The therapeutic challenge for nutrition for type 2 diabetes is not only for glucose control,  but to improve insulin sensitivity and reduce the level of risk factors for cardiovascular disease (CVD). Diet treatment can do more for each of these than is generally realized. Diet treatment is often focused on overweight and obesity, and if patients achieve little or no weight loss in the long term, they may lose interest. Working to decrease diet quantity for weight loss, although beneficial to most type 2 diabetic patients, may become confused by physicians and patients with better diet quality,  something different.  I focus on diet quality in this article.

Lowering carbohydrate to about 40% of daily energy and increasing fat and protein improves day-long glucose control and reduces serum insulin. Carbohydrate at 40% is far from a ketotic level,  and compatible with many types of diets. The immediate follow-up question is what type of fat and protein to emphasize. The answer on type of fat emerges from recent studies of large populations, metabolic studies, and basic science. Polyunsaturated fats commonly used in the diet (eg,  linoleic acid,  an essential omega-6 fatty acid from vegetable oils)  improve insulin sensitivity in animal models and in humans, as proven by euglycemic-hyperinsulinemic clamp.  In large US populations, a moderate intake of 6% of daily energy, the upper quintile of the population, predicts approximately a 40% reduced risk of type 2 diabetes.  Monounsaturated fats, found in vegetable oils and nuts, also are associated with reduced risk but not as much as polyunsaturated fats.

Linoleic acid produces other benefits and has the strongest effect of lowering low-density lipoprotein (LDL) cholesterol and the LDL/high-density lipoprotein (HDL)  ratio.  Diets rich in linoleic acid were tested in randomized clinical trials in the 1970s;  they lowered incidence of nonfatal and fatal CVD.  Polyunsaturated fats could be increased to at least 10% of daily energy, corresponding to a 35% reduced CVD risk. Thus, a healthy dietary pattern that emphasizes polyunsaturated fats from vegetable oil is well established in preventing and treating CVD and type 2 diabetes.
Polyunsaturated fats also include the omega-3 type.

The omega-3 class is composed of α-linolenic acid, from vegetables and vegetable oils such as soybean oil and canola (rapeseed) oil, and eicosapentaenoic acid, docosapentaenoic acid, and docosahexaenoic acid from fish oil.

Several clinical trials in Europe and Japan found that a small amount of omega-3 fats (eg,  1 g/d)  reduced coronary events.  Epidemiologic studies in the United States, Europe, Latin America, and Asia form a strong population basis for cardiovascular benefits of omega-3 fatty acids.  It is unknown if higher doses would confer more protection or perhaps even the opposite. It is reasonable to make sure that the diet has at least 1 g of omega-3 fatty acids from fish, or 3 or 4 g from α-linolenic acid.

A healthy dietary pattern can be described as rich in whole grains, fiber, fruits, vegetables, nuts, poultry, fish, low- or nonfat dairy products, and vegetable oils; low in sugar-containing drinks,  desserts,  and snacks;  and low (but not necessarily absent)  in red meat and other saturated fat and cholesterol-containing foods and fats.  The Dietary Approaches to Stop Hypertension (DASH) diet is one example. It lowered blood pressure substantially and lowered LDL cholesterol. The OmniHeart study modified the DASH diet so that 10% of energy from carbohydrate was replaced with 10% unsaturated fat or protein, about half from plant sources. The unsaturated fat and protein diets enhanced the dietary effects on blood pressure, LDL cholesterol,  and triglycerides.  The unsaturated fat diet had the best effect on HDL cholesterol, whereas protein lowered HDL, an unexpected result that needs confirmation.  A similar diet such as a traditional Mediterranean diet was studied earlier in Italy and found to reduce blood pressure and lipid risk factors.

Whereas it is fully established that the type of dietary fat is crucial, it remains contentious whether the type and form of dietary carbohydrate in the diet matters for glycemic control or for CVD. Carbohydrate-rich foods such as 100% whole-grain breads,  fruits,  and vegetables are an integral part of a healthy diet for diabetes and CVD. My view of the evidence is that these carbohydrate-rich foods, as described in the DASH or OmniHeart study diets, or traditional Mediterranean diets (except for white bread and white rice), are healthy for people with type 2 diabetes, regardless of their individual glycemic index. I also hypothesize that the better foods for glycemic control and insulin sensitivity will be those with low glycemic index. I readily concede that some respected colleagues would state these hypotheses differently. Thus, to study whether type of carbohydrate influences glycemic control and CVD risk, the current ‘DASH-type’ controlled diet trial is focusing on glycemic index as the major dietary variable.

A healthy dietary pattern that is low to moderate in carbohydrate,  increased in polyunsaturated fats,  and rich in fruits, vegetables, and nuts is strongly linked to a low incidence of type 2 diabetes and CVD, and improves insulin sensitivity, blood pressure, and lipid risk factors.

Reduced dietary sodium further lowers blood pressure.

These favorable effects occur in overweight or obese people who maintain their body weight, by force of study design or by their own actions. Although weight gain in adult life causes most type 2 diabetes and its adverse metabolic effects,  total reversal of weight gain is achievable by a few people, but not by most. However, optimal diet quality improves the metabolic pathology of obesity. This must be taught to patients so that they are encouraged to experience the many health benefits of nutritional,  non-pharmacologic treatment.

Disclosure
No potential conflict of interest relevant to this article was reported.

Corresponding author
Frank M. Sacks, MD
Cardiovascular Disease Prevention, Department of Nutrition,
Harvard School of Public Health, 665 Huntington Avenue, Boston,
MA 02115, USA.
E-mail: .(JavaScript must be enabled to view this email address)
Current Diabetes Reports 2008, 8:249–250
Current Medicine Group LLC ISSN 1534-4827

Disclosure
This work is supported in part by an unrestricted grant from the Research to Prevent Blindness, New York, NY.

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Frank M. Sacks, MD