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There’s a fuzz in my brain and an ache in my gut. My legs are leaden and my eyesight is blurry. But I have only myself to blame. Besides, I’ve been assured that these symptoms will pass. Between 10 days and three weeks from now, my body will adjust to the new regime, which entails fasting for two days each week. In the meantime, I just need to keep my eyes on the prize.

Fasting is most commonly associated with religious observation. It’s the fourth of the Five Pillars of Islam. Buddhists consider it a means to practice self-control and advocate abstaining from food after the noon meal. For some Christians, temporary fasts are seen as a way of getting closer to God. But the benefits I’m hoping for are more corporeal.

The idea that fasting might be good for your health has a long, if questionable, history. Back in 1908, “Dr.” Linda Hazzard, an American with some training as a nurse, wrote “Fasting for the Cure of Disease,” which claimed that minimal food was the route to recovery from a variety of illnesses including cancer. Hazzard was jailed after one of her patients died of starvation. But what if she was, at least partly, right?

A new surge of interest in fasting suggests that it might indeed help people with cancer. It could also reduce the risk of developing cancer, guard against diabetes and heart disease, help control asthma and even stave off Parkinson’s disease and dementia. Many of the scientists who study fasting practice what they research, and they tell me that at my age (39) it could be vital that I start now.

“We know from animal models,” says Mark Mattson at the U.S. National Institute on Aging, “that if we start an intermittent fasting diet at what would be the equivalent of middle age in people, we can delay the onset of Alzheimer’s and Parkinson’s.” Surely it’s worth a try?

Until recently, most studies linking diet with health and longevity focused on calorie restriction. They have had some impressive results, with the lifespan of various lab animals lengthened by up to 50 percent after their daily calorie intake was cut in half. But these effects don’t seem to extend to primates. A 23-year-long study of macaques found that although calorie restriction delayed the onset of age-related diseases, it had no impact on lifespan. So other factors such as genetics may be more important for human longevity too (Nature).

That’s bad news for anyone who’s gone hungry for decades in the hope of living longer, but the finding has not deterred fasting researchers. They point out that although fasting obviously involves cutting calories – at least on the fast days – it brings about biochemical and physiological changes that daily dieting does not.

Besides, calorie restriction may leave people susceptible to infections and biological stress, whereas fasting, done properly, should not. Some even argue that we’re evolutionarily adapted to going without food intermittently.

“The evidence is pretty strong that our ancestors didn’t eat three meals a day plus snacks,” says Mattson. “Our genes are geared to being able to cope with periods of no food.”

What is a fast?

As I sit here, hungry, it certainly doesn’t feel like that. But researchers do agree that fasting will leave you feeling crummy in the short term because it takes time for your body to break psychological and biological habits. Less reassuring is their lack of agreement on what fasting entails. I have opted for the “5:2” diet, which allows me 600 calories in a single meal on each of two weekly “fast” days.

The normal recommended daily intake is about 2,000 calories for a woman and 2,500 for a man, and I’m allowed to eat whatever I want on the five non-fast days, underlining the fact that fasting is not necessarily about losing weight. A more draconian regimen has similar restricted-calorie “fasts” every other day. Then there’s total fasting, in which participants go without food for anything from one to five days – longer than about a week is considered potentially dangerous. Fasting might be a one-off, or repeated weekly or monthly.

Different regimens have different effects on the body. A fast is considered to start about 10 to 12 hours after a meal, when you have used up all the available glucose in your blood and start converting glycogen stored in liver and muscle cells into glucose to use for energy.

If the fast continues, there’s a gradual move toward breaking down stored body fat, and the liver produces “ketone bodies” – short molecules that are byproducts of the breakdown of fatty acids. These can be used by the brain as fuel. This process is in full swing three to four days into a fast. Various hormones are also affected.

For example, production of insulinlike growth factor 1 (IGF-1), drops early and reaches very low levels by day three or four. It’s similar in structure to insulin, which also becomes scarcer with fasting, and high levels of both have been linked to cancer.

When it comes to treating cancer, Valter Longo, director of the Longevity Institute at the University of Southern California, thinks that short-term complete fasts maximize the benefits. He’s found that a 48-hour total fast slowed the growth of five of eight types of cancer in mice, the effect tending to be more pronounced the more fasts the animals undertook (Science Translational Medicine).

Fasting is harder on cancer cells than on normal cells, he says. That’s because the mutations that cause cancer lead to rapid growth under the physiological conditions in which they arose, but they can be at a disadvantage when conditions change. This could also explain why fasting combined with conventional cancer treatment provides a double whammy.

Clinical trials assessing the impact of fasting in people with cancer are ongoing. Early results are promising, says Longo, and patients in the advanced stages of cancer, who cannot wait for the results, might find it worth discussing fasting with their oncologist.

Less is more

Could fasting prevent cancers developing in the first place? Evidence is scant but there are “very good reasons” why it should, says Longo. He points out that high levels of IGF-1 and glucose in the blood, and being overweight are all risk factors for cancer, and they can all be improved by fasting. Another risk factor is insulin, says Michelle Harvie at the University of Manchester, UK.

Studying women whose family history puts them at high risk of developing breast cancer, she put half of them on a diet that involved cutting calories by about 25 percent, and half on a 5:2 fast. After six months, both groups showed a reduction in blood insulin levels, but this was greater in the fasting group. Harvie’s team is now analyzing breast biopsies to see whether this translates to fewer of the genetic changes associated with increased cancer risk.

High insulin is also associated with type 2 diabetes, so perhaps it is no surprise that fasting shows promise here, too.

At the Intermountain Heart Institute, Murray, Utah, Benjamin Horne has found that a 24-hour water-only fast, once a month, raises levels of human growth hormone, which 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 (American Journal of Cardiology).

Alternate day fasting (with a 500-calorie lunch for women and 600-calorie one for men on fast days) has similar benefits, says Krista Varady of the University of Illinois-Chicago. She has seen improvements in people’s levels of low-density lipoprotein cholesterol, sometimes known as “bad cholesterol”, and blood pressure, in volunteers eating either a low-fat or high-fat diet on “feeding” days.

For overweight people, any kind of intermittent fasting diet will probably help reduce the risk of diabetes and cardiovascular problems, says Mattson. In 2007, he found another benefit, too. He put 10 overweight people with asthma on an alternate-day incomplete fast and found that after just a few weeks their asthma symptoms improved.

Blood markers of inflammation, including C-reactive protein, also decreased, suggesting that the fast was helping to moderate their overactive immune system (Free Radical Biology and Medicine).

Whether fasting would benefit people with asthma who are in the normal weight range or those with other conditions associated with an overactive immune response, remains to be seen. There’s some evidence that alternate-day fasting can lower their levels of blood fat. However, Mattson suspects that when it comes to 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.

Mattson has, however, identified another effect of fasting that he believes can benefit everyone: It’s good for the brain.

“If you look at an animal that’s gone without food for an entire day, it becomes more active,” he says. “Fasting is a mild stressor that motivates the animal to increase activity in the brain.” From an evolutionary perspective, this makes sense because if you are deprived of food, your brain needs to begin working harder to help you find something to eat. His studies show that alternate-day fasting, with a single meal of about 600 calories on the fast day, can boost the production of a protein called brain-derived neurotrophic factor by 50 to 400 percent, depending on the brain region.

This protein is involved in the generation of new brain cells, and plays a role in learning and memory. It can also protect brain cells from the changes associated with Alzheimer’s and Parkinson’s (Neurobiology of Disease).