Controlled trials that assessed the effects of a walnut-enriched diet, compared with control, and reported on at least one blood lipid outcome were eligible for inclusion. The amount and frequency of walnuts included in the diet had to be stated. Studies that assessed the effects only after a meal, and those comparing the effects to those of other nuts, were excluded. The outcomes of interest were the mean serum total cholesterol, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and triglyceride concentrations. Other outcomes were weight change, markers of inflammation, oxidative stress, endothelial function, and antioxidant capacity.
Some of the included studies recruited people with normal blood cholesterol, others with modest hypercholesterolaemia. Some studies included people who were overweight or obese or who had diabetes or metabolic syndrome. Mean ages ranged from 24 to 66 years; between 28% and 100% were men; and where reported the body mass index ranged from 20 to 36. Baseline total cholesterol ranged from 159mg to 277mg per dL. Treatment lasted from four to 24 weeks. Walnuts contributed from 5% to 24% of the total calories in the diet. Control diets included low fat, Mediterranean, average for the country, and cholesterol lowering diets.
Study selection was carried out by one author and checked independently by a second author.