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The metabolic response to starvation can no longer be regarded as uniform as it has been in the past. Many concepts about starvation have developed from studies in obese individuals and are not universally applicable to lean individuals. For example, there appears to be major differences in the hyperketonaemic response to short-term starvation and differences in muscle metabolism between lean and obese subjects (ELIA, 1991; ELIA et al., 1990). There is also a higher N excretion and greater contribution of protein oxidation to energy expenditure in lean subjects undertaking prolonged starvation than in obese subjects. This last difference has important physiological implication with respect to survival. The greater protein economy in obese individuals also appears to apply to hypocaloric dieting, although particularly lean subjects (subjects unlikely to want to lose weight) have to be studied to demonstrate clear differences.
If very low calorie diets are to be prescribed to individual subjects, it is reasonable to do so in relation to a fixed intake of protein and energy or in relation to ideal body weight, so that the prescribed diets are not constantly changing as weight reduction occurs. However, there is still some disagreement about the optimal energy intake and composition of very low calorie diets. Some workers prefer to recommend modest reduction in energy intake while others recommend much lower intakes to achieve more rapid weight loss.
Since sudden cardiogenic deaths have occurred from total starvation and from very low calorie diets (often 0-350 kcal/d, usually when the quality of dietary protein has been poor, e.g., collagen hydrolysates and sometimes in the absence of adequate mineral micronutrient intakes), it is reasonable to recommend a minimum energy intake of 400 kcal/d (Department of Health and Social Security, 1987), an adequate amount of good quality protein and adequate quantities of micronutrients. Intakes of 0.15 g N/kg ideal body weight or more are commonly recommended.
In theory, the non-protein constituent of low-calorie diets should include some essential fatty acids, although essential fatty acid deficiency does not appear to occur even during prolonged total starvation, probably because endogenous fatty acids from adipose tissue stores are mobilized for use.
There are opposing views on the relative amounts of fat to carbohydrate in very low calorie diets. The workers who favour higher carbohydrate intake believe that such diets produce better exercise tolerance (e.g., BOGARDUS et al., 1981) and better N balances especially early after the start of dieting (HOFFER, 1988). The workers who favour a low carbohydrate intake remind us that acceptable changes in N balances or body composition can be obtained with diets containing little carbohydrate (e.g., CONTALDO and MANCINI, 1983 (trace - 26 g carbohydrate); FRICKER et al., 1991 (trace of carbohydrate); BOGARDUS et al., 1981 (2 g carbohydrate); HENDLER and BONDE, 1988 (2 g carbohydrate). Furthermore, several studies have reported little or no advantage of carbohydrate over fat in very low calorie diets with respect to N balance and body composition (e.g., HENDLER and BONDE, 1988; BOGARDUS et al., 1981).
It has also been claimed that dietary compliance is likely to be better with diets having a low carbohydrate content. This concept is based on the observation that carbohydrate reduces hyperketonaemia, which is alleged to reduce hunger. The hyperketonaemia of starvation is particularly sensitive to carbohydrate intake. Diets containing more carbohydrate have been reported to be associated with greater hunger (DAVIES et al., 1984), and therefore more likely to be associated with a poor dietary compliance. However, hard data to support the concept that ketone bodies have an important effect on hunger is difficult to find. Animal studies suggest that 3-hydroxybutyrate (but not acetoacetate) has anorectic effects (LANGANS et al., 1985), but care must be taken to consider the effects of acid-base disturbances, species differences and the unphysiological nature of some of the experiments.
In the presence of the opposing views, it is difficult to make definitive recommendations about carbohydrate intake, although it would seem reasonable to include up to 50 g carbohydrate even in very low calorie diets that provide as little as 400 kcal/d. There is little scope for further increments in such diets since the majority of the remaining energy is provided by the protein.
Very low calorie diets should also contain adequate quantities of minerals, vitamins and trace elements, as well as a balanced amino acid mixture. In a study of 17 individuals who died in the USA between 1977 and 1978 while consuming very low calorie diets, the amino acid mixture was not balanced (derived mainly from collagen hydrolysates), and some micronutrients were deficient. It is possible that the pattern of proteins lost during ingestion of these unbalanced diets is different from the pattern of protein loss associated with ingestion of other more balanced diets, or with starvation, and that this could affect tissue function. However, if the unbalanced, very low calorie diets were effective in limiting the loss of lean tissue to some extent, they may also have reduced the release of micronutrients, such as trace elements, which are constituents of lean tissues. Therefore, it is possible that the circulating micronutrient concentrations were reduced (and to a greater extent than starvation which is associated with greater loss of lean tissue), and in the absence of an adequate oral intake this may have predisposed to micronutrient deficiency. The above individuals who died, mainly in their third decade of life, lost a mean of 34% of their body weight, but the varation was large, with some dying after a loss of only 15% of body weight, which was achieved six weeks after starting the diet. Therefore, some individuals died with a substantial amount of fat remaining in their bodies.
These observations should not be extrapolated and linked to the general use of very low calorie diets, especially those that are well balanced. Incidental death from other causes may have also been responsible for some of the above deaths. It is also possible that medication (known or unknown to the physicians) may have contributed. Although it is obvious that the above observations on a small select group of patients should not be regarded as the typical outcome of patients on very low calorie diets (those that died represent a minute proportion of individuals taking very low calorie diets during the same period), the medical profession has been alerted to the potential dangers of using certain types of very low calorie diets. Dieting, especially prolonged dieting, should be undertaken under medical supervision (Department of Health and Social Security, 1987). Contraindications to very low calorie dieting in obese individuals include cerebral ischaemia (e.g., recent stroke or transient ischaemic attacks), ischaemic heart disease, especially in association with a poorly controlled blood pressure and pregnancy. In patients with disease, monitoring should be particularly close. A moderate amount of exercise in appropriate individuals is also to be recommended.