Imagine two people who share nearly identical DNA. One exercises regularly. The other moves significantly less. What happens to the body over time?
On the outside, they may still look strikingly alike. But beneath the surface, years of different habits can leave their mark on fitness, body composition, bone density, and blood markers. That is precisely what makes twin research so fascinating.
In Finland, researchers have followed rare identical twin pairs where one twin has been considerably more active than the other over many years. The comparisons offer an unusual window into the interplay between genes and lifestyle — while also reminding us how carefully research findings need to be interpreted.
Quick answer
What happens to the body when one twin exercises and the other does not?
When one twin exercises regularly for years and the other does not, research suggests differences may emerge in body composition, cardiovascular capacity, and metabolic markers — despite nearly identical DNA. The evidence comes from 17 twin pairs and reflects associations, not guaranteed outcomes for every individual.
Key takeaways
- The review draws on 17 rare identical twin pairs with long-term differences in physical activity levels.
- In the reviewed pairs, the more active twins had, among other things, higher physical capacity and less total body fat, abdominal fat, and liver fat.
- Differences were also reported in certain blood vessels, loaded sections of the skeleton, and large HDL particles.
- The small sample and observational study design mean the findings cannot be promised to every twin or used as individual exercise advice.
Why twin studies are so valuable
When researchers compare two unrelated people, differences in health could be due to almost anything: genes, upbringing, diet, work, income, or past illness. Identical twins offer a much closer comparison because they share essentially the same DNA and often much of their early environment.
When their lives later take different directions, researchers can ask: what do we observe in the twin who has moved more over many years? Each twin in a sense becomes a reference point for the other. It is not a perfect laboratory, but it can make certain associations more visible.
- Diet, sleep, and recovery may differ.
- Work, stress, illness, and medication can all have an influence.
- Prior health can affect both activity levels and later measurements.
The Finnish twin pairs
The 2022 review collects central findings from 17 Finnish identical twin pairs. In each pair, the twins had shown long-term differences in leisure-time physical activity. They came from two Finnish twin cohorts and underwent extensive clinical examinations.
Seventeen pairs may not sound like many — and it is not. But long-term activity-discordant identical twins are rare. That is precisely why the material is valuable, while the small sample also sets a clear limit on how far the conclusions can be stretched.
What the researchers found
In the reviewed pairs, the more active twins had higher physical capacity. They also had less total body fat, less fat around the internal organs, and less liver fat than their less active co-twins.
The researchers also reported larger lumen in certain major blood vessels to the legs, higher bone density in loaded parts of the skeleton, and more large HDL particles in the more active twins.
Taken together, the findings suggest that long-term differences in activity habits may coincide with differences across several parts of the body — even when the genetic foundation is very similar. But they do not show that every twin will get the same result, or that activity alone caused the entire difference.
What does this mean for twins?
Perhaps you and your twin already exercise in very different ways. Perhaps one is out running while the other struggles to find room for movement in daily life. Research shows why such differences are interesting — not that twins need to measure themselves against each other.
The most important thing is not who has the "best" numbers. It is that two genetically very similar people can still develop differently over time. Our habits may matter, but every body has its own history and its own starting point.
The TwinPare perspective
For TwinPare, this is the heart of why twin research is so compelling. Twins can be incredibly alike and yet shape very different lives. When researchers follow those differences, we gain a new way of understanding both human biology and the power of everyday habits.
But good science communication must hold two thoughts at once: the results may be meaningful, and they may still be uncertain. The Finnish twin pairs do not give us a recipe. They give us an unusually clear question to keep exploring.
Source and limitations
The source supports that the more active twins in the 17 reviewed pairs had higher physical capacity and differences in body fat, liver fat, certain blood vessels, bone density in loaded parts of the skeleton, and large HDL particles compared with their less active co-twins.
The source does not prove that physical activity alone caused all the differences. It also does not prove lower blood pressure, lower disease risk, a longer life, improved quality of life, or any specific outcome for an individual twin.
The small sample matters because unusual characteristics of a few pairs can influence the overall picture. Findings therefore need to be weighed against larger and different types of studies before broad health claims are made.
The study is still useful because activity-discordant identical twins are rare and offer a closer genetic comparison than studies of unrelated individuals. It helps TwinPare readers understand both the potential and the limitations of twin research.
Source notes
The source has been verified and editorially reviewed for this article. The limitations below show which level of conclusion the sources support.
- [kujala-2022] Physical activity and health: Findings from Finnish monozygotic twin pairs discordant for physical activity. Urho M. Kujala; Tuija Leskinen; Mirva Rottensteiner; Sari Aaltonen; Mika Ala-Korpela; Katja Waller; Jaakko Kaprio. Scandinavian Journal of Medicine & Science in Sports, 2022. Evidence type: Peer-reviewed summary of findings from 17 identical twin pairs with long-term differences in physical activity levels Limitation: The reviewed sample consists of 17 rare identical twin pairs with long-term differences in physical activity. The design is informative but small, and cannot make every observed difference universally causal or guaranteed. PubMed PMC DOI
Editorial source review
This section shows how the article's key factual claims are linked to the source.
Phrasings that require caution
- Write "suggests", "was associated with", "in the reviewed pairs", and "may help explain".
- Do not write that physical activity alone caused the differences.
- Do not promise that exercise will produce the same result for every twin.
- Do not link this source to lower blood pressure, lower disease risk, a longer life, or improved quality of life without additional verified sources.
| ID | Claim | Source support | Caution |
|---|---|---|---|
| C1 | Activity-discordant identical twin pairs can provide an unusual co-twin comparison in which genetic background is largely shared. | 2022 | The design reduces genetic variation but does not eliminate other possible explanations. |
| C2 | The review summarised findings from 17 long-term activity-discordant identical twin pairs from two Finnish twin cohorts. | 2022 | The rare and small sample should always be mentioned close to any description of results. |
| C3 | In the reviewed pairs, the more active twins had higher physical capacity and less total, visceral, and liver-related fat. | 2022 | Use "in the reviewed pairs" and "had", not a universal promise of training outcomes. |
| C4 | The more active twins had larger lumen in certain leg arteries, higher bone density in loaded areas, and more large HDL particles. | 2022 | Measurable differences are not the same as proven reduction in disease risk or a longer life. |
| C5 | The review improves understanding of possible site-specific and system-wide associations with long-term physical activity. | 2022 | "Possible associations" should not be rewritten as certain causality or individual guarantees. |