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Does Intermittent Fasting Work?

Fasting isn't just for the gym rats and health gurus. It's as easy as eating dinner early.

When it comes to nutrition and diet, we’re all looking for short-cuts. Too many of them lead to yo-yo dieting, plans without gains, and lots of expensive workout equipment or memberships, gone unused. So where does intermittent fasting, one of the booming diet trends out now fit into this world? It’s no fad. The science is promising, its history is long, and when tackled with patience and thoughtfulness, it can truly burn fat, promote weight loss, and improve overall health. Of course, there will be sacrifices. Here’s how to tackle intermittent fasting and what to expect. 

The Benefits of Intermittent Fasting

A study published in the New England Journal of Medicine last December summarized decades of research on the practice which found that Intermittent fasting leads to weight loss and improves blood pressure, cholesterol, asthma symptoms, and risk of cardiometabolic disease. Less definitive evidence, for example, clinical trials, suggest fasting could improve insulin resistance in patients with type 2 diabetes, improve surgical outcomes by reducing tissue damage, delay the onset of Alzheimers and Parkinsons disease, mitigate symptoms of multiple sclerosis, and tamper tumor growth. Animal studies have found intermittent fasting to improve the body’s response to stress, reduce the growth of tumors, reduce symptoms of multiple sclerosis, and mitigates cognitive symptoms after brain injury.  

So how does it work? When we eat regularly, the body relies on glucose, a simple sugar found in carbs, for energy. When we fast, glucose stores run out, forcing the body to turn to triglycerides, a type of fat, for energy. “Every time you eat you replenish the glucose stores in the liver.” says Dr. Mark Mattson, Professor of Neuroscience at Johns Hopkins University and author of the NEJM study. It takes about 10 to 12 hours of not eating to make this switch. During every hour after that, fat gets broken down into ketone bodies, which provide energy for the brain. This metabolic switch has a number of effects that are good for our health. 

Mattson says that most cancer cells rely on glucose, so making the body rely on ketones could deprive cancer cells of energy and inhibit tumor growth. There are currently ongoing clinical trials of intermittent fasting in patients with breast, ovarian, prostate, endometrial, brain, and colorectal cancers. And animal studies have found that fasting can reduce tumor growth and improve the body’s response to stress, which Mattson says provides reason to believe that it could improve the side effects of chemotherapy and radiation, both of which are enormously stressful for the body. 

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In Mattson’s research on rats, fasting activated the parasympathetic nervous system, the opposite of the fight or flight response. This explains intermittent fasting’s positive effects on blood pressure, cholesterol, and heart disease risk. The parasympathetic nervous system slows the heart rate and reduces blood pressure, similar to aerobic exercise, Mattson says. Intermittent fasting also seemed to protect the rat’s neurons (specialized cells that transmit information in the brain) from aging, which reduced their risk of Alzheimers and Parkinson’s disease, and stroke. 

In the future, intermittent fasting might even be used to treat some forms of diabetes. Emerging evidence suggests it helps regulate insulin, the hormone that controls how much glucose is in the bloodstream. When the body is sensitive to insulin, it can process food and clear sugar out of the blood quickly, says Dr. Felicia Stager, a Registered Dietitian and Postdoctoral Research Fellow at University of Alabama at Birmingham. When it’s desensitized to insulin, blood sugar remains high, which can cause issues with the eyes and kidneys, and lead to type two diabetes. In a study of men with prediabetes, eating the same number of calories as usual but restricting meals to a six-hour window, early in the day, increased insulin sensitivity and decreased blood pressure and oxidative stress — a kind of inflammation caused by an imbalance between free radicals and antioxidants which can lead to diabetes, high blood pressure, heart disease, Parkinson’s, Alzheimer’s, and cancer.

Why would earlier meals help? Stager says it’s all about the circadian rhythm, or sleep wake cycle. In the morning we’re more insulin sensitive, which helps with blood sugar control. She describes the circadian rhythm as an anticipatory system. Your body anticipates it being light out, and revs up its insulin sensitivity in the morning to prepare for the day, and reduces it at night in anticipation of sleep. “Eating in alignment with those daylight hours really aligns the brain clock with the body clock,” Stager says. 

How to Implement Intermittent Fasting

The fasting periods in the second study were severe — participants ate all of their meals within a six hour window, finishing dinner before 3 p.m. But Stager says it doesn’t have to be that dramatic. “In general we know that reducing someone’s normal eating patterns, so if someone normally eats over a 12 hour period, reducing that to a 10-hour period is probably going to provide some benefit. I think more so than the duration of the fast is the timing of the fast,” Stager says. To start, she recommends just moving dinner up a little earlier. 

After a few weeks of eating like this, participants in the early time restricted feeding study lost their appetite at night, so it’s likely that after you get over that initial 2-4 week hump, intermittent fasting could be relatively easy to maintain. But for those struggling to go long hours without eating, exercise can help speed up the time it will take for that metabolic switch to happen. “[Exercise] will accelerate the depletion of the liver energy stores and the switching to fat,” Mattson says. While many people find it easiest to stick to a short eating window by skipping breakfast, Stager says tha waiting all day to eat and then gorging at dinner won’t be as beneficial, since you’ll likely eat differently than you would have earlier in the day. 

If you’re looking to intermittent fasting to help you lose weight without changing what you eat, you should know that it’s not that simple. The NEJM study says that intermittent fasting is a weight loss method as effective as dieting, citing a literature review of six short-term studies. These studies relied on alternate day intermittent fasting, in which participants went a few days each week eating only a few hundred calories. It makes sense that cutting out entire days worth of food would lead to weight loss. But researchers noted that these kinds of eating patterns were “not well tolerated.” 

Early time restricted feeding, the kind of intermittent fasting that was the subject of the second study, in which subjects eat all their meals in a six hour period each day, didn’t lead to weight loss, though that was by design. Participants said that eating all of their regular meals within such a short period of time was harder than not eating at night, so this kind of behavior might not be replicated out of a lab setting. Either way, the results of the study indicate that “It’s really not a weight-loss method,” Steger says, noting that people who do lose weight on IF max out at around 1 to 2 percent of their body weight. “I kind of refer to it as a backup for people who are really struggling to make any other changes in their diet,” Stager says. 

Like other healthy but hard to maintain habits, the effects of intermittent fasting only works for as long as you do it. While benefits can be seen in as little as 2-4 weeks, they reverse just as quickly once you stop fasting. “It’s kind of a diminishing returns thing,” Mattson says, comparing it to exercise. Though you will have benefitted from any length of intermittent fasting, the benefits won’t be sustained when you go back to normal eating. If you do lose weight, you’ll regain it in the same way you would after going off of any other diet, Stager says. So if you do use fasting to lose a few pounds and then stop, the weight loss won’t be any more sustainable than doing a crash diet and then resuming your normal eating patterns.

Plus, the effects of intermittent fasting pale in comparison to those you’d see with significant weight loss. And it can’t stand in for healthy eating habits either. “If somebody is eating really poorly, they probably should target the what as opposed to the when,” Stager says. Children, the elderly, pregnant women, and anyone who has struggled with binging and purging should avoid intermittent fasting. In general, restricting your meals to a shorter time frame seems to have some benefits over grazing all day long. But nothing substitutes for a balanced, diverse diet.