Intermittent fasting has been gaining popularity among people looking to shed extra kilograms and maintain a healthy weight. Researchers argue that this type of diet may also slow down aging and disease.
In intermittent fasting, what essentially takes place in the body is that one source of energy — which can facilitate the accumulation of body fat — is switched for another.
Our bodies run on glucose, or simple sugar, but when we fast for a longer period of time, that energy source becomes unavailable.
Our system needs to identify a different kind of "fuel." That is when the body begins to convert certain types of body fat into fatty acids, which are easily absorbed by the blood.
Fatty acids, in turn, produce molecules called ketones, which the body uses as its new source of energy.
Stephen Anton, a researcher at the University of Florida College of Medicine in Gainesville, calls this process "flipping the metabolic switch."
"This switch," explains Anton, "can happen after a certain period of time fasting. It's a gradation in which your metabolism over time shifts to use higher and higher amounts of ketones for energy."
He and his team were interested to learn more about how this switch occurs, and whether it could bring other health benefits, alongside weight management.
For this purpose, they reviewed numerous recent studies focused on the mechanisms and benefits of intermittent fasting.
The team's review, published in the journal Obesity, suggests that intermittent fasting may be more healthful than other dieting strategies, as ketones put less stress on cells than the byproducts of other dieting styles.
Significant weight loss regardless of style
Anton and his colleagues explain that the switch usually begins to take place after 8–12 hours of fasting, though in the case of individuals who practice intermittent fasting, the fasting strategies vary.
The researchers focused on the two most common types of intermittent fasting diets, the first of which is based on time restrictions for eating.
In it, the dieter may fast for a number of hours per day — for instance, 16 hours — while allowing themselves to eat anything they'd like over the remaining hours.
For the second type of intermittent fasting, dieters may choose to alternate days of total fasting, with days when no food is off limits.
Or they may simply alternate days of frugal eating — when individuals limit themselves to foods that equal only about 500 calories in all — with days of unrestricted eating, or "feasting days." "Of course," Anton notes, "we recommend healthy food [during the feasting times]."
The team's review of existing studies revealed that, all in all, any type of intermittent fasting diets are associated with significant weight loss.
In all 10 clinical trials assessing the effects of alternate-day fasting, the results conclusively pointed to this strategy's effectiveness when it came to shedding extra kilos. And, 3 out of the 4 studies focused on the restricted timing type of intermittent fasting had similar results.
"So in my mind, it's not a question of whether it works for producing fat loss," says Anton. What's more interesting and more important is what kind of tissue is lost through intermittent fasting.
Additional potential health benefits
Most of the studies reviewed by Anton and team revealed that, while participants did lose body fat, no significant amount of lean tissue — which includes organ tissue, muscular tissue, and bone tissue — was lost.
This is important, since lean tissue allows our bodies to keep on functioning well, and other types of dieting strategies, Anton notes, lead to significant loss of both fat and lean tissue, which may affect health in the long run.
Studies into the effect of the switch from glucose-driven energy to ketone-driven energy in rodents and other animals suggests that intermittent fasting could also have other health benefits, the scientists say.
The researchers say that it could help to prolong the lifespan, improve the functioning of metabolic processes, protect cognitive function, enhance physical performance, reduce harmful instances of inflammation, and shield against cardiovascular diseases.
"An important takeaway is that we all have the ability to switch our metabolism from glucose to ketone utilization. And that switch has the potential to have profound health benefits for us, in addition to the positive changes in body composition."
Still, the authors warn against starting intermittent fasting without first asking for a doctor's advice. This dieting style may not be equally beneficial for everyone, and in some cases it could do more harm than good, he cautions.