Doctors and patients are increasingly seeking to reduce moderately high cholesterol levels through dietary means, rather than the prescription of drugs such as statins, which come with potential serious side effects.
Nuts, an important source of mono- and poly-unsaturated fatty acids which are known to have a favorable effect on blood lipids, have captured the attention of scientists in recent years. A number of epidemiological studies have linked them to a significantly reduced risk of dying from cardiovascular disease, and overall longevity.
The study is published in this month's issue of the Journal of Nutrition. It set out to be a systematic review of dietary intervention studies of the independent effect of nuts on lipid concentrations.
Researchers from North-West University in Potchefstroom, South Africa, looked at the results of 23 studies, but because of big differences in study designs no formal statistical analysis was performed. Rather, the published papers were given a rating based on their methodology.
In three studies where participants ate 50 to 100g of almonds a day their total cholesterol decreased by two to 16 percent and LDL ('bad') cholesterol by 2 to 19 percent, compared to those on a control diet.
The same was true in two peanut studies (consumption of 35 to 68g per day), one pecan study (72g per day) and four walnut studies (40 to 84g per day). The results for macadamia nuts (50 to 100g per day) were "less convincing", wrote the researchers.
Overall, they concluded that eating between around 50 and 100 g (1.5 to 3.5 servings) of nuts five or more times a week, as part of an overall diet with total fat content making up around 35 percent of energy and high in mono- and/or polyunsaturated fatty acids may significantly reduce total and LDL cholesterol.
"Future food-based strategies for improving plasma lipid concentrations should consider the lipid-lowering effect of nuts."
In 2003, the FDA approved a qualified health claim that eating 1.5 ounces (42.8g) of nuts a day may reduce the risk of coronary heart disease. According to the American Heart Association, 70.1 million Americans have one or more form of cardiovascular disease, and the prevalence of coronary heart disease is estimated at 13 million.
As far as future research on nuts goes, the North-West team said it would be useful to investigate the effects using larger sample sizes and over longer time periods, especially using mixed nuts and other varieties not yet considered.