most vividly in the Baltimore aging studies, where one can recalculate the data on the energy needs of men first when they were 25 years of age and then when the same men were aged 70. If one applies the energetic analyses to the fall in activity and tissue metabolism as the muscle mass slowly declines but adjust the data so that they were the same body weight, then the overall reduction in energy output amounts to a fall of 2100 per day. This means that over the 45 years the falling output is equivalent to an annual average reduction of nearly 50 kcal/day. So in effect the Baltimore men’s food intake needed to fall by nearly 50 kcals each day with further equivalent falls in daily intake each year decade after decade to avoid putting on weight as they age [62]. If extra food is eaten, then weight gain occurs, and the classic analyses suggest that about 20–25% of this is lean tissue which then contributes to increasing the maintenance energy requirement.

Using detailed analyses of lean body mass changes in men and women, James and Reeds showed many years ago that women do not build lean tissue as much as men [63], and so their maintenance energy expenditure does not rise to the same extent as men. Therefore women store more of the excess dietary energy as fat which in part explains why women are likely to gain more weight than men under the same energy imbalance conditions. It is also why women are usually more prone to obesity than men and particularly prone to extreme obesity, i.e. BMIs >40.

This interaction of secular falls in demand for physical activity, together with the natural reduction in physical activity with aging, explains why in the 1970s, obesity was mostly confined to older adults. To prevent obesity, the body’s appetite sensing system would need to progressively reduce our food intake as we grow older, and this has become far more difficult with such limited levels of physical activity.