Your Guide to Intermittent Fasting

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.

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.

The metabolic switch from using glucose to using fat for energy happens 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. This typically happens about 12 hours after you stop eating. Because this process metabolizes fat while preserving muscle mass and function, researchers believe it has the potential to improve body composition over time.

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), 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 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.

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.

The benefits of caloric restriction (CR) are a classic example of the body reacting productively to stress. There is significant evidence in animals and humans that eating fewer calories without reaching malnutrition can improve longevity, weight loss, cardiovascular health, insulin sensitivity, and mitochondrial function. However, CR diets are challenging to maintain long-term, and yo-yo dieting cycles can be harmful. Intermittent fasting (IF) offers similar benefits while being more sustainable for many people.

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 up to 4 liters of water per day. And 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.
• 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.”

Heart health:

• IF has been shown to reduce total cholesterol (TC), low-density lipoprotein (LDL) cholesterol, triglycerides, blood pressure, and inflammation.

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 can lower insulin resistance, fasting glucose, and leptin levels, and increase glucose tolerance.

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.
• Insulin resistance exacerbates amyloid β-protein accumulation that contributes to Alzheimer’s disease, so reducing insulin resistance can also help the brain. There is some evidence that intermittent fasting can slow down the transition from detectable cognitive impairment (DCI) to mild cognitive impairment (MCI).

Lifespan:

• While calorie restriction has been shown to benefit lifespan, specific anti-aging effects are dependent on your genes. 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.

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:

• 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.