The world of diet and nutrition is both fascinating and challenging because everyone seems to have an opinion about it. We also readily accept the opinions of friends, both in real life and virtually, without much scrutiny. My patients regularly tell me about the all-natural juice cleanses, herbal supplements and the latest diet fads they hear about from friends at the neighborhood barbecue or their children visiting from out of state. They often start these diets after the initial recommendation or after reading a couple blog posts from random people that “Dr. Google” finds for them. When it comes to a pill or vaccine I recommend, suddenly they require detailed statistics and lengthy scientific explanations about the side effects, risks, and benefits. What would happen if we applied that same level of scrutiny to the intermittent fasting diet?
Intermittent fasting diets involve not eating or severely restricting food intake for certain hours of a day, or for certain days of the week. Common patterns include not eating past a certain hour in the evening, alternating days (one day eating normally, the other eating about 30% of normal) or the “5:2” method of eating normally for 5 days, then severely restricting/fasting for 2 days. Proponents point out that intermittent fasting has been used in ancient cultures from India, China, and many parts of Europe and is still practiced today all over the globe.
The claimed health benefits of intermittent fasting are numerous, ranging from weight loss, diabetes, and cardiovascular disease prevention to preventing or even treating cancer. However, most if not all of these claims are not ready for prime-time use because the science behind them is faulty or incomplete.
The biggest and most surprising failure of the intermittent fasting diet is in helping people lose weight. A study in JAMA Internal Medicine compared groups of people trying to lose weight eating a normal calorie-restricted diet versus alternate day fasting. They were studied for one year, which included a 6-month weight loss phase and a 6-month weight maintenance phase.
After 6 months and 12 months, the alternate day fasting group showed no additional weight loss compared to the traditional calorie restriction group. More people quit the alternate day fasting diet compared to simple calorie restriction because they couldn’t maintain it. Cardiovascular health markers like blood sugars, insulin levels, and cholesterol were no better in the alternate day fasting group either. In fact, LDL cholesterol levels were higher in the fasting group. Basically, people are able to eat more on non-fasting days to make up for any weight loss on fasting days. Additionally, fasting can trigger a starvation mode in the body that prevents weight loss and promotes higher blood sugar and cholesterol levels.
The claims that intermittent fasting can prevent cancer or starve a tumor are also based on shaky science. Most of the studies supporting these claims were done in mice or other animals. These often don’t translate to humans. The few human studies available have included very small numbers of patients. Furthermore, they don’t prove that intermittent fasting prevents cancer, only that it affects a blood marker that may be associated with cancer. One such marker is insulin-like growth factor 1 (IGF-1). High levels of IGF-1 have been correlated with prostate cancer and colon cancer. A study in mice in 2007 showed that intermittent fasting can lower levels of IGF-1. So it would make sense that intermittent fasting would prevent cancer or starve tumors, right? Not necessarily.
Consider a similar, alternate scenario of angiogenesis in cancer. Angiogenesis is the process of forming new blood vessels, which allows tumors to attract nutrients to grow and spread. Early studies of tumor angiogenesis from the 1970s-1990s were very exciting. It appeared that scientists had discovered the Achilles’s heel of all cancers. Several drugs were developed to inhibit angiogenesis. Despite these drugs effectively blocking angiogenesis and decades of research showing how important angiogenesis is in tumors, the drugs failed to treat most cancers and are only modestly effective in a few.
The human body is complex and sometimes even the most logical conclusions prove to be wrong as was the case with angiogenesis inhibitors. Similarly, assuming the effect of intermittent fasting on IGF-1 or other hormone levels will prevent cancer or starve tumors is inaccurate and potentially dangerous. Many of the claims about preventing Alzheimer’s, strokes, diabetes and aging follow the same logical fallacy.
The good news is, the short-term harms of intermittent fasting are minimal and largely theoretical. People taking medication to lower blood sugar like insulin or medications taken with food could have serious side effects from intermittent fasting. Vitamin and nutritional deficiencies could be worsened by intermittent fasting. Those with eating disorders or obsessive/compulsive tendencies also might have issues with intermittent fasting. Again, these risks are largely theoretical.
The long-term side effects of intermittent fasting are unknown as they have not been studied well in humans. The fact that many cultures have practiced intermittent fasting for millennia doesn’t prove its safety because for most of human history, people died before age 50 of infections and other currently preventable/treatable diseases.
One thing is clear: the intermittent fasting diet might be a good option for your pet mouse. The health benefits for humans remain to be determined.