Obesity is increasing at an alarming rate in both adults and children worldwide. In the US, around 28% of men and 33% of women are obese (BMI>30). The prevalence is almost as high in some European countries. Countries going through rapid socioeconomic transitions in Africa, Asia and South America are also facing the health consequences of overweight and obese populations. People who are obese are more likely to develop certain illnesses such as diabetes mellitus, cardiovascular diseases, hypertension and some forms of cancer than normal-weight people.

Obesity occurs when more energy is consumed from food and alcohol than is required to meet biological and activity needs. Fat contains twice as many calories as carbohydrates such as sugars (9 kcalories per gram compared with 4 kcalories per gram). High fat foods are also very palatable and energy dense, making it easier to consume excess energy and put on weight.

Guidelines for reducing weight often recommend cutting down on both fat and sugar. However, it is very difficult to achieve a diet that is both low-fat and low-sugar because sugar intake tends to increase when fat intake is reduced and vice versa. This is known as the “fat-sugar see-saw” and it also explains why replacing sugar with artificial sweeteners to assist weight loss can be counter-productive as it may, in fact, lead to increased fat intake. Allowing sugar in a low-fat diet can make it more palatable and easier to follow.

Studies have shown that obese and overweight people generally consume more energy from dietary fat. In contrast, high sugar consumers are more likely to be slimmer.

The capacity for carbohydrate storage in the body is limited. Consumption of excess carbohydrates, including sugar, produces a rapid increase in their oxidation that is not observed with fat. In addition, conversion of sugar to fat is very limited in humans under normal conditions. On the other hand, the capacity for fat storage in the body is virtually unlimited and excess dietary fat is readily stored in fat deposits.

Fat is the most energy dense nutrient and has the weakest effect on satiety. However, eating carbohydrates, including sugar, triggers signals that promote satiety and decrease hunger. Studies have shown that artificial sweeteners do not provide the same restraint over intake as sugars.

At least 55% of food energy should be derived from a wide variety of carbohydrate sources for all ages except for children under 2 years. Replacing fat in the diet with carbohydrates (starch and sugars), and increasing physical activity levels, can assist in initial weight loss and help to maintain a healthy body weight.

The evidence suggests that a high-fat, high-energy diet, combined with inactive sedentary lifestyles, are the two principle factors increasing the risk of obesity. A combination of high-carbohydrate diets and regular physical activity can assist in the maintenance of an ideal body weight.

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