Intermittent fasting may not be healthy after all

Currently in vogue, intermittent fasting (IF) – which switches between periods of eating and fasting – may not be as beneficial as many have thought.

Controlling when you eat – not what you eat – has been shown to aid weight loss and improve metabolic health, blood pressure, cholesterol, and blood glucose in a plethora of studies.

Proponents of IF said our bodies have evolved to function well for extended hours, or days, without food.  Eating within a 6-hour window period and fasting for 18 hours can trigger a metabolic switch from glucose-based to ketone-based energy, which can increase stress resistance, improve longevity, and keep cancer and other diseases at bay. [N Engl J Med 2019;381:2541-2551] Inducing a ketogenic state has also been linked to decreases in cardiovascular risk factors. [Am J Med 2020; 133: 901–907]

However, a new study linked IF to problematic eating disorder (ED) behaviours. IF in the 12 months and 30 days in women were significantly associated with overeating, binge eating, vomiting, laxative use, and compulsive exercise, as well as higher scores on EDE-Q, a measure of ED psychopathology. Similarly, IF in the past 12 months in men was associated with compulsive exercise and higher EDE-Q. [Eat Behav 2022; 47:101681]

“The study shines a light on the fact that engagement in IF may be connected with ED behaviours, requiring healthcare professionals to be aware of this contemporary and popular dietary trend, despite proponents on social media touting its effectiveness and benefits,” said study investigator Dr Kyle Ganson from the University of Toronto in Toronto, Ontario, Canada.

He said evidence on the effectiveness of IF for weight loss and disease prevention has been mixed, but it is important to understand the potential harms of IF.

IF in teens, young adults

Ganson and team analysed data from the Canadian Study of Adolescent Health Behaviours, a national study of Canadian adolescents and young adults (n=2,762). Participants were recruited from social media ads in November and December 2021. They answered questions about weight perception, weight change behaviours, their IF engagement, and ED behaviours.

Multiple modified Poisson regression analyses were used to determine the link between IF (past 12 months and 30 days) and ED behaviours and psychopathology.

Overall, 47.7 percent of women, 38.4 percent of men, and 52 percent of transgender or gender non-conforming (TGNC) individuals reported adhering to IF in the past 12 months. Varying patterns of association between IF and ED behaviours were found across genders, including the TGNC cohort, but are more pronounced in women.

The study caveats included the method of recruiting which could pose a selection bias.  Another was the participants’ self-reporting.

Balancing benefit vs harm

“The reality is that IF may help some but harm others and is most likely not healthy for all,” commented Dr Angela Guarda, Associate Professor of Eating Disorders in the Department of Psychiatry and Behavioral Sciences, Johns Hopkins School of Medicine, and Director of the Johns Hopkins Eating Disorders Program at Johns Hopkins Hospital in Baltimore, Maryland, US. “Individuals at risk for ED would be ill-advised to go for IF.”

IF may hide restrictive behaviours and prolong the identification and treatment of anorexia, bulimia, or binge eating disorder.  

“Healthcare professionals should therefore assess individuals on IF for ED-related attitudes and behaviours. They should be aware of the ways in which ED behaviours are masked as IF engagement,” said Ganson. He added that clinical and public health efforts are needed to protect the health of adolescents and young adults.

“Continued research is needed to establish for whom IF may be therapeutic vs an iatrogenic recommendation,” advised Guarda.

Differing opinion

Fasting expert Brad Pilon, author of Eat Stop Eat, one of the first written books about the unique benefits of IF, has a different perspective. He said periodically going without food has advantages over traditional dietary interventions.

“Every couple of hours we are making decisions about the “what to eat” rather than the “when to eat”. Problem is, the “what” has a lot of variations. One expert suggests dropping carbs, another proposes taking in more fats. That makes traditional dieting difficult,” he said. “What I like about IF is you do not have to do that. I like to use the analogy of an on-off switch, instead of a dimmer.”

Still, experts said the claimed health benefits of IF are based on shaky science. The long-term side effects are also unknown. Hence, more studies on the outcomes of IF are warranted.