Can You Really Reverse Your Biological Age in Four Weeks?

A University of Sydney study found that some adults aged 65 to 75 finished a month on a controlled diet with lower "biological age" scores. That is easily read as four weeks of eating turning back the clock, but the researchers did not show that anyone's cells became younger, that they would live longer or that the change would last. What they did show is more useful: some measures described as biological age can move quickly when the health markers behind them change.

TL;DR

• Diet changed biological age scores within four weeks. The randomised trial involved 104 adults aged 65 to 75, with all food provided during the intervention.
• This was not an epigenetic clock. Biological age was estimated with the Klemera-Doubal Method, which combines clinical measures that change with age.
• The clearest result came from an omnivorous higher-carbohydrate diet. Compared with an omnivorous high-fat reference diet, it produced significantly lower scores in both versions of the model.
• The semi-vegetarian diets pointed the same way. Not every comparison reached statistical significance.
• This does not prove ageing was reversed. The authors warn the changes may reflect short-term physiological responses rather than a change in the underlying ageing process.
• There is no evidence yet that the effect lasts. The trial did not test whether the changes persist or reduce age-related disease or death.

What Is Biological Age?

Chronological age is simple; biological age is much less concrete. Researchers have developed many ways of estimating how the body appears to be ageing from measurements that change over time. Some, known as epigenetic clocks, use patterns of chemical tags on DNA. Others use proteins, metabolites or routine clinical measures such as blood pressure and cholesterol. We have looked at that distinction in more detail in biological age versus chronological age.

This study used the Klemera-Doubal Method, or KDM. Rather than measuring a single ageing mechanism, KDM combines biomarkers that vary with age to estimate a person's physiological state relative to their years. The researchers used two versions: a smaller model built on measures such as glucose, cholesterol, C-reactive protein and blood pressure, and a larger model that added markers including insulin, triglycerides, BMI and waist circumference. The resulting figure was compared with each person's chronological age to give δAge, or delta age. A lower δAge means a biomarker profile that looks younger than expected for someone's age, and that detail becomes important once the number starts to move.

What Did the Study Do?

The research, published in the peer-reviewed journal Aging Cell in April 2026, was a secondary analysis of the Nutrition for Healthy Living study, a randomised dietary trial of 104 adults aged 65 to 75. Participants had a BMI between 20 and 35, and none were vegetarian before joining. For four weeks, all of their food was delivered to them.

Everyone was randomly assigned to one of four diets. All four supplied 14% of energy from protein, but they differed in where that protein came from and in the balance of fat and carbohydrate. The two omnivorous diets took roughly half their protein from animal sources and half from plants, while the two semi-vegetarian diets took 70% from plants and 30% from animal sources. Within each pair, one diet was higher in fat, at around 37–41% of energy from fat and 41–43% from carbohydrate, and the other was higher in carbohydrate, at about 28–29% fat and 53% carbohydrate. The researchers calculated biological age estimates before and after the four weeks.

So Who Became "Younger"?

The omnivorous high-fat diet served as the reference because it most closely resembled what participants usually ate, and that group showed no meaningful change in estimated biological age. The other three groups generally moved towards lower estimates.

The most consistent result came from the omnivorous higher-carbohydrate diet. Compared with the reference group, and after accounting for starting values, its final δAge was about 3.5 years lower in the smaller model and 4.1 years lower in the larger one, and both differences were statistically significant. The semi-vegetarian high-fat group was about 3.5 years lower in the smaller model, which was significant, and about 3.2 years lower in the larger model, which narrowly missed conventional significance. The semi-vegetarian higher-carbohydrate group was about 2.9 years lower in both models, but neither result reached statistical significance. That is more nuanced than "carbohydrates reverse ageing" or "plant-based diets make you younger", and the trial was too small and short to support either claim.

Did They Really Reverse Ageing by Four Years?

No, and this is the most important point in the study. A difference of roughly four years in a biological age score does not mean four years of ageing disappeared in a month. The score is built from biomarkers such as cholesterol, glucose, insulin, inflammation, blood pressure and body measurements, and those can change far faster than the ageing process itself. If a diet improves some of those measurements, a formula based on them will produce a younger-looking age.

