FAT LOSS: PHYSICAL RESPONSES

In a comprehensive review of the issue in 1991, Dr Andrew Prentice and his associates at the Dunn Clinical Nutrition Centre in Cambridge, identified the physiological outcomes of slimming. These included: energy sparing adaptations, alterations in fat and lean body mass, altered energy substrate handling, changes in appetite, health related outcomes such as fitness and glucose tolerance, altered hormonal status, and altered fertility. These changes with slimming may also be moderated by a number of influences including genotype, degree of obesity at the onset of slimming, rate and duration of weight loss, composition of the slimming diet, use or non-use of exercise in the slimming regime and the influence of drugs if these are used.

Since many of the physiological adjustments are dependent on changes in body composition, we will first examine that before considering the adaptations themselves.

Body composition. Major changes in body composition occur with loss of body mass. Lean and fat tissue are almost inevitably lost together, although the proportions of each will differ widely. The greater the energy deficit and the lower the initial body fat level, the greater will be the percentage of fat-free mass (FFM) or muscle mass lost as the percentage of total weight loss.

For a very fat person, on the other hand, dieting is likely to be more effective for fat loss because of the greater stores available.

A more restrictive diet (500kcal v 1200kcal) will result in a greater proportion of lean mass being lost because the body simply cannot pull large amounts of energy out of the fat stores in a hurry. The lean mass represents the ‘quick release’ calorie compartment of the body so more energy is taken from the breakdown of muscle tissue. General estimates for ratios of lean to fat mass loss by obese people on reasonable diets according to English obesity expert Dr John Garrow, are about 25:75, i.e. 25 per cent lean body mass (LBM) to 75 per cent fat loss. This is used as the standard estimate for big people.

Myth-information. Individual foods such as grapefruit have been claimed to have special fat burning’ potential. There is no physiological reason why this is so, nor any positive evidence to this effect.

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