Читать книгу Eating for Sport - Shelly Meltzer - Страница 14

Fat is the most concentrated form of energy, providing double the amount of kilojoules (calories) as an equal amount of carbohydrate or protein (see p15). However, it is not a readily available source of energy during exercise. Even when you exercise at a low to moderate intensity, fat, because of its slow rate of utilization, can only provide about 50% of the energy needed – the other 50% still comes from carbohydrate. Furthermore, dietary fat is easily stored as fat in the body. It is only in rare situations that fat has a positive effect on performance. A fat-loading regime (see p176) may be of benefit prior to participating in an ultra-endurance event that would challenge the muscle glycogen stores. Carrying extra body fat can be an advantage in certain sports: in open water long-distance swimming the extra body fat offers buoyancy and insulation; and in certain positions of play in some team sports (such as a lock or prop in rugby) it protects organs. However, in most sports, extra body fat is simply additional dead weight, affecting the athlete’s speed and agility.

Eating too little fat also carries risk as fat provides fat-soluble vitamins and essential fatty acids, the latter specifically having important immune protective functions. Some fat in the diet is also needed to enhance the absorption of these nutrients and of course offers palatability.

High-fat meals or snacks are not recommended just before training or competing as they slow down the rate of stomach emptying. This means that the food remains in your stomach for longer, increasing the risk of stomach discomfort and, moreover, the fat crowds out the opportunity for much-needed carbohydrate.

In summary, in most situations, eating fat before an event is not beneficial and may be counterproductive. Only in certain events, such as polar expeditions, will fat help meet these requirements during the event. In the immediate post-exercise recovery period priority should also be given to carbohydrate and protein, keeping fat intake low.

The average lean athlete has 9–12kg (20–26 lb) of body fat and it has been estimated that this amount of stored body fat (adipose tissue) alone could supply sufficient energy for nearly four days of running. Despite these extensive fat reserves, there are still many factors that limit the body’s ability to use (oxidize or burn) fat as a fuel during exercise (see pp28, 30, 148).

‘Added’ fat should be limited to about a third of total daily fat allowance.

Different levels of body fat are required for different positions of play.

FOOD GROUP	less than 5g FAT	5–10g FAT	more than 10g FAT
MILK	250ml (1 cup) fat-free (skim) milk, 250ml fat-free yoghurt	250ml (1 cup) low-fat (2%)milk, 250ml (1 cup) low-fat yoghurt, buttermilk	250ml (1 cup) full-cream milk
ANIMAL PROTEIN IN 100G (3⅓ OZ) PORTIONS	Low-fat fish (hake, kingklip), calamari, crayfish, prawns/shrimps, oysters, mussels, tuna in brine, chicken breast (no skin), ostrich, venison, egg white	Snoek, salmon, pilchards, tuna in oil, chicken dark meat (no skin), lean veal, lean pork, lean beef, ham, egg yolk	Sardines in oil, fried fish, chicken with skin, mutton, sausages, polony, bacon, liver and organ meats
CHEESE	60g (2 oz) fat-free cottage cheese/Ricotta, 60g (2 oz) fat-free cream cheese, 60g (2 oz) low-fat cottage cheese, 30g (1 oz) low-fat cheese spread	30g (1 oz) low-fat Edam, 30g (1 oz) Mozzarella, 30g (1 oz) Feta	30g (1 oz) Cheddar, 30g (1 oz) Sweetmilk, 20g (½ oz) Parmesan, 60g (2 oz) low-fat cream cheese, 60g (2 oz) regular cream cheese
STARCHES (grains and cereals)	Bread, rolls, crackers, cereal, porridge, low-fat muesli, rice, pasta, potatoes, oil-free muffins	Homemade bran muffin	Pies, cakes, croissants, cake muffins
FRUIT	Fresh and dried fruit, fruit juice	¼ avocado pear	Coconut
VEGETABLES	All varieties	–	–
LEGUMES AND SOYA PRODUCTS	Portion of beans, lentils, soya mince	Portion of soya sausages and burgers	–
FATS	5ml (1tsp) extra light or light margarine, 15ml (1tbsp) low-oil and oil-free salad dressings	5ml (1tsp) regular margarine, 5ml (1tsp) butter, 10ml (2tsp) peanut butter, 5ml (1tsp) canola, olive or sunflower oil, 6 olives, 15ml (3tsp) sandwich spread	25ml (2tbsp) nuts
SUGAR & DESSERTS	Sugar, honey, syrup, jam, hard sweets, jelly babies, nougat, marshmallows, jelly, fruit sorbet, fruit lollies, low-fat frozen yoghurt, custard prepared with fat-free milk	–	50g (1.7 oz) chocolate bar, regular ice-cream, cream (dairy and and non-dairy) coffee creamers
BISCUITS	2 plain biscuits, 1 slice Swiss roll	2 biscuits with fillings, shortbread	–
SNACKS	45g (1 ½ oz) plain pretzels, unbuttered popcorn	45g (1 ½ oz) flavoured pretzels, 30g (1 oz) reduced-fat crisps	30g (1 oz) packet crisps, 30g (1 oz) cheese popcorn
OTHER	Fishpaste, yeast extracts	–	–
TAKE-AWAYS/CONVENIENCE MEALS	Home-made pizza prepared with low-fat base with fruit and vegetable toppings	Chicken burger (no mayonnaise), spicy rice, fish burger, lean burger	Fried chicken and chips, burgers
BEVERAGES	Portion of colddrink, juices, low-fat drinking chocolate	Malted drinks prepared with low-fat milk	Milkshakes

The fat content of commonly eaten foods according to portion sizes.

Recent research has, however, shown how in some situations these limitations can be overcome (see p30).

Your body fat and body shape is largely determined by your genetic make-up (determined by your parents) and is something you cannot change. Dietary and training strategies can, to a certain degree, remould your body shape, but you should ultimately choose a sport that suits your natural physique best! Recommended ranges for body fat levels do exist for certain sports, but it is important to have realistic and sensible body fat goals because of the following factors:

▪ Maintenance of lower body fat levels might not always be sustainable, often compromising overall nutritional status because it might involve highly restrictive eating patterns.

▪ Very low body fat levels (below 3–5% for males and 14% for females) are associated with negative health and performance outcomes. In females this can lead to irregular or cessation of menstruation which can lead to increased bone loss and bone diseases like osteoporosis (see p76). Athletes may also experience early fatigue, intolerance to cold and have an increased risk of infection.

▪ Improved performance is not always associated with very low body fat levels and in fact some athletes will perform better with body fat levels above the recommended range.