60th Scientific Sessions of the American Diabetes Association
Day 1 - June 10, 2000

The Effect of Fat vs Carbohydrates on the Etiology of Type 2 Diabetes

James Lenhard, MD

The diet and macronutrient composition that is most beneficial for preventing diabetes and obesity is becoming an increasingly complex and contentious topic of discussion. The heated public debates between the proponents of a high-carbohydrate diet and a high-protein diet are akin to the theatrics of professional wrestling. It is not surprising that the public is confused and that clinicians struggle to find sensible advice to provide to patients. To address this issue, organizers of the 60 ^th Scientific Sessions of the American Diabetes Association gathered a distinguished panel to explore the role of diet in the development of type 2 diabetes. ^[1] However, the speakers may have raised as many questions as they answered.

Diet and Type 2 Diabetes: Types of Fat and Types of Carbohydrates

The first speaker, Frank B. Hu, MD, PhD, of the Harvard School of Public Health began the session by pointing out that in the United States obesity and diabetes have become epidemic in the last decade, even though the percentage of fat in the US diet has decreased. ^[2]

Dr. Hu contends that more focus should be placed upon the type of fat and the type of carbohydrate consumed, rather than only the amount. The following table shows some of the reported physiologic effects of dietary fats obtained from different sources:

Table. Physiologic Effects of Dietary Fats

Type of Fat	Source	Effect
Saturated Fat	Meat, dairy	Increase low-density lipoproteins (LDL); decrease high-density lipoproteins (HDL)
Trans fatty acids	Vegetable oils	Increase LDL and triglycerides; decrease insulin sensitivity
Monounsaturated fats	Olive oil	Decrease LDL; increase HDL
n-3 fatty acids (fish)	Seafood	Decrease triglycerides; increase insulin sensitivity; antiarrythmic
n-3 fatty acids (plants)	Soybeans, walnuts	Antithrombotic; antiarrythmic

Dr. Hu also described clinical trials that have examined the association between the intake of dietary fat and the development of cardiovascular disease. The Nurses' Health Study, now in its 25 ^th year of follow-up, suggests that a diet high in saturated fat and trans fatty acids will increase the risk of coronary heart disease (CHD). The Nurses' Health Study, ^[3] as well as several others, also showed a reverse association between nut consumption and CHD. This underscores the point that the type of fat may be more important than the amount of fat. Dr. Hu presented data suggesting that fish consumption may offer protection against developing CHD and type 2 diabetes. Furthermore, replacing saturated and transunsaturated fats with unhydrogenated monounsaturated and polyunsaturated fats may be more effective in preventing CHD than reducing overall fat intake.

The type of carbohydrate also seems to be important. Carbohydrates were formerly classified as simple and complex. The new classification is based on the glycemic index (GI), which ranks foods based on their ability to increase blood glucose. For example, glucose has a GI of 100, baked potatoes have a GI of 93, carrots 49, pasta (remarkably) 39, and peanuts 14. Dr. Hu suggested that using the GI, or another similar measure called the glycemic load, could help predict risk of developing disease. The data from the Nurses' Health Study suggest that a high dietary glycemic load from refined carbohydrates increases the risk of CHD, whereas an increased intake of whole grains may protect against CHD.

In his concluding remarks, Dr. Hu recommended avoiding the mantras "fat is bad" or "carbohydrate is bad" and instead suggested focusing on the types of fat and carbohydrates rather than the total amounts.

Evidence for Dietary Fat as a Risk Factor for Type 2 Diabetes

Barbara Howard, PhD, of the MedStar Research Institute, Washington, DC, examined the connection between fat consumption and the risk of developing diabetes. ^[4] She presented data from 4 studies that examined the relationship between dietary fat consumption and the occurrence of type 2 diabetes. Although several prospective studies have shown a positive correlation between fat intake and diabetes other prospective studies, including the Nurses' Health Study, have failed to show this connection. The ongoing Women's Health Initiative may provide some answers.

Dietary Fat, Insulin Resistance, and Obesity

Dr. Howard remarked that studies have also failed to show a connection between the amount of dietary fat and in insulin sensitivity. However, she also presented data suggesting that the kind of fat consumed can make a difference in insulin sensitivity. People who consume a large amount of monounsaturated fat have demonstrated an increase in insulin sensitivity, whereas those consuming a large amount of saturated fat will be more insulin resistant. The association between dietary fat and obesity is more clear. Data suggest that increased fat consumption leads to obesity, although carefully controlled trials have suggested that the increase in weight is more modest than might be expected. However, the relationship between dietary fat and the development of diabetes is likely through the influence of obesity.

