The Role of Nutrition in the Etiology of Type 2 Diabetes
Andrey Nailor, M.D.·Published 12 September 2008·8 min read
Key points
Energy Intake There is now a considerable amount of evidence to suggest that rapid acculturation is associated with increased rates of type 2 diabetes (11).
Many studies have examined the role of sucrose and sugars in the etiology of type 2 diabetes.
In the San Luis Valley Diabetes Study a high intake of fat was associated with an increased risk of IGT and type 2 diabetes.
There are no firm epidemiological data with regard to the role of dietary protein in the etiology of type 2 diabetes.
Energy Intake There is now a considerable amount of evidence to suggest that rapid acculturation is associated with increased rates of type 2 diabetes (11). Several characteristics of the Western lifestyle predispose to overnutrition and obesity, which in turn increases the risk of developing insulin resistance and type 2 diabetes - particularly in individuals or populations with a genetic predisposition for diabetes. Physical inactivity and high intakes of energy-dense foods lead to an energy intake in excess of requirements (2,12).
An impressive decline in diabetes death rates in several places was reported during World Wars I and II in locations affected by food shortages.
The contribution of overnutrition to risk of diabetes has also been demonstrated. When food consumption per capita rose sharply in Japan, Taiwan, and some Pacific islands, there was also a sharp rise in the prevalence of type 2 diabetes (11).
A number of studies have demonstrated improvements in metabolic parameters among people with impaired glucose tolerance (IGT) after interventions aimed at reducing energy intake and increasing physical activity, suggesting that it may be possible to reduce the incidence of type 2 diabetes (13 - 15). Indeed, some recent intervention studies have demonstrated the potential for weight loss to reduce the risk of progression from IGT to type 2 diabetes. The Finnish Diabetes Prevention study included 522 overweight persons with IGT, randomized to a control group or to intensive lifestyle intervention.
The cumulative incidence of type 2 diabetes after 4 years was 11% in the intervention group and 23% in the control group. The risk of type 2 diabetes was reduced by 58%, and this outcome was directly related to changes in lifestyle (17,18).
The Diabetes Prevention Program in the United States included 3234 persons of diverse ethnic background with IGT (19).
Participants in the intensive lifestyle program reduced their risk of developing type 2 diabetes by 58% over 3 years of follow-up, and the risk reduction was 71% among persons over the age of 60 years. Of interest is the finding that metformin, the pharmacological agent tested in this study, resulted in a risk reduction of 31%, which was less than the risk reduction observed for lifestyle intervention (20).
Excess body fat is perhaps the most important modifiable risk factor for the development of type 2 diabetes (11,15). It is estimated that the risk of type 2 diabetes attributable to obesity is as great as 75%.
Written and revised by
Andrey Nailor, M.D.
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However, it is important to emphasize that in most intervention studies aimed at weight reduction there are major difficulties in disentangling the potential benefits of weight loss from the effects of altering intakes of individual foods and nutrients and increasing physical activity, all of which have the potential to reduce diabetes risk.
Energy intake is difficult to assess adequately in large-scale epidemiological studies, even when the best available instruments are employed, and it has been demonstrated that overweight or obese persons underestimate their energy intake. Nevertheless, the consistency of beneficial effects shown in intervention studies in which body weight was reduced strengthens the recommendation: energy intake in excess of requirement and overweight should be avoided, particularly among those with a familial predisposition. In addition, the Finnish Diabetes Prevention study recently could show beneficial effects of high fiber, low-fat diets in the prediction of long-term weight loss as well as in the risk reduction of type 2 diabetes (18). Such advice probably offers the best hope of reducing the risk of developing resistance to the action of insulin and progression to type 2 diabetes (8,14,21).{pagebreak}
Carbohydrate and Fiber
Many studies have examined the role of sucrose and sugars in the etiology of type 2 diabetes.
A few have suggested a positive association, but the majority of studies have shown no association. Some have even suggested an inverse association between diabetes incidence and sucrose intake (11,14). Poor assessment of dietary intake, inability to disentangle dietary and other confounding factors, as well as overinterpretation of data derived from observational studies characterize many of these studies. Despite the lack of direct evidence for the role of sugars in the etiology of type 2 diabetes, it is conceivable that excessive sucrose intake might predispose to obesity, and thus sucrose indirectly may be a predisposing factor for type 2 diabetes.
This has been suggested particularly in those who prefer to consume large amounts of sugar-sweetened beverages (22,23).
There is support for the suggestion that foods rich in slowly digested starch or high in fiber might be protective. Countries with high intakes of these foods have low rates of type 2 diabetes. In a cross-sectional study of normoglycemic men, intake of pectin was inversely associated with postprandial blood glucose levels, independent of total energy intake and body mass index (BMI) (5). In the Health Professionals' Follow-up study and the Nurses' Health study, diets low in cereal fiber and with high-glycemic loads were associated with an increased risk of type 2 diabetes after adjustment for other risk factors (13).
In the Iowa Women's Health study the risk of self-reported diabetes was highest in the group with the lowest whole-grain intake. In contrast, a higher consumption of refined grains was associated with an increased risk of type 2 diabetes after adjustment for age and total energy intake.
