FAT LOSS: ENERGY/FAT SPARING ADAPTATIONS

May
08
2009

Posted by: admin in Categories: Weight Loss.
Using Tags: Weight Loss

Fat has a protective role for survival. Studies carried out with native African women show that they can lose up to 50 per cent of their body mass in the hungry season and then regain this in good times, without any major long term ill-effects. However, in doing so, the body shifts to a lower level of body mass including FFM and this reduces the resting and non-resting metabolic rates.

One extrapolation of the effects of adaptations is that the metabolism might become ‘fixed’ in its energy/fat sparing mode from repeated weight loss and gain cycles. This is popularly expressed as an increasing difficulty in losing weight and increasing ease of regaining weight with each cycle. Usually these series of weight cycles take place over a period of several decades and the effects of increasing age on weight loss and gain may well explain most of this phenomenon. There is no evidence that the metabolism gets permanently stuck in an energy/fat sparing mode, but then again getting evidence to prove or disprove the hypothesis is very difficult. The theory, however, has been popularised by Cannon and Einzig in their best selling book Dieting Makes You Fat? and is also expressed in several recent ‘anti-dieting’ approaches. It is thus important to understand the extent of adaptation that occurs and possible ways of counteracting this.

Prentice and his group have studied energy sparing adaptations from a range of different energy level diets and concluded that the decrease in metabolic rate resulting from dieting, generally varies between 15-25 per cent. This is obviously large enough to slow down any ongoing fat loss. The decrease in metabolic rate is also generally proportionately greater than the decrease in body weight (at least in the early stages by about 3:1).

The changes in metabolic rate are probably mediated by a reduction in circulating thyroid hormone tri-iodothyrine (T3) and reductions in sympathetic nervous system activity. Thyroid hormone has been used to try to counteract the decline in RMR, but it has had so many negative effects on reducing muscle mass and causing medical problems that it is now not used for this purpose.

Prentice’s group have carried out a theoretical analysis on the long term effects on body weight of a number of different energy level diets. It is presumed that adaptive responses will be more vigorous if the energy deficit is greater and weight loss is faster, although there is not a lot of evidence to back this up. The response, however, does seem to be proportionally greater in leaner people and this was observed by Ancel Keys (whose quote is at the beginning of this chapter) who studied the metabolic responses to semi-starvation in lean men who were staging a hunger strike in the 1940s. He noted, as did Leibel, that one of the most marked adaptations to a significant negative energy balance is lethargy and a reduction in physical activity including spontaneous activity.

Dieting can elicit energy sparing adaptations almost immediately it is begun. The changes that occur over time to RMR during dieting and re-feeding (i.e. gradually increasing the food intake over time). There is an immediate decrease in RMR of around 5 per cent on day 1 of a diet. This continues at a decreasing rate over time to around 15-25 per cent. On re-feeding, as can be seen from the shaded bar on the right of figure 14.5, there is again an immediate jump in metabolic rate of around 5 per cent. This helps to explain the positive benefits of re-feeding, particularly in cases where patients have been on very low calorie diets for long periods. Gradual re-feeding can actually help them to lose fat, at least up to a certain point (which is probably around 1200-1500kcal per day) due to this gradual reduction of energy sparing and its effects on metabolism.