Nuts have cemented their place as a heart-healthy food, but what about their effect on weight? A major new review of the evidence, published in Obesity Reviews, sets the record straight (1).

The systematic review and meta-analysis of six prospective cohort studies and 86 randomised controlled trials (RCTs), involving more than half a million people, found nut consumption does not lead to increased body fatness.

Nuts were linked with a 7% lower rate of overweight/obesity in long-term prospective cohorts, and RCTs showed a ‘high certainty’ of no adverse effect of nuts on body weight.

The review also found that, with an overall negative energy (kilojoule) balance (such as is the case in many weight-loss diets), including nuts led to better results than omitting them – when it came to body weight, body mass index (BMI) and waist-to-hip ratio.   

“Health professionals and dietary guidelines may recommend nuts, for those without allergies, for their cardiometabolic benefits without stipulations or concern of an adverse effect on weight control”. (1)

Results from the Prospective Cohort Studies:

The six prospective cohort studies involved a total of 569,910 adults, from the United States and 10 countries in Europe, that were followed by researchers for an average (median) of 18 years (range: 2.3 to 24 years).

People who ate the most nuts, compared to those who ate the least, had a:

• 7% reduced rate of overweight/obesity
• 28% lower risk of an unhealthy waist size (classed as ≥94 cm for men and ≥80 cm for women)
• 5% reduced risk of gaining ≥5kg.

Results from the Randomised Controlled Trials:

The review included 86 RCTs (the ‘gold standard’ in studies), with a combined 5,873 participants. Most of the trials were run in the US, and eight were conducted by Australian-based researchers. The average (median) follow-up duration was eight weeks (range: three to 104 weeks).

The pooled results of the RCTs showed a ‘high certainty’ that nuts do not affect body weight, BMI or body fat, compared with control.

When it came to the amount (or ‘dose’) of nuts, greater reductions in body weight and body fat were seen with increasing nut dose. In categorical analyses, nut doses ≥45.5 g/day indicated lower adiposity measures, compared to lower doses.

One of the barriers to increasing the consumption of nuts is the perception that they contribute to weight gain more than other healthy foods, due to their high energy density.

Nut ‘caveats’ are outdated

Advice on nut intake, including through dietary guidelines, often comes with a ‘caution’ to limit the amount eaten.

The Australian Dietary Guidelines state that nuts, with a recommended serve size of 30g, are only to be used occasionally, as a substitute for other foods in the ‘protein-rich’ food group. The reason given is their energy-density.

But according to the researchers of this major new review, “the concern that eating nuts contributes to increased adiposity appears unwarranted”.

How do nuts help manage weight?

There are several biological mechanisms behind why nut consumption isn’t linked with overweight/obesity risk and other measures of adiposity, including:

1. The healthy fats in nuts. Nuts are rich in mono- and polyunsaturated fats, which are thought to be more readily oxidised (or burnt up) and are better at increasing the body’s metabolic rate after eating (that is, ‘diet-induced thermogenesis), compared with saturated fats.
2. The filling nature of nuts. Nuts are rich in plant-based protein, unsaturated fats, and fibre, which increase hormones that assist with satiety (such as cholecystokinin and peptide YY). Plus, needing to chew nuts back to a small enough size to swallow activates signalling systems in the body that, in turn, reduce appetite.
3. The physical structure of nuts. The fibrous plant cell walls of nuts stop them from being completely digested within the gastrointestinal tract, which means not all of the fat in nuts is absorbed by the body – resulting in reduced energy (kilojoule) availability.

The Atwater Factor, a system for determining the energy value of foods which was founded more than a century ago, may overestimate the kilojoules obtained by the body from eating nuts – by up to 26%, depending on the nut type and form.