That can still matter. KDM-based measures have been linked to illness and death in large population studies, which is why researchers take them seriously. But the score may also be capturing short-term physiological flexibility. The authors are clear on this: they caution against treating the result as evidence of biological age reversal, because the changes may reflect an acute response to diet rather than a shift in a person's ageing trajectory. Lead author Dr Caitlin Andrews has said it is too early to conclude that specific dietary changes will extend life, while describing the findings as an early sign that changing diet later in life may bring benefits.

The Wider Problem With "Biological Age"

There is no single biological age test. An epigenetic clock measures something different from a KDM score built from cholesterol, blood pressure and glucose, and other systems use proteins, metabolites or organ function. They all produce a result expressed in years, which makes it feel easy to understand, but those years are not necessarily interchangeable.

The unit also invites exaggeration. A between-group difference of four years in a biomarker calculation is easily retold as "four years younger", even though that is not what the trial showed. Saying that several metabolic and inflammatory markers moved enough to shift a composite score is less exciting, but it is much closer to what happened. The same caution applies to other longevity claims, from metformin as an anti-ageing drug to daily multivitamins and slower ageing.

Does the Diet Still Matter?

Yes, for a less dramatic reason: changing diet shifted a set of markers associated with ageing within weeks, which is worth knowing even if nobody calls it becoming younger. The result also sits awkwardly with current diet tribes. The most consistent reduction came from an omnivorous diet with less fat and more carbohydrate, not from cutting out animal foods. The semi-vegetarian diets tended to produce lower estimates as well, which suggests protein source may play a part, but the results were not consistent enough to name a single winning pattern. The researchers call for longer trials, and studies in other age groups, to establish whether these changes persist and whether they matter for long-term health. If you want a structured way to change what you eat over a few weeks, The Sugar Reset works through the same markers this study measured, and pairs with the Vitae Reset Companion for day-to-day support.

What This Actually Means

A biological age score can be more changeable than biological ageing itself. Some scores are built from biomarkers that respond within weeks to diet and other influences, so a lower result may say more about recent physiology than about how fast someone is ageing.

A younger score is not meaningless, because measures such as blood pressure, glucose, cholesterol and inflammation matter for long-term health. But proving that ageing has reversed would take longer trials showing lasting changes and lower rates of age-related disease, disability or death. This small four-week trial does not offer an anti-ageing diet.

Frequently Asked Questions

Can you reverse your biological age?

Studies can produce lower biological age estimates after lifestyle changes, but that is not the same as showing the ageing process has reversed. Different biological age tests measure different aspects of physiology.

How did this study measure biological age?

It used the Klemera-Doubal Method, which combines clinical biomarkers that change with age into a single estimate. It was not an epigenetic or DNA-based test.

Which diet performed best?

The most consistent statistically significant result came from the omnivorous higher-carbohydrate, lower-fat diet, compared with the omnivorous high-fat reference diet. The semi-vegetarian diets also tended towards lower estimates, but not every comparison was statistically significant.

Does this mean a high-carbohydrate diet slows ageing?

No. The study lasted four weeks and measured a biomarker-based estimate rather than ageing outcomes, so it cannot show that any diet slows ageing or extends lifespan.

Are biological age tests useful?

They can be useful research tools for summarising several age-related measures, and some have been linked to later health outcomes. There is no single accepted test, and a result expressed in years should not be read as a literal measurement of how old your body is.

The Bottom Line

The participants did not reverse ageing in four weeks. What changed was a composite estimate built from physiological markers, most clearly on the omnivorous higher-carbohydrate, lower-fat diet. Whether those changes last, reduce disease or alter the rate of ageing is still unknown.

So when a study, clinic or wearable says your biological age has fallen by four years, the useful question is not whether you are four years younger. It is what was measured, what changed, and whether that change predicts anything that matters years from now.

Related reading: Could Eating Less Protein Slow Ageing? · Does Coffee Slow Cellular Ageing? · Biological Age vs Chronological Age

This is general information rather than medical advice; speak to your GP or a registered dietitian before making significant dietary changes.

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