The health benefits of intermittent fasting are slowly being clarified as more evidence continues to emerge, say the authors of a new review of 21 studies. Initial findings suggest that fasting might be effective for mild to moderate weight loss for certain groups of people, at least in the short term.
And data so far at least dispel the myth that “people are going to feel weak and not be able to concentrate during fasting,” lead researcher Krista A. Varady, PhD, professor of nutrition in the University of Illinois at Chicago, noted in a press release from her university.
“We’ve shown it is the opposite,” she said. “They actually have a better ability to concentrate.”
Yet much longer-term data are needed on issues such as safety, Dr. Varady and colleagues note in their review in.
The trials so far have only been conducted in adults – generally with overweight or obesity and sometimes hypertension, dyslipidemia, and/or diabetes – but some have been performed in those of normal weight.
Dr. Varady and colleague recommend that those with type 1 diabetes, type 2 diabetes, or other comorbidities, or patients who need to take medications with meals at certain times of the day, should seek clinical supervision when considering intermittent fasting.
And currently, based on existing evidence, intermittent fasting is contraindicated for children under age 12 and those who have a history of an eating disorder or a body mass index <18.5 kg/m2. Opinions vary about the safety of supervised fasting in adolescents with obesity. Also, safety has not been evaluated in those older than age 70, and in women who are pregnant or lactating.
‘A few studies’ show 3%-8% weight loss over 2-3 months
Despite the recent surge in the popularity of intermittent fasting, “only a few studies have examined the health benefits of these diets in humans,” Dr. Varady and coauthors emphasize.
They identified 21 clinical trials of three types of intermittent fasting strategies:
Alternate day fasting (alternating between consuming 0-500 kcal on “fasting” days, followed by unlimited food on “feasting” days), six trials.
5:2 diet (“feasting” on 5 days and “fasting” on 2 days), seven trials.
Time-restricted eating (eating during a 4- to 8- hour window), nine trials.
The trials were short (mostly 5-12 weeks long) and small (10-150 participants), and mostly conducted in the United States.
They found these strategies can all produce a mild to moderate 3%-8% weight loss during 8-12 weeks, similar to that attained with a calorie-restricted diet.
Some studies found that patients had improvements in blood pressure, LDL cholesterol, triglycerides, insulin resistance, and hemoglobin A1c.
These weight-loss strategies produced few gastrointestinal, neurological, hormonal, or metabolic adverse effects; “however, as adverse outcomes are not regularly assessed in human trials of fasting, definitive conclusions regarding the safety of these diets are difficult to draw at present,” the researchers caution.
Practical advice, great anecdotes
Typically, 1-2 weeks of adjustment is needed when individuals start intermittent fasting, the researchers say.
While following this eating pattern, patients should be encouraged to consume plenty of fruits, vegetables, and whole grains to boost their fiber and micronutrient intake.
On fasting days, they should consume at least 50 g of lean protein to help control hunger and prevent excessive loss of lean mass. On those days, alcohol is permitted but not recommended. Energy drinks and coffee or tea without sugar, milk, or cream are allowed, and diet soda should be limited to two servings a day because it can increase sugar cravings.
Ideally, clinicians should regularly assess patients for adverse effects during the first 3 months of intermittent fasting. They should also monitor patients for deficiencies in vitamin D, vitamin B12, and electrolytes, as well as for changes in medications for blood pressure, lipids, and glucose that may be needed if patients lose weight.
Patients who reach their weight-loss goals and wish to stop intermittent fasting need to transition to a weight-maintenance program, possibly by increasing energy intake on fasting days to 1,000-1,200 kcal/day or widening the eating window to 12 hours in time-restricted eating.
“I get lots of emails from people saying that they have been on the diet for 10-15 years, and it reversed their type 2 diabetes, and they lost 60 pounds, and it was the only diet they could stick to,” Dr. Varady noted.
“That is always nice to hear, but we really do need long-term data to see if people can do intermittent fasting for the long term,” she reiterated.
The review was funded by the National Institute of Diabetes and Digestive and Kidney Diseases. Dr. Varady received author fees from the Hachette Book Group for the book, “The Every Other Day Diet.” The other authors have declared no relevant financial relationships.
A version of this article first appeared on.