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term it is preferable to increase energy expenditure whilst attending to the quality of the
food
eaten (see WE SHOULD CHOOSE FOOD ACCORDING TO
FOOD GROUPS). In the
long-term the 'prudent diet' low in animal fat, is to be encouraged (see WHAT WE KNOW
ABOUT FATS AND OILS IN FOOD).
The question is whether, in the short term, there is favourable effect on body composition
(i.e.
relative quantities of fat, muscle and other lean body mass, body water and bone) when on a
low-energy diet. The diet may contain one macronutrient (protein, fat, carbohydrate, alcohol)
more than another. The advantage of relatively high protein content in such diets is that it
might
reduce the loss of high-protein tissues like muscle. At very low energy intakes (less than 3360
kilojoules or 800 kilocalories per day) this appears to be the case. The increased protein in
these
diets ought to be accompanied by a reduction in fat content, since art from providing essential
polyunsaturated fatty acids and enabling absorption of fat-soluble vitamins, has no recognised
nutritional value. Obviously there should be no alcohol, which, like fat, is high in kilojoules!
There
is not much opportunity within a 3360 kilojoule diet to include carbohydrate, since 100 grams
of
protein already contributes 1680 kilojoules of energy.
It should be emphasised that this reduction from the usual dietary intake of carbohydrate
will be
associated with a loss of body carbohydrate stores (as glycogen) liver and muscle and the loss
of
water associated with the glycogen. The initial loss of body weight as water is almost
inevitable
with reduction of energy intake; it lasts a few days, contributes nothing to loss of body fat, and
the weight returns when usual eating habits are resumed.
In an every-day diet for weight maintenance, it is healthy to include low-fat versions of high-protein foods, especially those derived from meat (trimmed, lowfat cuts) and reduced and low-fat dairy products rather than their high-fat counterparts. A high-protein diet itself is of no particular value.
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