Is Intermittent Fasting Harmful for Someone with Type 2 Diabetes?

With Caroline Apovian, MD, FACP, FACN; Scott Isaacs, MD, FACP, FACE; and, Angela Fitch, MD, FACP

Eat all that you want one day, cut way back the next. This so-called intermittent fasting diet seems to be trending. It may sound like a great way to lower your hemoglobin A1c as the pounds melt off. But is it a good idea?

A new study from New Zealand suggests it might be, while three US doctors who reviewed the findings for EndocrineWeb urge caution, saying there are much better ways to achieve a healthy weight.

The New Zealand researchers found the risk of low blood sugar (hypoglycemia) was increased during the fasting days, as expected, but that the two intermittent fasting plans they tested still managed to produce weight loss and a drop in A1c levels, which is used to assess your risk for diabetes;1 the study is published in Diabetic Medicine.

However, three endocrinology experts who reviewed and commented on the study for EndocrineWeb say other approaches are less hazardous and just as, or more effective in producing a healthy weight loss.

Dieting only on some days will cause greater struggles with glucose levels.

Evaluating the Research on Intermittent Fasting  

Researchers from Wellington Hospital and the University of Otago in New Zealand defined intermittent fasting a bit differently than we do here in the US. Participants could eat whatever they wanted five days a week and then they were instructed to fast for the next two days.1 They had to do this for 12 weeks. In this study, the ''fast'' was actually a very low-calorie diet. Men could eat about 600 calories on a “fast” day and women were limited to 500 calories.

At the start, 41 participants who had been living with type 2 diabetes (T2D) for years were randomly assigned to this 5:2 plan, as it is sometimes called. About half of them were instructed to fast for two days in a row during the week; the others were instructed to fast on non-consecutive days.

In the consecutive fasting group, the average age was 62 years and their diabetes diagnosis had been made 13 years ago, on average. In the non-consecutive group, the average age was 58 years, and their diagnosis had been made 9 years earlier.1

The men and women were on a variety of medications, including insulin, sulfonylureas, metformin, and oral hypoglycemic agents.1 The researchers made adjustments to their medications in response to lower blood sugar levels on the fasting days, to avoid worrisome hypoglycemic episodes. The main study aim was to look at low blood sugar events and to monitor the amount of weight loss, A1c levels, and other parameters.1

Implementing the Intermittent Fasting Study 

In all, 15 participants experienced low blood sugar events, for a total of 53 such incidents. The rates of low blood sugar were no differences between the consecutive and non-consecutive groups, but the chance of having low blood sugar was doubled on fasting days than on non-fasting.

Both groups lost weight. At the study start, the group that fasted on non-consecutive days weighed, on average, 242 pounds. By the end of the 12-week study, they had lost on average of 8 pounds. Those in the consecutive fasting group weighed, on average, 239 pounds at the study start, and by the end, they had lost nearly 7 pounds on average, which isn’t statistically significant meaning both groups had similar results.

The level of A1cs declined from 8.2 to 7.5 in the non-consecutive group and from 8.4 to 7.8 in the consecutive fasting days group; also not significantly different.

The final answer on the worth of this approach is still up for debate, the researchers write, "Our study protocol could be adopted for the longer-term studies that will be required to assess the tolerability and sustained efficacy of an intermittent fast.”Unfortunately, they didn’t respond to our questions about how the groups differed and whether any of the participants were willing to stay with the diet long-term.

Intermittent Restricted Calorie Dieting Not Encouraged

While the study demonstrates that the following a very low-calorie diet can be safe for those with type 2 diabetes, at least for 3 months, it is ''not very effective," says Scott Isaacs, MD, FACP, FACE, an endocrinologist and adjunct instructor of medicine at Emory University School of Medicine, in Atlanta.

"The study showed minimal weight loss over the 12-week period of about 6 to 7 pounds, which is less than the typical weight loss on a standard low-calorie diet," Dr. Isaacs says. His advice: "Focus on a healthy low-calorie meal plan instead. Clearly following a low-calorie meal plan every day is superior to being on a very low-calorie diet for two days out of the week."

Angela Fitch, MD, FACP, associate professor of internal medicine and pediatrics at the University of Cincinnati College of Medicine/UC Health, says that while intermittent fasting can be an option, patients who choose this approach must work closely with their doctors to figure out how best to lower their hypoglycemic medications on fasting days.

Overall, however, Dr. Fitch says, "the most consistent way to lose weight and keep it off when you have type 2 diabetes is limiting the carbohydrates in your diet." Eating less than 150 grams of starchy carbs a day improves blood sugar levels and helps to promote weight loss for most people with T2D, she says. Eating less than 20-50g is even better, but harder to maintain long-term, she says.

The quality of those carbs is crucial, she says. She advises eating carbohydrates with higher fiber content. Choose cauliflower, for instance, not white rice.

Despite the findings that the intermittent fasters lost some weight loss and lowered their A1c levels, the study did find a higher risk of hypoglycemia on the fasting days, points out Caroline Apovian, MD, FACP, FACN, professor of medicine and pediatrics at the Boston University School of Medicine and director, Center for Nutrition and Weight Management, Boston Medical Center.  

"The reason is people on insulin and sulfonylureas really cannot safely do intermittent fasting is because of the risk of hypoglycemia,'' she tells EndocrineWeb.

With any of my patients who choose to follow a low-calorie diet, Dr. Apovian warns them that they will need to see her often to monitor their gluocose levels and to adjust their medications, and they will need to check their blood sugars 3 times a day to avoid the risk of hypoglyemia.

"My guess is, people who try this [intermittent fasting] long-term are going to end up gaining weight," Dr. Apovian says. ''They need something more consistent."

None of the researchers or physicians interviewed have any financial conflicts.

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