8/20/2026
This is the seventh piece in my series about Eric Topol's book, Super Agers: An Evidence-Based Approach to Longevity.
One of the most popular ideas in the longevity world today is calorie restriction and fasting. Could simply eating less—or eating only during certain hours—help us live longer and healthier?
Topol's answer is intriguing, but far from a simple yes.
1. What is calorie restriction?
Calorie restriction generally means reducing daily calorie intake by 20–50% while maintaining a balanced proportion of macronutrients. It is different from a ketogenic diet, which drastically reduces carbohydrates and derives most of its calories from fat.
During World War I, a natural experiment in Denmark found that dietary restriction was associated with a substantial reduction in mortality.
Calorie restriction has also extended lifespan in many laboratory organisms, including yeast, fruit flies, and mice. But studies in rhesus monkeys have produced conflicting results: one found a significant increase in lifespan, while another found no difference.
And strict calorie restriction has its drawbacks. It can be difficult to sustain and may lead to loss of muscle mass and bone density, as well as impaired brain function.
At the same time, animal and human studies suggest potential benefits involving metabolic health, immune function, and possibly anti-tumor mechanisms. So it's complicated.
2. Milder alternatives: time-restricted eating and intermittent fasting
Instead of continuously eating less, people can restrict when they eat. Some common approaches include:
16:8: eating within an eight-hour window and fasting for 16 hours
5:2: eating normally five days a week while substantially restricting calories on two days
Alternate-day fasting
Other forms of intermittent fasting
These approaches are generally more practical than severe long-term calorie restriction.
But there is an important complication: restricting the hours in which you eat often results in eating fewer calories anyway.
That makes it difficult to determine whether the benefit comes from when people eat, how much they eat, or both.
3. Human trials have produced mixed results
The key part here is different randomized trials have produced very different results.
Some found little or no additional benefit from time-restricted eating compared with ordinary eating or simple calorie restriction.
Others found benefits, particularly when the eating window was shifted earlier in the day. Some participants experienced improvements in:
weight loss
body fat
blood pressure
glucose regulation
sleep
mood
Time-restricted eating also produced weight loss and improved glucose regulation in one trial involving people with type 2 diabetes.
Intermittent fasting has produced similarly mixed but sometimes promising results, including modest improvements in glucose regulation among people at risk of diabetes.
Overall, however, the evidence is not yet consistent enough to draw firm conclusions.
4. Anti-aging benefits remain unproven
This is where Topol is particularly cautious.
Despite the enormous enthusiasm surrounding fasting in the longevity community, no calorie-restriction, time-restricted-eating, or intermittent-fasting regimen has yet been definitively shown to slow human aging or extend healthspan.
There are promising findings involving metabolic and biological-age markers, but these are not the same as proving that people will actually live longer or remain healthier for longer.
Topol also warns against turning fasting into a guaranteed “wellness” or weight-loss solution.
5. The circadian rhythm may be just as important as the calories
I think the most interesting part of this discussion is the connection between meal timing and our circadian rhythm.
Our bodies operate according to biological clocks that influence:
digestion
nutrient absorption
gut hormones
metabolism
immune function
inflammation
Even the gut microbiome changes throughout the day. Irregular meal timing can disrupt the synchronization of these systems and impair glucose regulation.
So the important question should be “When should we eat?”
Maintaining a relatively consistent eating schedule may help keep our biological clocks synchronized.
6. A surprisingly simple practical recommendation
Until stronger evidence emerges, Topol's practical advice is much less dramatic than some of the fasting regimens promoted online:
Eat dinner early—ideally three to four hours before bedtime—and avoid calories overnight until breakfast.
In other words, rather than pursuing extreme fasting, establish a regular daily eating rhythm and avoid late-night eating.
The bigger picture
Topol is quite cautious about the popular claim that “fasting is good for longevity.”
Perhaps the most practical lesson is：
Eat reasonably and regularly, avoid eating late at night, and don't confuse a promising theory with proven longevity science.
