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Intermittent fasting that focuses on an earlier eating window is more effective at reducing blood sugar levels than other diets such as calorie restriction, a study finds. Johner Images/Getty Images
  • Researchers compared the effects of time-restricted intermittent fasting, caloric restriction, and a weight loss booklet on weight loss.
  • They found that time-restricted intermittent fasting reduced blood sugar levels and increased insulin sensitivity more than the other methods.
  • Further research may refine how time-restricted intermittent fasting may be used to manage type 2 diabetes.

Type 2 diabetes (T2D) is a condition that affects how the body uses glucose for energy. Over time, T2D can damage nerves and blood vessels. More than 37 million Americans have the condition.

Studies show that intermittent fasting (IF) can help manage the condition. One study found that IF can lead to weight loss. Another study reported that IF can reduce cholesterol levels in older adult men.

Some research even shows that IF can reverse T2D. In one study, participants no longer had the condition after three months of IF.

Understanding more about how IF affects T2D risk could lead to the development of preventive strategies for the condition, as well as new treatment options.

Recently, researchers compared the effects of time-restricted IF and a reduced calorie diet for people prone to developing T2D.

They found that time-restricted IF led to greater improvements in post-meal blood sugar levels than a reduced-calorie diet.

The study was published in Nature Medicine.

For the study, the researchers recruited 209 adults with an average age of 58 years old and an average BMI of 34.8, which is considered obese.

They were then split into three groups at random:

  • Time-restricted IF: 30% of energy requirements between 8 a.m. and midday followed by a 20-hour fasting period on three nonconsecutive days every week, and unrestricted eating on other days
  • Calorie restriction (CR): 70% of energy requirements per day without set times for eating
  • Standard care (SC): a weight loss booklet

Participants partook in their assigned diets for six months and were followed for 12 months thereafter.

Their fasting blood sugar levels were assessed two and six months into the dietary interventions and 12 months afterward.

After the six-month intervention, the researchers found that glucose levels fell by 10.1 mg.dl-1, 3.57 mg.dl-1, and 4.15 mg.dl-1 in the IF, CR, and SC groups.

A year after those measurements were taken, those on the IF diet still had lower average blood sugar levels than those in other groups: -4.71 mg.dl-1 compared to – 3.79 mg.dl-1 and -3.57 mg.dl-1.

The researchers further found that those on the IF diet were more sensitive to insulin, but this was not statistically significant and experienced larger drops in cholesterol than those on the CR and SC groups.

Those in the IF and CR groups initially lost more weight than those in the SC group, however by month 18, there was no difference between the groups.

At the year follow-up, however, more people continued with CR than IF: 78% of those in the CR group continued a CR diet, while only 42% of those on the IF diet continued IF for 2-3 days per week.

The researchers noted that during the follow-up period, 45% of those in the IF and CR groups reported at least one adverse event, such as fatigue, constipation, and headache, compared to 19% of those in the SC group.

While the authors did not discuss how time-restricted IF may lower T2D risk, they noted that eating earlier in the day is linked to health benefits such as better 24-hour glucose control and insulin sensitivity.

Dr. Dana Ellis Hunnes, assistant professor at UCLA Fielding School of Public Health, who was not involved in the study, told MNT that while the authors do not fully explain the underlying mechanisms, their findings may mean that circadian rhythms, hormones, and activity levels may be involved in T2D risk.

MNT also asked Dr. Mark Guido, an endocrinologist with Novant Health Forsyth Endocrine Consultants in Winston Salem, North Carolina, not involved in the study, about what may explain the link between intermittent fasting and reduced markers for T2D. He said:

“It is hard to say for sure, but we do know that insulin levels are reduced during a fast and that fat starts to break down, both of which are beneficial for the delay or prevention of type 2 diabetes.”

When asked about the study’s limitations, Dr. Hunnes noted:

“One of the biggest limitations is that the 4-hour window for eating was less sustainable for some of the participants than perhaps a slightly longer window for eating might be.

“Another limitation- as always- is the sample size. [It remains to be seen whether these findings could be] generalizable to a larger population, [and whether they can be] turned into a ‘greater public health’ goal. But, it’s a nice and interesting start.”

Dr. Guido added that other limitations include that the researchers only looked at lab changes and body composition as opposed to how many patients were or were not diagnosed with diabetes. He added:

“The early beneficial effects of fasting vs calorie restriction appear to disappear after six months, which seems to imply that while fasting is better initially, both fasting and calorie restriction are just as beneficial long term.”

MNT asked Dr. Guido about the biggest takeaway from the study. He said:

“These findings add to the growing body of evidence that lifestyle — including healthy diet and exercise — is the best medicine for the prevention and treatment of type 2 diabetes mellitus.”

To the same query, Dr. Hunnes said: “The more we learn about nutrition and chronic diseases, such as T2D, the more we learn just how important nutrition, timing, quality of foods etc. are in the development of chronic disease, such as T2D. I think this [study] is a nice stepping stone to further demonstrate the importance of nutrition on disease risk.”