Is Surplus Carbohydrate Converted to Fat?

Marc K. Hellerstein, MD, PhD, of the University of California at San Francisco ^[5] examined the metabolic consequences of consuming a high carbohydrate diet on the manufacturing of new fat, or de novo lipogenesis. Eucaloric replacement of dietary fat by carbohydrate does not induce hepatic de novo lipogenesis to any substantial degree. But how are carbohydrates metabolized? Carbohydrates may either be stored as glycogen or oxidized as fuel. Because there is a limited capacity to store carbohydrates as glycogen, the majority of carbohydrates are oxidized. Dr. Hellerstein presented a schematic showing how carbohydrates induce metabolic processes that allow carbohydrate oxidation to increase and match carbohydrate intake. However, there are no "free calories." If the amount of carbohydrate consumed exceeds the ability to burn it as fuel, it will be converted to fat.

Dr. Hellerstein presented research showing that consumption of high levels of carbohydrate may lead to a decrease in LDL, but an increase in triglycerides and a decrease in HDL. Carbohydrates may also impair clearance of lipids from the circulation. It appears that dietary carbohydrate is a double-edged sword, with both advantageous and deleterious effects.

Dietary Regulation of Insulin Resistance

Gerald Shulman, MD, PhD, ^[6] of Yale University School of Medicine, New Haven, Connecticut closed the symposium by reviewing data which suggest that healthy offspring of parents with type 2 diabetes are most prone to developing diabetes themselves. In addition, the offspring who have an increased level of free fatty acids are most susceptible. Dr. Shulman presented some very detailed research that suggests that fatty acids directly inhibit insulin action. His research indicates that this occurs through glucose transporter-4 (GLUT-4), the transporter that brings glucose into the muscle cell.

Summary: Implications for Clinical Practice

• The type of fats consumed may be more important than the amount. Saturated and trans fatty acids should probably be consumed in small amounts, whereas monounsaturated fats should be emphasized.

   

• Similarly, the type of carbohydrate is important. Carbohydrates with a low ability to raise the blood glucose (low GI) should be emphasized over carbohydrates that lead to a large increase in glucose levels (high GI).

   

• Monounsaturated fat may improve insulin sensitivity.

   

• Consumption of dietary fat has not yet been proven to lead to a risk for developing type 2 diabetes, but it does seem to lead to the development of obesity.

   

• High consumption of carbohydrate, in a eucaloric setting, does not lead to storage of the carbohydrates as fat. Instead, the carbohydrates are preferentially utilized as oxidizable fuel.

   

• Dietary carbohydrates may lead to a decrease in LDL, an increase in triglycerides, a decrease in HDL, and a decrease in insulin sensitivity.

   

References

1. Mayer-Davis EJ. Does diet play a role in the etiology of type 2 diabetes? Fats versus carbohydrates. Presented at the 60th Scientific Sessions of the American Diabetes Association; June 10, 2000; San Antonio, Texas.

2. Hu FB. Diet and type 2 diabetes: types of fat and types of carbohydrates. Presented at the 60th Scientific Sessions of the American Diabetes Association; June 10, 2000; San Antonio, Texas.

3. Hu FB, Stampfer MJ, Manson JE, et al. Frequent nut consumption and risk of coronary heart disease in women: prospective cohort study. BMJ. 1998 Nov 14;317(7169):1341-1345.

4. Howard BV. Evidence for dietary fat as a risk factor for type 2 diabetes. Presented at the 60th Scientific Sessions of the American Diabetes Association; June 10, 2000; San Antonio, Texas.

5. Hellerstein MK. Metabolic influences of dietary carbohydrate: lipogenesis, triglycerides, metabolism, fat balance and insulin sensitivity. Presented at the 60th Scientific Sessions of the American Diabetes Association; June 10, 2000; San Antonio, Texas.

6. Shulman G. Dietary regulation of insulin resistance. Presented at the 60th Scientific Sessions of the American Diabetes Association; June 10, 2000; San Antonio, Texas.