Body fatness is probably the principal modifiable risk factor for the development of diabetes and metabolic syndrome. Traditionally, hypocaloric diets intended for weight loss are high in CHO and low in fat. A common perception is that dietary fat of any kind is fattening, while low-fat diets have slimming properties.
Thus, in spite of the accumulating evidence of the cardiovascular health benefits of diets high in MUFAs, nutrition experts are still reluctant to recommend them as an alternative to low-fat diets. However, as reviewed (Shah and Garg, 1996, Seidell, 1998), there is no evidence of weight gain with high-fat compared with high-CHO diets under isoenergetic conditions. This concept is supported by the results of parallel-design studies that compared high-CHO and high-MUFA energy-restricted diets in obese subjects with (Heilbronn et al., 1999) or without diabetes (Golay et al., 1996; McManus et al., 2001) for outcomes of weight loss and metabolic control. These studies included various sources of MUFA (oleic acid-rich oils and fats, peanuts and tree nuts) in the high-fat diet groups, with between-diet differences in fat content ranging from 15% to 27% of energy. All three studies showed that it was energy restriction, not diet composition, that determined weight loss, which was similar with the two dietary approaches. The study of McManus et al. (2001) showed superior long-term participation and adherence, with consequent improvements in weight loss, in the high-fat (35% of energy) group than in the low-fat (20% of energy) one. This was due to the higher palatability of a diet containing daily portions of products that are not traditional 'diet foods', such as olive oil, peanuts, peanut butter and mixed nuts. Two meta-analyses (Yu-Poth et al., 1999; Astrup et al., 2000) compiled data on body weight from randomized, controlled studies that compared ad libitum energy diets high in either total fat or CHO on a number of health outcomes in non-obese, non-diabetic subjects. The results favour low-fat diets for weight maintenance in normal-weight individuals or weight loss in the overweight, but a reduction of SFA, not MUFA intake, was the major component of the CHO-for-fat exchange in the studies reviewed.
The long-term outcome, however, of ad libitum reduced-fat diets for weight control is dismal (Swinburn et al., 2001), supporting the notion that attaining a permanent change in eating habits related to obesity is a most difficult task. In addition, moderate-fat weight loss (from MUFA) and weight-maintenance diets improve the cardiovascular disease risk profile on the basis of favourable changes in lipids and lipoproteins (Pelkman et al., 2004). At the molecular level, Rodriguez et al. (2002) recently demonstrated upregulation of uncoupling protein genes (that is, enhanced mitochondrial fatty acid oxidation and thermogenesis) by olive oil feeding in rat adipose tissue and skeletal muscle. This is important, as increased heat production by specific fatty acids provides a mechanism to improve energy balance by decreasing the efficiency of fat deposition, thus targeting the core of the problem in obesity and diabetes. Studies comparing high-MUFA diets and high-CHO diets with ad libitum energy intake are needed to evaluate their efficacy in weight reduction and maintenance of weight loss.
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