Diets Low in Fat are not Solution to Obesity Epidemic
For the Chairman of the Harvard Nutrition Department, nutritionists and health officials were wrong to demonize fats: “The explosion of obesity in developed countries may be due to this belief that carbohydrates do not grow “.
President of the Department of Nutrition at Harvard School of Public Health (Boston, Massachusetts), Professor Walter Willett is considered one of the best, if not the best, nutritionist in the world. At his side works indeed the team of epidemiologists most prestigious of this field. For almost 30 years, they have followed two huge cohorts: the 80,000 women of the Nurses ‘Health Study and the 50,000 men of the Health Professionals’ Follow-up Study (“Study of Health Professionals”) ). The information accumulated by Walter Willett and his team from these unprecedented prospective studies provided valuable insights into the influence of diet on health. He sums them up here.
What to attribute the epidemic of obesity that affects Western countries?
Prof. Walter Willett: Many factors contribute to it, but all of them come down to either a decrease in physical activity or an increase in calories consumed. Unfortunately, our society encourages inactivity; School physical activity programs are too often reduced or eliminated. The number of hours spent watching television is enormous, and in many studies it is the factor most strongly associated with being overweight. Of course, the agri-food industry has its share of responsibility. She conducts extensive research programs to find out how to make us eat more, for example by maximizing practicality, by playing on color, sugars, calories. In other words, it creates hooks to exploit human weaknesses and we bite into them. Unfortunately, the scientific community contributed to this problem by spreading the message that only calories from fat are counted, and this led many to believe that large amounts of sugar, cereals, starchy foods could be consumed without pay the price in terms of weight.
Many people try yet on the advice of their doctor, lose weight by eating less fat.
I published a synthesis of studies on the relationship between dietary fat and body weight. It is true that some, when prescribed a diet low in fat, will lose a few pounds in a few months. But most of them return before one year. Considering studies of this type that have lasted more than a year, they show no appreciable reduction in body weight. One of the best controlled studies was reported by Dr. Sacks and McManus: after 18 months, people who followed a low-fat diet actually lost less weight than those who followed a Mediterranean diet with a contribution of moderate fat. (1) Thus, there is now strong evidence that low-fat diets will not be the answer to the obesity epidemic.
For many nutritionists, carbohydrates have no impact on waist circumference.
There is a widespread belief among the public and some doctors that it is impossible to get fat by consuming carbohydrates. It’s silly ! You can get fat with excess calories, no matter where it comes from. The explosion of obesity in developed countries may be due to this belief that carbohydrates do not make you fat! We have studies that suggest that refined carbohydrates disrupt the appetite control mechanisms. After a meal rich in fast carbohydrates, one feels satiated at first. The blood sugar rises, but under the surge of insulin, it then goes down to a low level, and you need to eat again. This is one of the mechanisms by which fast carbohydrates maintain hunger. It is very possible that, depending on whether one consumes carbohydrates with a high or low glycemic index, one favors or not the gain of weight.
Do the current nutritional recommendations take into account these differences?
In the United States, there is a dietary “pyramid” which makes it possible to visualize the relative share of each food group in an ideal diet. This pyramid is not even worth the paper on which it is printed. The message is that carbs are good, and fats are bad. This is not correct. By following such recommendations, one can increase one’s cardiovascular risk, by abandoning good fats and overloading carbohydrates. The problem is that the scientific community is having the hardest time questioning itself.
Many doctors and nutritionists believe that carbohydrate consumption does not increase the risk of diabetes. What did you find on your side?
We have known for a long time that highly refined carbohydrates cause blood glucose levels to rise dramatically and cause insulin excretion. We therefore hypothesized that people who consume too much refined food have a chronic excess of insulin that can deplete the pancreas and lead to diabetes. We have shown that women on high glycemic index (GI) diets high in fiber and low in fiber have a 2.5-fold increase in diabetes. In men, the risk is of the same order. Foods that have a high glycemic index (GI) are, to schematize, those that have a significant impact on blood glucose. The baguette of white bread, fried potatoes, some breakfast cereals have a higher glycemic index.
Do refined carbohydrates have other health consequences?
We found an association between high glycemic index diets high in carbohydrates and the risk of cardiovascular disease in women. Those who consume the most of these carbohydrates have a risk multiplied by 2. One thing seems clear, the harmful effects of a diet rich in refined carbohydrates depend on individual resistance to insulin, that is to say ability of cells to capture blood sugar.Jefferson’s study of US Post employees has shown that diets high in refined carbohydrates increase insulin levels and triglycerides, especially since insulin resistance is pronounced.
You have published nutritional recommendations that rehabilitate fats since they occupy a place as important as carbohydrates. Are not fats dangerous for your health?
There has never been any serious evidence that the caloric contribution of fat plays a big role in long-term health. That’s why I put “good” fats next to “good” carbohydrates in my recommendations.
What advice can we give to stay healthy?
It is important to stay lean and active, to practice regular physical activity such as walking. It is necessary to replace each time saturated fats and “trans” fats can be replaced by polyunsaturated fats as contained in vegetable oils. Consume little processed, unrefined, high fiber carbohydrates. Potatoes should be consumed in moderation: this food is not suitable for a sedentary lifestyle. You have to alternate red meat and white meat. The first is not good when it is consumed daily. The fish appears protective. Simple sugars should be used sparingly: they tend to replace healthier foods and may contribute to the glycemic load. High glycemic index carbohydrates are especially problematic for sedentary individuals.
(1) McManus K : A randomized controlled trial of a moderate-fat, low-energy diet compared with a low fat, low-energy diet for weight loss in overweight adults. Int J Obes Relat Metab Dis 2001, 25(10 : 1503-1511.