The effect on diabetes
High insulin is also associated with Type 2 diabetes, so perhaps it is no surprise that fasting shows promise there, too. At the Intermountain Heart Institute in Murray, Utah, Benjamin Horne has found that a 24-hour water-only fast, performed monthly, raises levels of human growth hormone. That hormone triggers the breakdown of fat for energy use, reducing insulin levels and other metabolic markers of glucose metabolism. As a result, people lost weight, and their risk of getting diabetes and coronary heart disease was reduced. Alternate-day fasting (with a 500-calorie lunch for women and a 600-calorie meal for men on fast days) has similar benefits, says Krista Varady of the University of Illinois. She has seen improvements in people's levels of low-density lipoprotein cholesterol, or "bad cholesterol," and blood pressure in volunteers eating either a low-fat or high-fat diet on "feeding" days.
For people who are overweight, any kind of intermittent fasting diet will probably help reduce the risk of diabetes and cardiovascular problems, Mattson says. In 2007, he found another benefit, too. He put 10 overweight people with asthma on an alternate-day incomplete fast and found that their asthma symptoms improved after just a few weeks. Blood markers of inflammation, including C-reactive protein, also decreased, suggesting that the fast was helping to moderate their overactive immune system.
Whether fasting would benefit normal-weight people with asthma or other conditions associated with an overactive immune response remains to be seen. There is some evidence that alternate-day fasting can lower their levels of blood fat. However, Mattson suspects that with diabetes and cardiovascular disease, fasting may not be as beneficial for people of normal weight as it is for people who are overweight, simply because they are already likely to be in pretty good shape, metabolically speaking.