DIETARY FIBER AND RISK OF TYPE 2 DIABETES

11.8 DIETARY FIBER AND RISK OF TYPE 2 DIABETES

There are no trials focused on fiber intake alone in modifying the risk of diabetes in nondiabetic individuals. A multifactorial intervention including increasing fiber intake (mainly insoluble) as a component has been shown to prevent the development

of type 2 diabetes. 2 In the Finnish Diabetes Prevention Study (DPS), 523 obese persons with IGT were randomized into an intervention or control group. During the trial the risk reduction of diabetes was 58%. When the results were analyzed according to the success score, none developed diabetes in either group if they achieved four or five intervention goals (weight loss at least 5%, physical activity at least 4 h/week, fiber intake > 15 g/1000 kcal, intake of total fat < 30% of energy, and intake of saturated fat <10% of energy). Post hoc analyses of individual inter- vention goals in the prevention of diabetes have not been reported.

11.8.1 E PIDEMIOLOGICAL S TUDIES

The best epidemiological evidence for dietary fiber in the prevention of type 2 diabetes is ironically for insoluble dietary fiber. Two large-cohort studies, one in

over 70,000 American nurses 35 and the other in over 42,000 American male health professionals, 34 showed that cereal fiber, but not fruit or soluble fiber, decreased the risk of diabetes during follow-up by 28 to 30% for the upper vs. lower thirds, even

Functional Food Carbohydrates

when taking into account the glycemic load of the diet and other potentially con- founding factors. Findings in 36,000 women from Iowa confirmed these results. 51

The mechanisms by which cereal fiber, which in the diets of these cohorts contains only small amounts of soluble fiber, such as β-glucan and arabinoxylan, may prevent diabetes independently of glycemic load and body weight is unclear, because insoluble fiber does not affect insulin and glucose metabolism, at least in the short term. It is possible that the association is due to other compounds in whole grains, but most of the apparent preventive effect of whole-grain foods against diabetes 51,103–105 seems to be mediated by cereal fiber or components closely asso- ciated with it. Epidemiological studies suggest that cereal fiber and whole-grain products also appear to prevent obesity and weight gain, in addition to independent effects on decreasing the risk for diabetes, 106 but trial evidence is conflicting. This topic will be covered in greater detail in the chapter on obesity (see Chapter 9, this book), but mechanisms that decrease obesity and weight gain are also critical in the prevention of type 2 diabetes. There is also currently controversy over whether soluble fiber can aid in weight loss.

Epidemiological evidence for soluble fiber in the prevention of diabetes is lacking. This may be because high quantities of soluble fiber, much more than normally consumed in Western diets, are required to alter postprandial insulin and glucose responses. Furthermore, dietary assessment in epidemiological studies is imprecise.

11.8.2 P OSTPRANDIAL S TUDIES

Oat β-glucan at sufficient doses decreases postprandial insulin and glucose responses in healthy individuals. 76,85,88,107 Five grams or more of psyllium per dose also decrease postprandial glucose and insulin responses in nondiabetic subjects, 108,109 but doses

of only 1.7 66 or 2.3 g 110 do not. Guar gum has decreased postprandial insulin levels, but not glucose concentrations, in nondiabetic individuals. 111,112 However, guar gum has decreased both postprandial insulin and glucose levels in several studies. 113–116

A low-GI (53) barley meal rich in dietary fiber eaten in the evening improved glucose tolerance at breakfast, whereas an evening meal with pasta (GI 54) had no effect,

suggesting the importance of dietary fiber for second-meal effects. 61 Similar findings have been shown for breakfast and lunch. 117

11.8.3 M EDIUM - TO L ONG -T ERM S TUDIES

Longer-term studies on the effects of oat bran, psyllium, and guar gum on insulin and glucose metabolism in nondiabetic individuals are sparse. In 36 overweight and obese middle-aged and older nondiabetic glucose men, effectiveness improved, but insulin sensitivity and the acute first-phase insulin response as measured by the minimal model test remained unchanged in men who consumed oat cereal (14 g of dietary fiber, 5.5 g of β-glucan) for 12 weeks, compared with wheat cereal. 118

A 4- week β-glucan-enriched (8 to 12 g/day) barley diet did not have a significant effect on glucose tolerance in a crossover trial in 18 somewhat overweight hyperlipidemic men. 119 Oat bran (100 g/day) or oat bran containing β-glucan (10.3 g/day) did not

The Role of Carbohydrates

affect fasting glucose or insulin levels in hypercholesterolemic individuals partici- pating in a randomized crossover trial (n = 8) 120 or a randomized parallel trial (n = 52). 121 Psyllium at a dose of 3.4 g with meals did not lower fasting glucose levels in two 8-week randomized controlled trials with 26 122 and 75 123 mildly to moderately hypercholesterolemic patients.

In summary, soluble dietary fiber such as β-glucan, psyllium, and guar gum at sufficient doses (3.7 to 14 g/meal 124 ) decreases postprandial insulin and glucose responses in healthy individuals. A second-meal effect has also been described. Longer-term studies on the effects of oat bran, psyllium, and guar gum on glucose and insulin metabolism are few and have shown no benefit. On the other hand, studies in groups in which such benefits might be expected to occur, e.g., in persons with IGT or the metabolic syndrome, have not been carried out. Furthermore, studies lasting longer than 1 or 2 months may be needed. The methodology in assessing insulin and glucose metabolism has often been restricted to fasting glucose and insulin determinations, and has not included oral and i.v. glucose tolerance tests or the euglycemic hyperinsulinemic clamp.