8-Hour Eating Window Improved Glycemic Control, Small Study Shows

Eating within an 8-hour window improved glycemic variability in adults at risk for type 2 diabetes, an interim analysis of a randomized crossover study of 15 people showed.

Compared with eating over 14+ hours per day, eating only within an 8-hour window over 3 days significantly increased time spent in euglycemia (70-140 mg/dL) by 3.3% (95% CI 2.4-4.2), reported Kelly Bowden Davies, PhD, MSc, of Manchester Metropolitan University in England.

“In 3 days, that’s quite striking,” she said at the European Association for the Study of Diabetes (EASD) annual meeting in Madrid.

The study measured the effects of an early 8-hour eating window — 8 a.m. to 4 p.m. — and a late window of noon to 8 p.m.

“We didn’t see a benefit of early or late time-restricted eating,” Bowden Davies pointed out. “The important point from this study is that we see a benefit of restricting your eating window to 8 hours a day. Both conditions had a positive effect on their glucose control.”

Time-restricted eating over the course of 3 days also significantly reduced:

• Mean absolute glucose by 0.6 mmol/L (10.8 mg/dL; 95% CI 0.2-0.9)
• Coefficient of variation by 2.6% (95% CI 1.6-3.7)
• Mean amplitude of glucose excursions by 0.4 mmol/L (7.2 mg/dL; 95% CI 0.2-0.6)

These measures were reflective of how well individuals were able to regulate their blood glucose levels.

“Time-restricted eating in this study … even in 3 days produces small, subtle differences that are subclinical at this point but … they’re statistically significant,” said Bowden Davies. “We recognize that it is quite a promising bit of data to suggest a really applicable, feasible intervention that could be translated across different populations.”

Several prior studies have highlighted intermittent fasting as a weight-loss method, but fewer studies have looked at its effect on glucose levels.

“There are some researcher groups that recognize … that if we manipulate the time of which we eat, it can better align with circadian rhythms,” said Bowden Davies. “The way in which our body secretes hormones typically changes throughout the day. If we eat earlier in the morning, it can be hypothesized that it may be better aligned such that it would cause a greater improvement in glucose control for these people who are at-risk or have the disease themselves.”

Only 15 adults (nine females and six males) were included in the randomized, cross-over study. Average age was 52, baseline weight was 175 lb, BMI was 27.7, HbA1c was around 5.5%, fasting glucose was around 84.7 mg/dL, and fasting insulin was 7.6 µIU/mL.

Inclusion criteria included having typical eating and drinking habits spanning over 14 hours per day, not meeting physical activity guidelines, and not on a current lifestyle intervention.

“These people were generally normoglycemic — they had good glucose control,” she said. “But these individuals were overweight and obese and what we classify as having risk factors for the development of type 2 diabetes.”

All participants completed both study conditions, starting with 3 days of their unrestricted diet (14+ hour window), followed by 3 days of an early or late time-restricted window. This was followed by a 7-day washout period, followed by another 3 days of eating in at least a 14-hour window, and another 3 days of an early or late 8-hour diet.

The early time window scheduled the first meal at 8 a.m., first snack at 10 a.m., second meal at 12 p.m., second snack at 2 p.m., and third meal at 4 p.m. The late time window scheduled the first meal at noon, first snack at 2 p.m., second meal at 4 p.m., second snack at 6 p.m., and last meal at 8 p.m.

Glycemic measures were assessed with FreeStyle Libre continuous glucose monitors (CGMs). Physical activity levels didn’t change during the study.

Participants lost weight during the intervention, with body weight declining from 175.7 lb to 173.3 lb in the early time-restricted condition and from 177.5 lb to 174.6 lb in the late condition.

Other outcomes including triglycerides, total cholesterol, HDL cholesterol, LDL cholesterol, fasting glucose, and fasting insulin didn’t change. “This was only 3 days, so that was unsurprising,” said Bowden Davies.

Two members of the audience questioned the generalizability of the methods outside of the small study, since most patients with overweight or obesity without diabetes don’t have access to CGMs to keep track of glycemic fluctuations — though over-the-counter CGMs have recently hit the market.

But Bowden Davies said time restricted eating is “a really simple strategy” that patients can try even without glucose monitoring.

“Typically, individuals who progress to metabolic disorders and disease are eating around the clock,” she said. “We need to tell those people that there’s a benefit to eating for less hours a day.”

This was an interim analysis, she emphasized. Her group plans to look more closely at how participants respond to certain meals during time-restricted eating.

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