The Australian Dietary Guidelines recommend we eat 30 grams (1 ounce) of nuts – a small handful – each day. But many of us know nuts are high in calories and fat.
So should we be eating nuts or will they make us gain weight?
In short, the answer is yes, we should eat them, and no, they won't make us gain weight if eaten in moderate amounts. The fats in nuts are mostly the "good" fats.
And aside from that, our bodies don't actually absorb all the fat found in nuts. But we do absorb the nutrients they provide.
Dietary fat: friend or foe?
Nuts do contain fat, and the amount of fat varies between nut types. For example, a 30g serving of raw cashews or pistachios contains around 15g of fat (0.5 ounces), whereas the same amount of raw macadamias contains around 22g of fat (0.7 ounces).
There are different kinds of fats in our diet and some are better for us than others. Nuts contain mainly monounsaturated and polyunsaturated fats.
These types of fats are known as "good fats". They can help lower cholesterol when we eat them in place of saturated fats.
The type of fats present varies between nuts. For example, walnuts are rich in polyunsaturated fats, whereas other types of nuts such as hazelnuts and macadamias have more monounsaturated fat.
What the evidence says
Even if the type of fat in nuts is good for us, they are still high in fat and calories. But this doesn't mean we should be avoiding them to manage our weight.
Studies that looked at people's eating habits and body weight over a long period have found people who regularly eat nuts tend to gain less weight over time than people who don't.
We see a similar pattern in clinical studies that asked people to include nuts in their diets and then looked at the effects on body weight.
A review of more than 30 studies examined the effects of eating nuts on body weight. It did not find people who ate nuts had increased their body weight, body mass index (BMI), or waist circumference, compared to a control group of people who did not eat nuts.
In fact, one study found that when people ate a pattern of food aimed at weight loss, the group of people who ate nuts lost more body fat than those who didn't eat nuts.
Let's nut this out
There are several possible explanations for why eating nuts doesn't seem to lead to weight gain.
We don't absorb all of the fat in nuts: The fat in nuts is stored in the nut's cell walls, which don't easily break down during digestion. As a result, when we eat nuts, we don't absorb all of the fat. Some of the fat instead is passed out in our faeces. The amount of calories we absorb from eating nuts might be between 5 percent and 30 percent less that what we had previously thought.
Nuts increase the amount of calories we burn: Not only do we not absorb all the calories in nuts, but eating nuts may also increase the amount of energy and fat we burn. It's thought this may partially be explained by the protein and unsaturated fats in nuts, although we don't yet know exactly how this occurs. Increases in the number of calories burnt can help us maintain or lose weight.
Nuts help us feel full for longer: As well as fat, nuts are rich in protein and fibre. So, nuts help to keep us feeling full after we eat them, meaning we're likely to eat less at later meals. Recent studies have also suggested providing people with nuts helps improve the overall quality of the types of foods they eat. This may be because nuts replace "junk foods" as snacks.
People who eat nuts have healthier lifestyles in general: We can't rule out the idea that eating nuts is just a sign of a healthier lifestyle. However, randomised controlled trials, which can control for lifestyle factors like eating habits, still find no negative effect on body weight when people eat nuts. This means the favourable effects of nuts are not just the result of nut eaters having healthier lifestyles – the nuts themselves play a role.
Overall, the evidence suggests nuts are a healthy snack that can provide us with many of the nutrients our bodies need. We can confidently include the recommended 30g of nuts a day in a healthy diet, without worrying about the effect they will have on our waistlines.
Elizabeth Neale, Career Development Fellow (Lecturer), University of Wollongong; Sze-Yen Tan, Senior Lecturer in Nutrition Science, Institute for Physical Activity and Nutrition (IPAN), School of Exercise and Nutrition Sciences, Deakin University, and Yasmine Probst, Senior lecturer, School of Medicine, University of Wollongong.