Are There Downsides to Intermittent Fasting?

Can intermittent fasting help you lose weight?

Can fasting for short periods of time improve our health and even slow the aging process? Intermittent fasting is a hot topic right now. It’s a style of eating that alternates periods of fasting with periods of eating normally. The idea is that giving your body a “rest” from digesting and storing food offers health and anti-aging benefits when you do it in moderation. It’s a practice that’s gaining steam, and even some healthcare practitioners advocate occasional fasting.

But, is fasting for health beneficial or overhyped? Although the verdict is still out, some studies suggest that fasting intermittently, rather than eating around the clock, turns on proteins involved in cellular repair. Activating these cellular processes may help protect cells against the assaults and damage they’re constantly exposed to. The hope is that this will slow cellular aging and protect against health issues, possibly even some types of cancer. However, we’re still a long way from showing that fasting lowers the risk of cancer.

Another way intermittent fasting could be beneficial is by reducing blood glucose and insulin. When you eat throughout the day, your pancreas is forced to pump out more insulin in response to the food you take in, particularly carbohydrates. Unfortunately, high levels of insulin and insulin resistance is associated with health problems, including type 2 diabetes and cardiovascular disease. Some studies even show that fasting may help with weight loss. Interestingly, in rodents, intermittent fasting, prolongs life. But, we don’t know yet whether the same is true of humans.

How Do You Do an Intermittent Fast?

There are a variety of ways to structure intermittent fasting. One of the most popular ways is to restrict food for a certain number of hours during the day a few days per week. Most advocates say that a 16-hour fast is sufficient to offer health benefits. Some intermittent fasters are more ambitious and tackle a 24-hour fast, although that’s not necessary to get some benefit. For example, an intermittent faster might stop eating after dinner and not eat breakfast or lunch the next day. They might use this approach once or twice a week and eat normally the rest of the time. But, because a 16 hour fast is more manageable than going a full 24 hours without food, it’s the type of fast that’s most popular. It’s a little harder to not eat for an entire day!

What Are the Drawbacks to Intermittent Fasting?

Keep in mind that the benefits of intermittent fasting are still somewhat theoretical. We don’t know for sure that it lowers the risk of health problems, prolongs life, or slows the aging process. Most of the data showing health benefits from fasting is from animal studies. However, small studies in humans that look at health biomarkers and weight loss also suggest that intermittent fasting may have health benefits. Plus, the human body is adapted to fasting. If you look at our primitive ancestors, they went without food for substantial periods of time. Even today, people fast for religious reasons, sometimes for several days without ill effects.

How might fasting exert potential health benefits? One theory has to do with the body’s stress response. Fasting imposes stress on the body, and stress in controlled doses may ramp up the body’s natural defense mechanisms. In other words, what doesn’t kill you makes you stronger. Much like the body adapts in a positive way to the stress of exercise, cells adapt to short-term stress by turning on protective pathways that protect them against damage.

But, are there downsides to fasting? If you stop eating for 12 to 16 hours, your body breaks down fat stores and produces ketone bodies as an alternative fuel source. Although ketone bodies aren’t harmful short-term to healthy people who don’t have diabetes, they can make you feel bad until your body adapts to using them as a primary fuel source. The same happens when people go on a very low-carb diet. The body of a carb restricter pumps out lots of ketone bodies and the sometimes feel tired or flu-like for a few weeks. However, these ketone bodies may have benefits, particularly for the brain. Studies suggest that they may help protect neurons in the brain and, possibly, slow brain aging.

It’s important to focus on eating nutrient-dense foods when you aren’t fasting. Fasting for 16 to 24 hours and then eating a junk food meal is a recipe for nutritional deficiency. Every calorie that you eat when you’re not fasting should count nutritionally. No empty calories or sugar!

Another concern is that intermittent fasting might lead to muscle loss. If you lose weight through fasting, you will also lose a certain amount of muscle, but there’s no evidence that intermittent fasting boosts muscle loss more than traditional dieting. In fact, a study published in Obesity Review showed that intermittent fasting may trigger less loss of muscle tissue than conventional calorie restriction to lose weight.

Who Shouldn’t Fast

If you have certain health conditions, particularly blood sugar problems like diabetes or a tendency toward low blood sugar, you shouldn’t fast. In fact, if you have any medical condition or take medications, consult your physician before abstaining from food, particularly a 24 hour or longer fast. Also, don’t fast if you’re pregnant.

Here’s a suggestion. If you’re healthy and intrigued by the idea of fasting, try a less ambitious 16-hour fast and see how you respond. To do this, eat an early dinner and skip breakfast the next morning. If you eat dinner at 7:00 P.M., you’ll be ready to eat at 11:00 A.M. the next day, just in time for an early lunch. Be sure to drink lots of calorie-free fluids. You still need to stay hydrated when you’re fasting.

Finally, don’t get involved with fasting if you have a history of an eating disorder. One concern that health professionals have is that fasting could foster or worsen an eating disorder. If you have a tendency to obsess over calories or a history of an eating disorder, intermittent fasting isn’t for you. In fact, it’s not for everyone, regardless of the potential health benefits. So, keep that in mind before undertaking fast, even one of short duration. Also, realize that the verdict is still out as to whether it’s truly beneficial in humans.



CMAJ. 2013 Jun 11; 185(9): E363–E364. doi:  10.1503/cmaj.109-4451.
MedPage Today. “Intermittent Fasting: The Science is Growing”
Obes Rev. 2011 Jul;12(7):e593-601. doi: 10.1111/j.1467-789X.2011.00873.x. Epub 2011 Mar 17.


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