There was paleo, Whole30, then keto—all diets that tried to restrict what and how you ate. But the latest dieting trend is not eating at all.
Fasting is becoming increasingly popular. But, as with almost all diets, the health claims made by proponents—inflammation reduction, lower blood pressure, less cancer, even extended lifespan—stand on shaky scientific ground. Worse, the trend of detoxing or cleanses marketed as forms of fasting may actually be dangerous.
Here’s a look at the truth behind the fast.
Science Doesn’t Support Fasting Claims
The research on fasting is lacking: While some research hints at a few benefits, most studies haven’t been performed on humans. And the ones that have are too small to produce definitive results. “A lot of those studies were done in animals and we're not mice," said Caitlin Van Dreason, a registered dietitian and clinical nutrition manager for the Cambridge Health Alliance.
Even these animal studies are “not conclusive. They haven’t found any evidence strong enough to recommend it as a healthy diet approach. Could we find out more down the line about metabolic health and weight loss effects? Potentially,” she said. But we’re not there yet.
That’s not to say that fasting doesn’t work for some people. Nancy Becker, a registered dietitian and a fellow of the Academy of Nutrition and Dietetics, said, “Some people who fast one day every week manage to keep their weight stable. It’s a technique to get less calories in you. Whether it works or not depends on you, your goals, and what your body’s like.” (Full disclosure: Becker is related to the author of this story.)
For many though, fasting is attractive because it clearly can lead to weight loss. But if calorie restriction is your goal, there are plenty of more pleasant options. And fasting can have the unintended effect of making you more hungry, which means you might be likely to binge afterwards and negate the caloric reduction.
“Are you going to stick on a diet where you’re hungry?” Van Dreason asked.
More worrisome, fasting and bingeing border on eating disorders such as anorexia and bulimia, which means people who fast regularly need to be careful about treading on that edge.
There’s No Such Thing as a Cleanse
Fasting may be unproven, but its cousin the cleanse is flat-out fiction. “There is no such thing as a cleanse,” says Becker. “It’s a meaningless word.” If your body contains toxins that need to be removed, then you are very ill. That’s because your liver’s sole purpose is removing toxins. If it’s not functioning properly, you don’t need a cleanse—you need a liver transplant or a blood transfusion.
“I can’t stress enough that your liver is a very important organ,” says Van Dreason. “If it wasn’t working, you would know it. You would be very sick. Cleaning is its job.”
On top of that, cleanses depend on pseudoscience that can encourage unhealthy or even dangerous nutrition. Take juicing. The sugar content of fruit juice is about the same as soda. And when you drink only juice, you remove fiber from your diet, which would otherwise help you feel full, improve your digestive health, and be good for your heart. It’s much healthier to eat the whole fruit or vegetable, Van Dreason says.
Another popular diet is the alkaline cleanse (popularized by celebrities like Tom Brady) in which people consume lemon juice and other acidic foods to boost their pH level and help arthritis, cure cancer, eliminate kidney stones, and solve heart problems. But it’s actually impossible to change the pH of your body. “Your stomach has hydrochloric acid in it. That’s going to reset the pH of everything you eat,” says Van Dreason. “Your blood pH is tightly controlled, and if it’s off you’ll know it because you’ll be extremely sick.”
Cleanses also often involve selling you a product. That’s a dietary red flag. “The weight loss and wellness industry is a billion-dollar industry and they feed off people’s anxieties,” says Van Dreason. “I teach my patients to be smart consumers: Is this company trying to sell me something? What are the medical qualifications of the person selling this? And does it make sense?” Van Dreason advises that if you have questions about your diet choices, don’t take advice from celebrities. Instead check out the Academy of Nutrition and Dietetics’ website called Eat Right.
The Keto Diet and its Fasting Relationship
One of the latest and most popular diet fads is the Keto diet, similar in many ways to the Atkins diet. It requires cutting out carbohydrates and replacing them with proteins and fat. What happens to your body under these conditions is similar to fasting: You go into a state of ketosis. According to Becker, “Your brain needs carbohydrates to run. When you’re deprived it takes 20 hours or so to run out of what’s stored. Your brain adjusts by using ketones [produced by your liver] and adapts to running on them instead. Many people feel high when they have a little mild ketosis. It affects you emotionally. You get a little loopy, maybe you enjoy that. That’s maybe the appeal of fasting.” (Though she notes this is variable and some people don’t get loopy, they just get a headache.)
Van Dreason says there is no research that details the long-term effects of ketosis because it’s difficult to remain on the Keto diet or a diet of fasting. “We don’t have a group of people that have followed it for a long time. We don’t have the studies. What we know is we have this huge body of evidence that tells us a diet rich in fruits and vegetables and whole grains promotes good health. We have so many good, well-done scientific studies [about that],” she says. Conversely, she says, we also have a lot of evidence from many studies that show red meat is linked to colon cancer and heart disease. And we also know that very high levels of ketones in the body can be extremely dangerous and cause ketogenic shock, which can put people into a coma (something that happens with diabetics).
Fasting and History
All that said, there are historical and cultural connections to fasting. “Many different cultures use fasting as a path to spiritual awareness,” says Becker. “It’s clear that not putting any food in your body for a day affects a person deeply, and there may be spiritual reasons to do that.”
If you feel that you are stuck in a food rut or want to shock yourself out of a bad eating habit, Becker says a short fast might help—emphasis on the might.
But as a long-term solution, fasting is much more problematical. A useful diet, says Van Dreason, is one that is easy to implement, enjoyable, and can be done for a lifetime.
“If we’re thinking about a sustainable lifestyle change to promote health,” Van Dreason says, “intermittent fasting can get in the way of enjoying eating and socializing. Food is about nutrients but it’s also about pleasure and enjoying company socially. When you’re looking at making healthy diet changes, it’s important to ask if this is something that fits with your lifestyle,” she says.
