Walnuts considerably improved diet quality, the inner lining of blood vessels, and cholesterol levels in people who participated in a recent study. Walnuts were able to achieve these improvements without negatively affecting blood pressure or glucose levels in individuals who participated in the research.
In a randomized trial, researchers found that when 112 study participants ate walnuts for six months -- followed by six months of a diet void of walnuts -- individuals of the study in the walnut group had a significantly better diet with or without diet counseling.
Thirty-one of the participants in this new walnut study were men ranging from ages 25 to 75, and all were recruited in Connecticut. Newspaper advertisements and flyers were used to recruit the participants. All of the study subjects were nonsmokers with a high risk of diabetes, which was determined by fasting blood glucose, metabolic syndrome, or waist circumference.
Participants were excluded if they were found to be allergic to walnuts or other nuts, in addition to those with restricted diets by their own choice, those with diabetes or certain cardiovascular complications, those on supplements like fish or flaxseed oil, those receiving pharmacotherapy for obesity, and individuals with a current eating disorder. Notably, 15 of the participants did not complete the study.
Total and low-density lipoprotein (LDL) cholesterol and endothelial function improved with the walnut diet of about a handful, or 56 grams of walnuts per day, according to the lead author Dr. David Katz, at Yale University School of Medicine.
However, the authors of this latest study found no evidence of an increased insulin response or a decrease in hemoglobin A1c levels. The authors wrote, "This may relate to variations in study populations, study duration, or the treatment dose, among other potential explanations."
Dr. Katz offered a brief comment about the health benefits and value of walnuts.
"Walnuts are uniquely nutritious, so they likely confer benefits that other nuts do not. 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 diets in this latest study, which included consuming a handful of walnuts each day, neither significantly improved nor worsened body mass index, percent body water, percent body fat, or visceral fat when the two walnut groups were compared.
However, in the group that consumed walnuts for the six-month period, participants' waist circumference improved.
The researchers note that the walnut diets did not have a significant effect on fasting blood glucose or blood pressure. The authors wrote, "Compared with most other nuts, walnuts have a higher content of polyunsaturated fatty acids."
Besides improving cholesterol levels, diet quality, and endothelial function, walnuts also have a high energy density. This insight suggests that there's the possibility that walnuts contribute to a positive energy balance and to weight gain, though there was no evidence for that in this recent study.
In addition to this study on the health effects of walnuts by Yale University, Harvard's Nurses' Health Study -- the largest and longest running investigations of factors that influence women's health -- found that increased nut consumption is associated with a lower risk of type 2 diabetes.
Dr. Katz told MedPage Today that this latest research on walnuts has its limitations, including the fact that most participants were white women, so the results cannot be generalized. Additional limitations included that the results of the study relied on self-reports of dietary intake by the study's participants -- which other studies have shown to be greatly limited. In addition, participants were not given a restricted diet.
The authors concluded their research by noting that "Further investigation is warranted in a more diverse population to replicate these findings."
On Monday, the authors published their findings of this latest study on walnuts in BMJ Open Diabetes Research & Care.