Energy-Dense Walnuts Tied to Better Diets
Walnuts significantly improved diet quality, endothelial function, and cholesterol without negatively affecting blood pressure or glucose levels in a new study.
In a randomized trial of 112 participants who either spent 6 months eating walnuts followed by 6 months on a diet with no walnuts, or vice versa, the walnut group had a significantly better diet both with dietary counseling (9.14±17.71 versus 0.40±15.13 on the 2010 Health Eating Index; P=0.02) and without it (7.02±15.89 versus −5.92±21.84; P=0.001).
In addition, endothelial function and total and low-density lipoprotein (LDL) cholesterol improved from baseline with the walnut diet of about 56 g of walnuts per day -- about a handful -- though these measures didn't significantly vary when the two arms were directly compared, according to lead author David Katz, MD, at the Yale University School of Medicine.
They published their findings in BMJ Open Diabetes Research & Care on Monday.
"Walnuts are uniquely nutritious, so they likely confer benefits that other nuts do not," said Katz in an email to MedPage Today, though he added that research is limited. "In general, nut consumption has been associated with health benefit, including reduction in all-cause mortality, but few nuts have been studied in isolation -- mostly walnuts and almonds, and to a lesser extent, pistachios."
The walnut diets neither significantly improved or worsened body mass index, percent body fat, percent body water, or visceral fat (P>0.05) when the two arms were compared. But waist circumference improved in the walnut plus dietary counseling group.
The walnut diets also had no significant effect on blood pressure or fasting blood glucose.
"Our findings persisted after controlling for age, gender, caloric intake, fiber intake, monounsaturated fatty acid intake, polyunsaturated fatty acid intake, omega-3 intake, and physical activity level," wrote the authors.
"Compared with most other nuts, walnuts have a higher content of polyunsaturated fatty acids," wrote the authors. The benefits vary, according to previous research, from improved semen quality to better cardiovascular health.
But walnuts also have a high energy density, so there's the possibility that it contributes to a positive energy balance and to weight gain, wrote Katz and colleagues, though there was no evidence for that in this study.
The Nurses Health Study has found that increased nut consumption is associated with a lower risk of type 2 diabetes, but the authors of the present study found no evidence of an increased insulin response or a decrease in HbA1c levels. "This may relate to variations in study populations, study duration, or the treatment dose, among other potential explanations," wrote the authors.
There were no caloric restrictions on participants' diets aside from the walnuts provided each week. Thirty-one of the participants were men and all were recruited in Connecticut via flyers and newspaper advertisements. Ages ranged from 25 to 75, and all were nonsmokers with a high risk of diabetes as determined by waist circumference, fasting blood glucose, or metabolic syndrome.
Exclusion criteria included those who were allergic to walnuts or other nuts, those with a current eating disorder, those with restricted diets by their own choice, those receiving pharmacotherapy for obesity, those on supplements like fish or flaxseed oil, and those with diabetes or certain cardiovascular complications. Fifteen participants did not complete the study.
In between the walnut stage and the no-walnut stage, participants had a 3 month washout phase. A dietitian customized diets to make room for walnuts in the diet, but participants were given freedom to determine how they consumed the walnuts, wrote the authors.
The Healthy Eating Index is based on the 2010 Dietary Guidelines for Americans and is used to look at how effective a nutritional intervention is.
Limitations of the study include the fact that most participants were white women, so the results may not be generalizable. The study was powered for the primary outcome, the Healthy Eating Index, and may have been underpowered for other reported measures. It relied on self-report of dietary intake -- which other studies have shown to be severely limited -- and participants were not given a restricted diet.
"Further investigation is warranted in a more diverse population to replicate these findings," concluded the authors.
Katz disclosed compensation from the California Walnut Commission for a public speech.