In the realm of human physiology and diet, few practices have garnered as much popular interest in recent years as intermittent fasting (IF). IF is an umbrella term for several practices that involve switching between periods of fasting and eating. Often this can look like finishing dinner before 8 p.m. and not eating until breakfast the next day after 10 a.m., thus creating a 14-hour window without food.

Studies have shown that this kind of time-restricted eating can have significant health benefits in animals, and a growing body of research indicates it may work in humans, too. Intermittent fasting may also help some people manage weight, reduce some disease symptoms, and even slow the pace of aging.

The science: How does intermittent fasting work?

IF works through hormesis—moderate stress that improves the body's stress response. It triggers "metabolic switching," where the body shifts from using glucose to burning stored fat for energy.

This typically happens about 12 hours after you stop eating, when your body reaches a negative energy balance—the glucose from your food is gone, forcing your body to metabolize fatty acids and turn them into ketone bodies.

Note, however, that much of the initial research was done in male animals. Human trials of IF have included more females, but more research is needed to understand if the same benefit is conferred across genders over time.

It’s also important to remember the importance of the word intermittent. Robert Kachko, ND, LAc, Director of Integrative Health at Atria, thinks IF can be beneficial for many people, but cautions that if you fast for too long (such as days at a time), when you go back to eating, your body may begin storing fat instead of burning it, slowing your metabolism.

“Our bodies are wiser than we are, and what our evolutionary biological mechanisms experience when we restrict too much is that famine is coming and it’s time to prepare,” says Kachko.

The potential benefits of intermittent fasting

While intermittent fasting can help offer a range of benefits, it’s important to note that most of the proven benefits come from the practice leading to a reduced calorie intake. This is useful, as many people find it easier to limit their eating to a specific window of time than to count calories, so they may be more likely to stick with IF and see these benefits. But for people who are not trying to reduce their calories, we still need more research to fully understand IF’s benefits.

Heart health:

  • IF methods that result in cutting down calories have been shown to reduce total cholesterol (TC), low-density lipoprotein (LDL) cholesterol, triglycerides, blood pressure, and inflammation. (Intermittent fasting without a calorie deficit may offer some of these benefits, but the research is not yet clear.)

Metabolic health:

  • IF has been found to reduce obesity in mice, and some short-term studies have found it effective in humans too. For people with type-2 diabetes, studies show that IF that reduces calorie intake can lower insulin resistance and fasting glucose, increase glucose tolerance, and improve levels of leptin, a hormone produced by fat cells that regulate appetite, metabolism, and energy balance.

Brain health:

  • Intermittent fasting increases ketone bodies, which are a good source of energy for the brain.
  • Studies in mice have shown that intermittent fasting decreases oxidative stress, which can contribute to brain aging, and that IF decreases inflammation in the brain and can have other protective effects against neurodegenerative disease.
  • Insulin resistance exacerbates amyloid β-protein accumulation that contributes to Alzheimer’s disease, so reducing insulin resistance can also improve the health of the brain. There is some evidence that intermittent fasting can help reduce  mild cognitive impairment (MCI) or slow cognitive decline, but more research is needed in humans.

Lifespan:

  • While calorie restriction has been shown to benefit lifespan in animals, similar results have not been shown in humans, and until biomarkers of aging are identified, it will be difficult to link IF to lifespan. We also need more research to determine whether these benefits are a consequence of tweaking the aging process itself or are the result of avoiding obesity.

Weight loss:

  • Research has shown intermittent fasting is as effective as calorie restriction for weight loss and can be particularly helpful for people with overweight and obesity. In clinical trials, IF has led to a reduction of 0.8%-13% in body weight and 3-8 cm in waist circumference over several months. Longer-term studies are needed to understand how sustainable this weight loss is over time.

The body's circadian rhythm also plays a role. IF can increase the expression of circadian rhythm-regulating genes that lower fat accumulation in the liver and adipose tissues, reducing inflammation, and decreasing susceptibility to metabolic syndrome. Metabolic syndrome is defined as having at least three of the following five risk factors: abdominal obesity, low HDL (“good”) cholesterol, high triglycerides, high fasting blood sugar, and high blood pressure.

Improving the regulation of your circadian rhythm may also have anti-aging effects because it controls compounds such as TOR and the cellular production of NAD+, which are important for slowing aging.

Guidelines for trying intermittent fasting

Intermittent fasting can encompass a variety of methods, all of which involve choosing regular intervals to fast and eat. No matter what protocol you want to try, there are a few things to keep in mind:

  • Stay hydrated and aim for 3 to 4 liters of water per day. (Usually some of this water intake comes from your food, but when fasting, this will all come from drinking water.) 
  • It can be useful to guide water intake by clinical hydration markers, such as looking for urine color that is pale to light yellow.
  • During non-fasting periods, it’s best to eat nutritious food, following something like the Mediterranean diet. Consume healthy fats, vegetables, fruit, lean meat, and complex carbs.

Some of the most popular approaches include:

  • Time-restricted eating (TRE) — A protocol that involves fasting every day by eating only in a period of about 6-10 hours, so each fast can last 14-18 hours.
    • Fasts can occur during whichever hours are most sustainable, though avoiding food late at night can help align eating with the body’s circadian rhythm. There is also some evidence that eating earlier in the day reduces appetite, which can lead to consuming fewer calories. 
  • Alternate day fasting (ADF) — An eating pattern that involves a 24-hour fast on alternate days. This can also include modified alternate day fasting, during which you eat up to 25% of your normal caloric needs (about 500 calories) on fast days. On non-fast days, people consume whatever they want, with a target of 125% of caloric needs.
  • 5:2 fasting — A method in which people fast for two days each week on non-consecutive days. For example, you might fast on Monday and Thursday, and then eat on the other five days.

While all three methods are similarly effective, Kachko prefers the consistency of daily time-restricted eating because it gets closest to natural human eating patterns. “In our modern world, we're eating all the time, and that's not appropriate. But historically, we also didn’t usually do these systematized fasts and then eat copiously the rest of the time,” Kachko says. “Every system thrives on knowing what to expect, so I prefer the same thing every day as opposed to these ups and downs.”

Intermittent fasting isn’t for everyone

Always check with your care team before starting any new dietary strategy. Most physicians do not recommend for people with the following conditions:

  • Untreated hypothyroidism
  • Blood sugar dysregulation
  • A history of disordered eating
  • Type-1 diabetes
  • Hormone imbalances
  • Kidney disease or cancer

Clinical trials have shown that intermittent fasting is safe for people with type-2 diabetes, but you should speak with your doctor before beginning any fast. People who take insulin or blood sugar-lowering medication should also plan with their health care provider, as fasting could lead to hypoglycemia.

Women who are pregnant or breastfeeding should also avoid IF.

More research is coming out about intermittent fasting all the time. In addition to speaking with your care team, you may want to try out different methods to find one that works for your lifestyle. As Kachko says, “intermittent fasting is best when personalized so that it’s sustainable for each person.”