Hazelnuts and Cholesterol: What Do Clinical Trials Show?

The claim that hazelnuts are good for the heart gets repeated a lot. But what is behind it? We looked at the clinical trials of the last twenty years and the meta-analyses that pooled them.

First, the basics: why is a fatty nut considered "good"?

About 61% of a hazelnut is fat. Most of that fat, however, is monounsaturated, mainly oleic acid. In an inspection study of Giresun Tombul hazelnuts, oleic acid was the dominant fatty acid, with linoleic acid at 9.45%, palmitic acid at 4.86% and stearic acid at 2.55%. The low saturated fat share puts hazelnuts closer to olive oil in nutritional profile.

This matters because the effect on blood lipids depends far more on the type of fat than on the amount.

A randomised controlled trial: the 2013 data

In a study with hypercholesterolaemic participants, 18-20% of daily energy intake came from hazelnuts. Compared with the control diet, the results were:

Total cholesterol fell 7.8%, LDL cholesterol 6.17% and triglycerides 7.3%, while HDL cholesterol rose 6.07%. Flow-mediated dilation, a measure of blood vessel function, also improved markedly.

What the researchers stressed was that the improvement was not limited to cholesterol figures. Oxidised LDL, hs-CRP (an inflammation marker) and sVCAM-1 all fell, pointing to a broader picture than simply "lowering cholesterol".

What does the meta-analysis say?

In a 2016 Bayesian meta-analysis that screened nine studies and pooled three randomised trials, a significant reduction in LDL cholesterol was found in favour of the hazelnut-enriched diet. Total cholesterol also showed a marked downward trend.

In the same analysis, HDL cholesterol, triglycerides and body mass index remained substantially unchanged. That last finding matters: although hazelnuts are calorie dense, they did not cause weight gain in these trials.

A separate study on oxidation resistance

In healthy participants with normal cholesterol, 30 days of hazelnut consumption raised the lag time for LDL oxidation from 54.6 to 65.2 minutes. Alpha-tocopherol (vitamin E) content within LDL increased and oxidised LDL levels fell.

This appears directly linked to the vitamin E content of hazelnuts. Because oxidation of the LDL particle is regarded as one of the early steps in arterial disease, the finding has drawn attention.

The critical detail: the effect is not permanent

Perhaps the most striking result of the 2013 trial was this: four weeks after participants stopped eating hazelnuts, the measurements returned to baseline.

That tells us hazelnuts are not a food to be eaten as a "course" and then dropped. The researchers' recommendation was to add hazelnuts permanently to the daily diet without increasing total calorie intake.

The study in children

In an eight-week randomised trial with children and adolescents with primary hyperlipidaemia, LDL cholesterol fell and the HDL/LDL ratio improved in groups given hazelnuts with and without skin. Interestingly, no clear difference was found between the skin-on and skin-off groups.

So how much per day?

The amounts used in trials corresponded to 15-20% of daily energy, roughly 30-60 grams for an adult. The common recommendation in nutrition sources is a handful a day, about 30 grams.

The critical condition, emphasised repeatedly in the studies, is not to increase total calories when adding hazelnuts. They should replace a less nutritious snack rather than be added on top of existing meals.

Unfortunately it is not that simple

Most of these studies used relatively small groups over short periods. Only three randomised trials could be included in the meta-analysis. The findings are promising, but saying "eat hazelnuts and your cholesterol will drop" overstates them.

Hazelnuts are meaningful as part of a balanced diet. If you have a cholesterol problem, treatment decisions belong with your doctor; this article is for information and is not medical advice.

Sources

Orem A. et al., "Hazelnut-enriched diet improves cardiovascular risk biomarkers beyond a lipid-lowering effect in hypercholesterolemic subjects", Journal of Clinical Lipidology, 2013.

Perna S. et al., "Effects of Hazelnut Consumption on Blood Lipids and Body Weight: A Systematic Review and Bayesian Meta-Analysis", Nutrients, 2016.

Yücesan F.B. et al., "Hazelnut consumption decreases the susceptibility of LDL to oxidation", Anatolian Journal of Cardiology, 2010.

Deon V. et al., "Effect of hazelnut on serum lipid profile and fatty acid composition of erythrocyte phospholipids in children and adolescents with primary hyperlipidemia", Clinical Nutrition, 2018.