What are the health effects of different diets?
The relationship between diet and health has been very complicated to pin down. Long-term controlled experiments are more or less impractical. While scientists can observe with great accuracy everything a person eats over a few days, most diseases develop over years or decades. On the other hand, it is impossible to monitor precisely what a person eats over a period of 20 or 30 years. One approach is to use large prospective studies. In the late ’70s, Harvard began tracking the diets of hundreds of thousands of health professionals. The study participants regularly fill out food questionnaires and complicated formulas are used to convert the data into percentages of saturated fat, cholesterol and so on. The participants are also followed to see what diseases they contract. Then, after controlling for all possible confounders (such as smoking, for example), the scientists look for associations between dietary intake and disease. The head of Harvard’s Herculean effort in nutritional epid
