Nancy A. Melville

February 22, 2010

February 22, 2010 (Crystal City, Virginia) — The daily consumption of walnuts has the potential to improve endothelium function and plasma lipids in patients with type 2 diabetes, according to a study presented here at Preventive Medicine 2010: the Annual Meeting of the American College of Preventive Medicine.

With a high polyunsaturated and monounsaturated fat content, many types of nuts have gained favor as a healthy addition to any diet, and walnuts, in particular, have a high level of omega-3 fatty acids. Researchers at Yale University in New Haven, Connecticut, theorized that the addition of walnuts could provide cardiovascular benefits to diabetics.

"We know nuts are rich in fiber, and have a variety of micronutrients, minerals, B vitamins, magnesium, and a number of properties that make them a likely candidate for benefits in people with cardiovascular risk," David L. Katz, MD, a coauthor on the study and director of the Yale University Prevention Research Center, told Medscape Public Health & Prevention.

"We asked if this could prove a vascular benefit in an at-risk population — adults with type 2 diabetes," Dr. Katz explained.

The randomized controlled crossover trial involved 24 subjects (14 women and 10 men) with type 2 diabetes with a mean age of 58 years. The subjects were randomly assigned to receive an ad libitum diet enriched with 56 g of walnuts per day or an ad libitum diet without walnut supplementation.

The study involved an 8-week washout period between each 8-week treatment phase. Before and after each phase, participants underwent endothelial function testing and assessment of cardiovascular biomarkers.

The primary outcome measure for the study was a change in flow-mediated dilatation (FMD) after 8 weeks, and secondary outcome measures included changes in plasma lipids, glycated hemoglobin (HbA1c), fasting glucose, insulin sensitivity, and anthropometric measures.

There was a significant improvement in FMD among the participants consuming the walnut-enriched diet, compared with those not consuming diets with walnuts (FMD, 2.2% ± 1.7 % vs 1.2% ± 1.6 %; P = .04).

Although the subjects consuming walnuts showed reductions from baseline values in total cholesterol (–9.7 ± 14.5 mg/dL; P < .01) and low-density lipoprotein (LDL) cholesterol (–7.7 ± 10.0 mg/dL; P < .01), the reductions were not significant compared with those on the diet without walnuts (total cholesterol, –9.7 ± 14.5 vs –4.5 ± 23.0; P  .38; LDL cholesterol, –7.7 ± 10.0 vs –7.8 ± 20.6; P = .97).

The study also did not find any significant differences in anthropometric measures, HbA1c, fasting blood glucose, or insulin sensitivity between the 2 diets.

However, Dr. Katz said, the endothelial function findings were key.

"We did not see significant improvement compared with placebo in a variety of the other cardiac risk factors we measured as secondary outcomes, but when it comes to endothelial function, I'm reminded of The Lord of the Rings. It's the 1 ring to rule them all," he said.

"What it tells us is how the blood vessels are feeling in light of everything that is flowing by. You can measure everything you can think of that is flowing by, such as LDL cholesterol, triglycerides, or stress hormones, or you can simply ask the blood vessels, in essence: 'Given everything that is floating by, how are you feeling today?' "

"And the blood vessels in this study said, 'we feel better after we've been fed walnuts daily.'"

The study also evaluated whether the supplementation of walnuts, representing about 365 calories per day, would cause weight gain in a patient population that is already prone to obesity. The results showed no significant weight gain among subjects receiving walnut supplementation.

"If you're adding a nutritious food to the diet, the benefits of the food might sometimes be offset if weight gain occurs, and obviously, with diabetics, the last thing you want to do is cause weight gain," Dr. Katz explained. "But our study suggests you can make room for a highly nutritious food in your diet, particularly if it tends to fill you up, as nuts do."

Although the study focused on diabetics, Dr. Katz said his team has the more ambitious goal of showing the benefits of walnuts in healthy individuals.

"To prove something works, you often have to begin with something that is broken and show that you can fix it," he said. "So we're working with diabetes first, then prediabetes, then pre-prediabetes, and then hopefully we can make the convincing argument that this should be incorporated in everyone's diet."

"I've long held the view that in order to peddle prevention, we have to work backward, and my interest is keeping healthy people healthy. I am a doctor and I take care of sick people, but I'd prefer people don't get sick in the first place."

Almonds have also shown important benefits, and diabetics may better benefit from a mix of nuts, said Wael Al-Delaimy, MD, PhD, associate professor of epidemiology and chief of the Division of Global Health at the University of California, San Diego.

"A combination of different nuts is always healthy and should be recommended as part of a healthy lifestyle, since each nut has certain unique components," he said. "That being said, walnuts and almonds do stand out with significant benefits because of the type of fats they contain."

Almonds, too, appear to have their own unique benefits for diabetes, he added.

"Omega-3 fatty acids that are mostly in walnuts are very good for nerve development and the brain, and also the heart, but some research suggests that almonds have specific sugar-control qualities."

Dr. Al-Delaimy noted that the walnut study did not show any similar benefits.

"The dose of the walnuts in this study is double the daily required value, yet it still did not control the glucose-related measures in participants," he pointed out.

Dr. Katz concurred that almonds are highly nutritious and likely offer benefits as well. "On the NuVal scale of overall nutritional quality from 1 to 100, both walnuts (at 82) and almonds (at 79) stand out as nutrition superstars."

In the meantime. Dr. Katz and his team are moving ahead to the next step of evaluating the benefit of walnuts in prediabetics.

"We just received funding to study a larger cohort of prediabetics as we pursue this notion of food as medicine. Then the question is, 'Who should take this medicine?', and we don't think the answer is just diabetics."

Dr. Katz's lab is funded in part by the Centers for Disease Control and Prevention in Atlanta, Georgia. The study received funding from the California Walnut Commission. Dr. Katz and Dr Al-Delaimy have disclosed no relevant financial relationships.

American Academy of Pain Medicine (AAPM) 26th Annual Meeting: Poster abstract 212645. Presented February 19, 2010.

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