Most of today’s trendy diets have a lot of rules about what you can eat. Whole 30 restricts sugar, alcohol, grains, legumes, soy and dairy; Keto says no to carbs and limits protein. But intermittent fasting doesn’t dictate what you should be eating at all—only when to eat.
In intermittent fasting, you fluctuate between periods of eating and fasting. This might mean eating during an eight-hour window then fasting for the other 16 hours of the day, fasting one full day a week or limiting food intake to 500 calories per fasting period.
“But nothing in these diets suggests what to eat,” says Robert Hutchins, MD, MPH, an internal medicine doctor with UNC Health Care. “Other diets say, ‘You should be eating X, Y and Z.’ Instead, intermittent fasting says, ‘Eat whatever you want, but it should be at certain times or limited to a certain number of calories.’”
How does intermittent fasting actually work, and is it healthy? Dr. Hutchins explains.
The Science Behind Intermittent Fasting
“One of the major keys to understanding fasting, to understanding really any diet, is understanding the role insulin plays,” Dr. Hutchins says. Insulin, the hormone that regulates blood sugar, is made in the pancreas and released in the bloodstream in response to eating. Once released, insulin causes the body to store energy as fat. “Insulin makes fat, so the more insulin made, the more fat you store,” he says.
During intermittent fasting, the periods when you are not eating give the body time to lower insulin levels, which reverses the fat-storing process. “When insulin levels drop, the process goes in reverse and you lose fat.”
Two other hormones, ghrelin and leptin, are also at work. Ghrelin is the hunger hormone and tells you when you’re hungry. “Some data suggest that intermittent fasting can decrease ghrelin,” Dr. Hutchins says. “There’s also some data that says there’s an increase in leptin, which is the satiety hormone. That’s the one that says, ‘Hey, I’m full.’”
With more leptin and less ghrelin, people will feel fuller faster and hungry less often, which could translate to fewer calories consumed and, as a result, weight loss.
Health Benefits of Intermittent Fasting
Besides losing weight, people who fast may experience improved heart and brain functions because of reduced insulin.
“Think of your insulin level as the first domino in the whole-body cascade of what can happen with intermittent fasting,” Dr. Hutchins says. “If you have high insulin levels, that can lead to obesity, diabetes, high blood pressure and elevated triglycerides. Those things together are called metabolic syndrome, which increases risk for cardiovascular disease. So, if your insulin level decreases, you will probably lose weight, and your cholesterol, blood sugar and blood pressure will improve.”
The drop in cholesterol can also decrease the inflammation caused by metabolic syndrome, namely plaque buildup in the arteries and overall cardiovascular inflammation. “As a result of these improvements, people can also reduce their risk of cardiovascular disease,” he adds.
This process can happen even in the absence of weight loss, and Dr. Hutchins notes that it’s not specific to intermittent fasting. “Some people can intermittently fast and be on a strict regimen, but if they eat a bunch of junk on the days they can eat, the diet won’t have an effect on heart health,” he says. “This effect is probably more specific to what they’re eating than to when they’re eating.”
When it comes to the brain, Dr. Hutchins says that some data has shown that lower insulin levels can also reduce dysfunction in a person’s brain cells, called neurons, which could possibly decrease the risk of Parkinson’s and Alzheimer’s diseases.
“If there’s less neuronal dysfunction, that could possibly lead to less risk of those conditions,” Dr. Hutchins says. “But this is still only theoretical. Risk of Alzheimer’s, for example, has been shown to be significantly higher in people who have metabolic syndrome.”
Should You Try Intermittent Fasting?
Although intermittent fasting might bring health benefits, Dr. Hutchins emphasizes that whether you see results in weight loss or overall health comes down to nutrition.
“Based on the data I have looked at, I can’t say that intermittent fasting is more successful than other diets that aren’t on a fasting pattern,” he says. “The problem I have found with intermittent fasting is that people can stick with the fasting, but when they start a period where they are eating again, they end up eating a lot of junk and processed food instead of healthy food.”
Ultimately, Dr. Hutchins says the best diet is the one that works best for you. He says there’s no perfect candidate for intermittent fasting, but if you want to try the diet, find a fasting schedule that fits in with your lifestyle. For example, if fasting one full day is too hard, try eating for eight hours and fasting for 16 instead.
Always, the main emphasis of your diet should be on what you’re eating. “The bottom line is you have to be eating real food, not processed stuff,” Dr. Hutchins says. “I’d recommend eating a lower-carbohydrate diet, limiting both sweets and starches, because those are the things that cause your insulin level to rise most.”
It’s also important during periods of fasting that you remain hydrated. “When people fast, they aren’t fasting from water,” he says. “You have to stay well-hydrated.”
Like any other diet, people should also take medications into consideration when altering their eating habits. For example, people on blood pressure medicine or medication that lowers their blood sugar should avoid fasting, as periods of not eating could cause blood sugar levels to drop too low, resulting in hypoglycemia, a serious complication of low blood sugar levels. Talk to your doctor about the diet that is best for you, and before you pursue any kind of fasting.
Need a referral to a dietitian? Ask your doctor, or find one near you.