The ratio of whole grain to refined grain was related to a significantly lower risk of diabetes, suggesting a potential benefit for replacing refined grains with whole grains (24).
It is of interest that, in the studies quoted, cereal fiber appeared to contribute most to the protective effect; however, experimental studies have repeatedly demonstrated a more marked beneficial effect of soluble fibers than insoluble fibers on several measures of carbohydrate and lipid metabolism. Thus, fiber from fruit and vegetables might have been expected to have exerted a more marked protective effect than cereal fiber.
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Although further research is needed to investigate the effects of different types and sources of fiber, it seems to be prudent to encourage an increased consumption of total dietary fiber from different sources: whole grains, fruits, vegetables, and legumes.{pagebreak}
Types of Fat
In the San Luis Valley Diabetes Study a high intake of fat was associated with an increased risk of IGT and type 2 diabetes. In the 1 to 3 year follow-up of this study, fat consumption predicted the progression to type 2 diabetes in persons with IGT (25).
A positive association has also been found between saturated fat and hyperglycemia or glucose intolerance in cross-sectional studies (5,25). However, large cohort studies with diagnosed type 2 diabetes as an end point did not show an appreciable association with saturated fat intake (26).
Conversely, a high intake of vegetable fat was inversely associated with the risk of type 2 diabetes during 6 years of follow-up among participants in the Nurses' Health Study who were not obese (26). With the exception of the San Luis Valley Diabetes Study, other epidemiological studies did not find a significant association between intakes of monounsaturated fat and risk of type 2 diabetes (25). However, fats of a different nature are often highly correlated in the diets, and therefore confounding by one type of fat may have hindered the analysis for another type of fat.
The relationship between nature of dietary fat and type 2 diabetes has been studied in persons with IGT and undiagnosed type 2 diabetes patients who were reported to have higher proportions of saturated fatty acids in serum cholesterol esters than persons with normal glucose tolerance (27).
Perhaps the best evidence for the potentially deleterious effect of saturated fatty acids comes from the KANWU Study. In this study, replacing an appreciable proportion of dietary saturated fatty acids with monounsaturated fatty acids was associated with an increase in insulin sensitivity (28).
It is also of interest to note that both in the Finnish Diabetes Prevention Study and in the US Diabetes Prevention Project, which showed a reduced risk of progression from IGT to type 2 diabetes, the dietary recommendations included advice to appreciably reduce saturated fatty acids. No studies are yet available to suggest conclusive associations between trans-fatty acids and the risk of type 2 diabetes.
The suggestion that n - 3 polyunsaturated fatty acids may also play a role in the development of type 2 diabetes came from a prospective study of 175 elderly men and women who were habitual fish eaters. They were shown to have a 50% lower risk of developing glucose intolerance over a follow-up period of 4 years compared with persons who were not regular fish eaters (29).
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It now appears that the effects of various fatty acids on the risk of type 2 diabetes are similar to their effects on lipoprotein-mediated risk of coronary heart disease (30 - 32).
According to the available data, modifying intake of dietary fats towards consuming less saturated fat and more unsaturated fats may reduce the risk of developing type 2 diabetes - in addition to reducing cardiovascular risk. {pagebreak}
Other Nutritional Factors
There are no firm epidemiological data with regard to the role of dietary protein in the etiology of type 2 diabetes. Although vegetarians present with lower rates of type 2 diabetes compared with persons who eat meat, it is impossible to disentangle the association of animal protein with the risk of type 2 diabetes from other dietary factors, such as saturated fat and fiber intake (11). The relationship between alcohol and other dietary variables similarly complicates attempts to evaluate a potential role for alcohol in the etiology of type 2 diabetes.
In the Rancho Bernardo Study, increasing intakes of alcohol in obese men were associated with an increased risk of type 2 diabetes (33). On the other hand, moderate alcohol intake has been shown to be associated with enhanced insulin sensitivity (34,35).
So far, no epidemiological studies have provided convincing support for the role of micronutrients in the etiology of type 2 diabetes.
The suggestion that low-birth weight infants, especially those who show rapid catch-up growth, are at increased risk of developing IGT and type 2 diabetes later in life is fairly consistent, but the possible relation of this phenomenon to maternal malnutrition needs further research.
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Recent knowledge regarding the potential of nutritional factors in the prevention of type 2 diabetes can be summarized as follows:
Structured programs on lifestyle modification that emphasize a reduction in total energy and saturated fat intake, and encourage an increase in fiber consumption, together with increased physical activity and regular contact with the healthcare team, are the most promising approaches to reducing the risk of developing type 2 diabetes.
Avoiding being overweight, treating overweight and obesity as well as prevention of weight regain once weight loss has been achieved are particularly important for those with a familial predisposition for type 2 diabetes.
Monika Toeller
German Diabetes Research Center, Heinrich-Heine University, Düsseldorf, Germany Jim I. Mann Department of Human Nutrition, University of Otago, Dunedin, New Zealand
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