Scientists conducted a dietary experiment lasting 31 months on 38 macaques (there were 43 at the start, but two did not stick to the diet and three died during the experiment). During the experiment, the monkeys that were on a Western diet rich in animal fats and proteins gained twice as much weight as the group on a Mediterranean diet. Most of the excess weight of the ‘Westerners’ was fat, even though they ate less by the end of the experiment. The Mediterranean diet consists mainly of vegetables, fruits, seafood and poultry, unlike the Western diet, which is rich in animal fats and proteins, red meat and processed meat products (which put consumers at increased risk of cancer). The Mediterranean diet is considered to be beneficial to health, for example, improving cognitive abilities. However, until recently, there had been no long-term studies examining the effect of this diet on weight gain and calorie consumption. Scientists from the United States decided to remedy this and compared the Mediterranean diet with the Western diet by conducting a study on macaques (article by Shively et al. Mediterranean versus Western Diet Effects on Caloric Intake, Obesity, Metabolism, and Hepatosteatosis in Nonhuman Primates published in the journal Obesity). The study lasted 31 months, which, according to the scientists, corresponds to 8–12 years of human life for macaques. The macaques were divided into two groups, which were first fed regular laboratory food for monkeys to ‘equalise’ them, and then, after 6 months, switched to Western and Mediterranean diets. The scientists calculated the composition of the Western diet so that it was as similar as possible to what the average American woman aged 40–49 eats (according to data from the US Department of Agriculture). It included foods such as high-fructose corn syrup (an ingredient widely used in the United States as a source of sucrose, for example, in Coca-Cola), corn oil, lard (fat rendered from pork) and beef fat. The Mediterranean diet included fish meal, walnuts, beans, bananas, applesauce, tomato paste, and olive oil, among other things. Overall, both diets were designed to have the same number of calories and approximately the same ratio of carbohydrates/proteins/fats and cholesterol content. During the diet trial, the scientists looked at how the animals’ body weight, fat mass, calorie intake and physical activity changed. Immediately after the macaques were switched from their laboratory diet to the experimental diets, consumption jumped sharply in the Western diet group: their consumption was 80 kcal per kilogram of body weight, compared to 68 kcal/kg in the Mediterranean diet group. But then the appetite of the Mediterranean group began to grow, while that of the Western group began to decline, and at about the halfway point of the experiment, in the 15th month, the groups were equal in energy consumption, and then the ‘Mediterranean’ group surpassed the ‘Western’ group. Calorie consumption per kilogram of body weight during the experiment. Blue – group on the Western diet, red – on the Mediterranean diet. Figures from an article in Obesity. The weight of the test subjects changed quite differently. The macaques from the Western diet group immediately took the lead and maintained it until the end. By the end of the experiment, their body mass index was on average 12% higher than that of the macaques in the Mediterranean diet group. The macaques were examined in a tomograph, which revealed that most of the weight gain was due to adipose tissue. Changes in body mass index over the period of the experiment. Blue – group on Western diet, red – group on Mediterranean diet. Observation of the animals’ behaviour showed that macaques on the Western diet were more active, but this did not affect their weight gain. Scientists believe that obesity resulting from the Western diet can be explained mainly by the composition of fats in both diets. Although the two diets did not differ in terms of calories and the ratio of carbohydrates to proteins and fats, there were significant differences in the composition of fats. The Western diet contained about a third more saturated fatty acids than the Mediterranean diet. In turn, the Mediterranean diet contained the most monounsaturated fatty acids, and the content of polyunsaturated fatty acids did not differ as much: only 6% more than in the Western diet. Perhaps the good appetite of the ‘Western’ macaques at the beginning of the experiment was related to the composition of fats – previous studies on rodents have shown that a diet rich in saturated fats leads to overeating. In addition to the unpleasant consequences of the Western diet, the scientists also noted an increase in liver fat in individuals on the Western diet – 40% compared to 14% in the Mediterranean diet group – and an increase in blood insulin levels, which can be characterised as a prediabetic condition. The authors of the study believe that the Mediterranean diet protects against overeating and obesity, reduces the risk of diabetes, and may be beneficial for people with enlarged livers.