Intermittent fasting has attracted plenty of attention over the past decade, and now a group of US endocrinologists and nutrition experts say the data is in to confirm it’s a useful lifestyle option for patients with type 2 diabetes.

But after 225 clinical trials focused on approaches like time-restricted eating and the 5:2 diet, there’s not yet enough evidence to back the approach for helping patients manage their type 1 or gestational diabetes, Harvard Associate Professor of Nutrition Courtney Peterson and colleagues said.

The authors published their review in The Lancet Diabetes and Endocrinology, looking at what the evidence has shown for how fasting affects weight loss, glycaemic control and the risks of hypoglycaemia according to hundreds of clinical trials on the subject.

They found:

• The data showed that when sustained for a few months, intermittent fasting lowers HbA1c by 0·3–1·2% among type 2 diabetes patients,
• Fasting diet patterns delivered modest to moderate weight loss,
• The evidence is strongest for time-restricted eating and the 5:2 diet, and
• Trials have shown no increased risk in hypoglycaemia among patients who fast, though insulin dosing likely needs to be reduced on fasting days.

Neither the American Diabetes Association or the International Diabetes Federation (IDF) Global Clinical Practice Recommendations back intermittent fasting.

But given it appeared to be as effective as calorie restriction at achieving glycaemic control, there was a case to include it as an option for patients in guidelines for type 2 diabetes, the authors argued [link here].

Clinicians should “offer intermittent fasting as one of several lifestyle options or as an alternative to calorie-counting or caloric restriction”, telling patients they should expect a 0.3% to 1.2% reduction in HbA1c and 4-7% weight loss over three to 12 months, they said.

Caution with T1D, gestational diabetes 

Associate Professor Courtney Peterson.

The review only found one small trial of 10 patients with T1D. This showed patients on the 5:2 diet had a greater reduction in body weight than those using caloric restriction, but it was not enough to be statistically significant.

“Given only one small trial, there are insufficient data on efficacy and safety to recommend intermittent fasting for people with type 1 diabetes,” the authors said.

It was a similar situation for prediabetes, with five RCTs showing the approach could help glycaemic control in these patients, but the evidence was still too preliminary.

Meanwhile, no trials have evaluated intermittent fasting for women with gestational diabetes.

Two studies of women at risk of GDM showed time-restricted eating had little impact on glycaemic control. At the same time, the authors acknowledged intermittent fasting was currently not recommended for women who were pregnant or breastfeeding. Clinicians should not suggest fasting approaches for pregnant women, the authors argued.

Other groups like patients with a history of hypoglycaemia, those with a history of eating disorders and adolescent patients should not be recommended intermittent fasting as an option for managing their T2D, they said.

There were also some patients whose lifestyles were not well suited to fasting approaches, Associate Professor Peterson and colleagues said.

“In our clinical experience, individuals with erratic or inflexible schedules (eg, long work shifts with no breaks), a lack of family support, or constraints around family mealtimes tend to struggle the most with intermittent fasting.”