Carbohydrate metabolism

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The primary question under consideration in the current review is whether diets high in carbohydrate (CHO) or unsaturated fatty acids have a more beneficial influence on the metabolism of glucose. Studies examining the effects of modulating total dietary fat and CHO can be difficult to compare because investigators have employed variable end-points, diet composition, duration, methodologies and different design.

For years, the development of type 2 diabetes mellitus, metabolic syndrome and insulin resistance have been found to be related to high caloric intake, especially saturated fatty acids (SFAs), and it is usually associated with excess weight and obesity (Reaven, 2003). For this reason, research in the 1960s, pointed out that a diet higher in CHO (up to 60-70% of total energy), complemented by a reduction in the daily intake of cholesterol and an increase in the consumption of fibre, could be used by people with diabetes. The studies have usually shown that substituting total fat (with high content of SFA) with CHO results in an improvement in mediated-glucose disposal and insulin secretion (Brunzell et al., 1971; O'Dea et al., 1989; Borkman et al., 1991).

The possibility that a diet with a high complex CHO intake might improve glucose metabolic control and lipid profile was of great interest. However, it soon became a concern that these diets could have negative effects on diabetic patients since they could raise the plasma levels of triglycerides and lower HDL cholesterol plasma levels (NIH, 1987). Although the findings were initially thought to be transitory, they were confirmed by several subsequent studies (Garg et al., 1992; Parillo et al., 1992; Blades and Garg, 1995). This led to the belief that CHO intake could favour the appearance of features of insulin resistance syndrome, owing to an increase in CHO intake, which would necessitate higher levels of hormone to maintain glucose homeostasis. Furthermore, recently studies have produced variable results with no consistent detrimental effects of high-CHO diets on insulin sensitivity. Thus, Gerhard et al. (2004) have shown, an ad libitum, low-fat, high-fibre, high-complex CHO diet resulted in greater weight loss than did a high-MUFA diet, and the former did not increase plasma triglyceride concentrations from baseline or worsen glycaemic control in patients with type 2 diabetes mellitus. However, this result is in contrast to previous reports that low-fat, high-CHO diets may cause deterioration in glycaemic control in type 2 diabetes (Parillo et al., 1992).

Experience with healthy Mediterranean populations, having a low rate of ischaemic coronary heart disease, explains the great interest placed on the study of the effects of MUFA on CHO metabolism. One of the first studies, carried out by Garg et al. (1988), showed that feeding type 2 diabetic patients with a high-fat diet enriched in MUFA (50% fat and 33% of calories as MUFA) resulted in a lower insulin requirement, lower plasma glucose concentration and lower triglyceride plasma concentration versus a low-fat, high-CHO diet (60% CHO, 25% fat and 9% as MUFA). Using a similar design, Bonanome et al. (1991) failed to demonstrate this improvement. However, different studies since 1995 have confirmed the initial data, showing that MUFA-enriched diets reduce the requirement for insulin and decrease plasma concentration of glucose and insulin. By replacing complex CHO with MUFA in the diet, Parillo et al. (1992) found a decrease in plasma triglyceride, postprandial plasma glucose and insulin concentrations, with higher insulin mediated glucose-disposal using the euglycemic hyperinsulinaemic clamp method. Additional studies in people with diabetes have shown lower fasting blood glucose, lower average blood glucose and lower peak blood glucose levels, during a normal meal cycle and 24-h urinary excretion (Rasmussen et al., 1993).

At the end of the 1990s, a meta-analysis of various studies comparing these two approaches to diet therapy, low-fat, high-CHO diet or high-MUFA diet, in patients with type 2 diabetes, revealed that high-MUFA diets improve lipo-protein profiles as well as glycaemic control, while having no effect on fasting insulin and glycated haemoglobin concentration (Garg, 1998) (Table 4.1). Since Garg's meta-analysis, the results of various randomized crossover studies comparing the effect of the two dietary approaches on glycaemic control in diabetic patients have been reported. These studies, carried out by Rodriguez-Villar et al. (2000) and Luscombe et al. (1999), showed no differences in glycaemic control after a high-CHO diet with fat limited to 23-30% of energy compared with a high-MUFA diet with a total fat content up of 40%. Furthermore, Thomsen et al. (1999) have demonstrated that isocaloric diets rich in MUFAs or CHO, respectively, seem to have similar effects on cardiovascular risk factors in persons at high risk of developing type 2 diabetes mellitus. No differences in insulin sensitivity were found in healthy young subjects (Perez-Jimenez et al., 2001) after a high-CHO diet and a high-MUFA diet.

Many aspects of the diet composition have been considered to be important in the modulation of insulin resistance, but in the past years, more attention has been given to the ability of the quality of dietary fat, independent of the total amount, to influence insulin sensitivity and, throughout this, the risk of type 2 diabetes. The fact that a diet rich in CHO is accompanied by an increase in glycaemia and insulin, when compared with the MUFA-enriched diet, may be due to the fact that CHO increases peripheral insulin resistance. However, studies performed on insulin sensitivity, using the euglycaemic, hyperinsulin-aemic glucose clamp method, have demonstrated that CHO-enriched diets, compared with MUFA-diets, caused either no change or a decrease in insulin sensitivity in type 2 diabetic patients and in healthy subjects (Garg et al., 1992; Garg, 1994; Berry, 1997; Lichtenstein and Schwab, 2000; Perez-Jimenez et al., 2001). Thus, an alternative explanation for the decrease in insulin requirements, with MUFA diets, is the reduction in glucose availability and, consequently, the needs for insulin. Moreover, in the KANWU study, Vessby et al. (2001) showed that a change in the proportions of dietary fatty acid, decreasing SFA and increasing MUFA, improves insulin sensitivity but has no effect on insulin secretion in healthy subjects. In conclusion, with respect to dietary fat composition, diets enriched in SFA may increase insulin resistance, whereas a MUFA-enriched diet appears to improve insulin sensitivity. On the other hand, a high-CHO diet is an adequate alternative for improving glucose metabolism in healthy young men and women.

In summary, there is substantial evidence that in patients with type 2 diabetes mellitus, metabolic syndrome and insulin resistance, diets with a relatively high fat content based on MUFA provide a degree of metabolic control that is similar or even better than that obtained with high-CHO diets.

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