A University of Illinois Chicago team has summarized research on intermittent fasting to provide insights into its effects on the body and to provide advice for incorporating these diets in everyday life. They have also presented recommendations for future research into these popular diet methods.

"Clinical application of intermittent fasting for weight loss: progress and future directions," was recently published in Nature Reviews Endocrinology. Krista Varady, professor of nutrition in the UIC College of Applied Health Sciences, is the lead author.

The three main forms of intermittent fasting were reviewed: alternate-day fasting—consuming 0–500 calories on alternating feast days; the 5:2 diet—two fast days and five feast days per week; and time-restricted eating—eating only during a prescribed time window each day. These diets produce mild to moderate weight loss, 3% to 8% loss from baseline, over short durations of eight to 12 weeks.

The review also states that intermittent fasting is on par with traditional calorie-restricted diets and shows results in improving some cardiometabolic risk factors. Additionally, intermittent fasting is generally safe, producing few gastrointestinal, neurological, hormonal or metabolic effects. Other findings included:

• Fasting works for individuals of normal weight as well as those with obesity.
• People with insulin resistance or prediabetes benefit from intermittent fasting, losing similar weight amounts as those without those conditions.
• Body composition for weight loss during intermittent fasting is similar to calorie-restriction diets, with 75% of the weight lost being fat and 25% lean mass.

The research also dispelled some myths about intermittent fasting.

"The main myth is people are going to feel weak and not be able to concentrate during fasting. We've shown it is the opposite: They actually have a better ability to concentrate," Varady said, adding the increased energy may be an evolutionary response to give strength to seek food.

Additionally, current research shows intermittent fasting does not harm metabolism.

"With any diet, as you lose weight, your metabolism, like your calorie needs, will go down because they're correlated tightly with your muscle mass. As you lose weight, people tend to lose a little bit of muscle. But fasting doesn't tank your metabolism at all. We've shown that it is the same that would happen with like traditional dieting," Varady said.

The review also outlines areas for future research on intermittent fasting including:

• Long-term, randomized controlled clinical trials of all three fasting diets.
• Trials and qualitative studies that examine the effects of fasting diets on people with conditions such as diabetes, polycystic ovary syndrome and thyroid disorders.
• Studies that compare the three diets with each other.
• Studies that look at the effects of fasting to learn more about the mechanisms that underlie the metabolic improvements observed with fasting.

"We really do need long-term data to see if people can do intermittent fasting for the long term," Varady said. "I get lots of emails from people saying that they have been on the diet for 10 to 15 years, and it reversed their Type 2 diabetes, and they lost 60 pounds, and it was the only diet they could stick to. That is always nice to hear, but we need actual data to support that."

For those who want to try intermittent fasting, and for their clinicians, the review offers these guidelines:

Who can do intermittent fasting?

• Adolescents with severe obesity.
• Adults with normal weight, overweight or obesity.
• Adults with hypertension or high cholesterol.
• Patients with insulin resistance or prediabetes.
• Patients with Type 1 or Type 2 diabetes.

Advice for starting intermittent fasting:

• Plan on a one- or two-week adjustment to fasting. Headaches are common but can subside with increased water intake.
• Boost fiber through eating fruits, vegetables and whole grains.
• Eat at least 50 grams of lean protein on the fast days when alternating feast days to control hunger and prevent excessive lean mass loss.

What should be monitored during intermittent fasting?

• Adverse effects: Clinicians should assess adverse effects during the first three months of the diet.
• Nutrient deficiencies: Clinicians should monitor vitamin and mineral levels.
• Medications: Medications to control blood pressure, cholesterol and glucose should be monitored and may need to be reduced if the patient loses weight.
• Therapy: Patients should participate in behavioral change programs to help achieve long-term weight management.