Peanuts are not normally considered a diet food - primarily because they are known to have a relatively high caloric level - but a new study in the US has shown that they can be an effective diet food because they act as an appetite suppressant.
In what is claimed to be the first clinical study designed to confirm and explain a body of epidemiological data showing that nut-eaters tend to have a lower body mass index (BMI) than non-nut-eaters, researchers from Purdue University in the US studied the effects of daily peanut consumption on dietary intake, satiety, energy expenditure and body weight. Their research is published in the August issue of the International Journal of Obesity.
The team led by Dr Richard Mattes of Purdue's department of foods and nutrition, found that the high protein and fibre content in peanuts may play an important role in curbing hunger and can thus help prevent weight gain. Earlier research had shown that people who regularly consumed about an ounce of peanuts, nuts and peanut butter had lower BMI scores than those who did not.
The Purdue study focused on three diet treatments, each including 500 calories of peanuts. The first group included the peanuts without any dietary guidance, while the second group were told to add the peanuts to their usual diets. The third group followed individualised diets, substituting the peanuts in place of 500 calories in the diet.
According to Dr. Mattes, in both the first and second groups, the subjects spontaneously remarked that they felt full and could not eat all of the food they would normally eat on a daily basis. The men and women in the study compensated for most of the additional calories by eating less than usual, even without dietary instruction to do so. This resulted in lower actual weight gain than was expected from including the extra calories into their routine.
In the first group, the weight gain was just 2.2 pounds (998 grams), far below the expected weight gain of eight pounds (3.6 kilograms). In addition, total daily energy intake was significantly lower than predicted, as the subjects naturally compensated for 66 per cent of the energy provided by the peanuts.
In the second diet group, subjects were instructed to add peanuts to their usual diets for three weeks. Because this treatment period was shorter, expected weight gain with peanuts added to their usual diet was about three pounds (1.4kg) while actual weight gain was only about one pound (454g). This strong dietary compensation may be due to the high satiety value of peanuts, the researchers said.
In the third group, the researchers discovered that fat intake was decreased by 50 per cent and the calories were replaced with an equivalent amount of energy from peanuts. Essentially, subjects were instructed to follow a low-fat diet and include the peanuts each day. Weight gain was predicted to be zero because this was a calorie substitution diet. As predicted, no significant change in body weight was seen when peanuts were substituted for other fats in the diet and overall diet quality improved.
"Peanuts have a great mix of features, such as high protein, high fibre, and a crunchy texture, which enhance satiety," said Dr Mattes. The eating satisfaction is partly responsible for the lack of actual weight gain in this study. The results of this study are consistent with earlier research at Purdue showing that peanuts and peanut butter satisfy hunger up to five times longer than some high-carbohydrate snacks such as rice cakes. There is also some previous research that demonstrated that whole peanuts are inefficiently absorbed, which may be another possible factor contributing to the prevention of weight gain.
The researchers also looked at the hedonics, or the enjoyment factor, of eating peanuts, as the lack of enjoyment is often cited as reason for giving up on traditional diet foods. They discovered that the enjoyment ratings for peanuts stayed high over the five-month trial period.
In addition to containing good unsaturated fat, peanuts provide plant protein and fibre, as well as vitamin E, folate, potassium, magnesium and zinc, all which are thought to be important to health. Peanuts also contain bioactive components such as phytosterols, flavonoids and antioxidants.