Can fasting turn back your biological age?
Fasting, particularly intermittent fasting and ketogenic diets, moves multiple biological ageing markers in a favourable direction. Whether this truly translates into a younger biological age over the long term has not yet been proven, but you can already start with a fasting method that suits you.
A ketogenic diet followed for 6 months and intermittent fasting followed for 8 weeks both clearly lowered what is called metabolomic age in people with multiple sclerosis. Daily calorie restriction without any fasting periods had no measurable effect on that measure. This is a biological clock value based on chemical patterns in the blood, not a direct measurement of how old your body is.
Alternate-day fasting improved several markers of biological ageing in healthy, middle-aged adults: less belly fat, lower inflammation and better cholesterol levels. This remained safe for more than 6 months, but the group consisted of healthy, non-obese people, so it says nothing yet about people with health conditions.
Whether fasting also reduces the number of so-called 'ageing cells' is less clear. An analysis combining 27 studies shows that calorie restriction lowers certain proteins in the blood that are linked to cell ageing. But the direct markers of ageing cells and the length of the protective caps on chromosomes barely changed. The researchers point out that those proteins are not specific enough to treat this as proof.
A long-running study into moderate calorie restriction (12 to 24 months) in healthy young to middle-aged adults did show a clear drop in blood markers of cell ageing, and that drop was linked to better blood sugar control. One important difference from animal studies: in people, calorie restriction does not lower a growth-related protein in the blood, whereas in animals that is a key way calorie restriction extends life.
In short: fasting can move ageing markers in a favourable direction, but whether this amounts to a genuine reversal of your biological age has not yet been established. The effects depend on the fasting method: short fasting cycles appear to have more influence on biological clocks than ordinary calorie restriction.
This answer draws on multiple studies in people, including a study in people with multiple sclerosis, a study on alternate-day fasting in healthy middle-aged adults, a large long-term study on calorie restriction, and an analysis combining 27 studies on diet and cell ageing. The evidence is strongest for short-term changes in metabolic markers and body fat. The direct evidence that fasting genuinely 'reverses' biological age is weaker, because most measurements are indirect markers rather than long-term health outcomes.
Can you really lower your biological age, and does research support this?
There are promising hints that biological age is measurable and perhaps steerable, but this has not been proven. Wait for larger studies in people before basing major decisions on this.
How quickly can you change your biological age with exercise?
Regular moderate to vigorous exercise is associated with a biological age that comes out 1 to 3 years lower, and molecular changes in muscles are measurable after just a few weeks. Whether a lower score on an epigenetic clock truly means living healthier or longer has not yet been proven: treat it as an encouraging signal, not a guarantee.
How can I measure my biological age?
DNA clocks, and GrimAge in particular, give the most reliable picture of your biological age. These measurement methods are currently used mainly in research.
How often does it make sense to remeasure your biological age?
Clocks change slowly: two years is a realistic minimum to see the effect of a lifestyle change, but whether a lower score truly means you will live longer has not been proven. Track your daily weight, activity and blood pressure more frequently, and treat a remeasurement as a two-year check-in, not a weekly dashboard.
Does fasting reset your immune system?
Fasting lowers inflammatory markers and has measurable effects on the gut microbiome that appear to be beneficial for the immune system. For healthy people who want to address low-grade inflammation, time-restricted eating is a practical starting point.
Can fasting really rejuvenate your cells?
Animal studies show convincing signs of cellular rejuvenation through fasting, and the first studies in people point in the same direction.