Teen Fitness, BMI, and Severe Infection Risk Decades Later

Teenage fitness may predict more than future heart disease. In a Swedish cohort of nearly one million men, low cardiorespiratory fitness and higher BMI at about age 18 were each associated with a higher risk of hospitalization for sepsis, bacterial pneumonia, and infective endocarditis decades later. The result is striking because these are severe bacterial infections, not vague immune "wellness" outcomes.

The study does not show that a bad bike test at 18 programs a weak immune system for life. It does show that adolescent fitness and body size identify people at higher long-term risk, even after the researchers accounted for each factor separately. That is a finding worth taking seriously—and not turning into a causation story before the evidence earns one.

A fitness measure reaches into infectious disease

Birger Sourander and colleagues' 2025 Swedish registry study followed 995,091 male military conscripts, enrolled at a mean age of 18.3 years, through December 2019. At conscription, participants had measured BMI and a standardized bicycle-ergometer assessment of cardiorespiratory fitness (CRF). The researchers then linked those baseline measures to national inpatient diagnoses of sepsis, bacterial pneumonia, and infective endocarditis.

All three outcomes required hospitalization. The study therefore addresses the severe end of bacterial infection, not colds, minor skin infections, or every antibiotic prescription someone might receive over 30 years.

Higher adolescent BMI and lower CRF each predicted higher risk of all three infections after the investigators entered both variables into the same models. They also adjusted for muscle strength, parental education, and asthma. Excluding men who developed a severe bacterial infection within four years of conscription reduced one obvious source of reverse causation: an illness already present at baseline could otherwise lower both exercise capacity and later infection risk.

The BMI result did not begin only at clinical obesity. Compared with a BMI of 18.5–19.9 kg/m², risk was already higher in the 22.5–24.9 kg/m² range, which many people would still consider comfortably normal. That pattern fits a graded association rather than a neat biological cliff at BMI 25 or 30.

The publication reports hazard ratios and 95% confidence intervals in its full results, but the available evidence summary does not provide those values. The study's size makes chance an unlikely explanation for every association; it does not supply an absolute risk for an individual teenager, nor does it prove that changing BMI or CRF would change later infection risk.

Why the result is plausible, but still unexplained

Excess adiposity can coincide with chronic low-grade inflammation, altered adipokine signaling, and impaired innate immune-cell function. Lower aerobic fitness may travel with metabolic dysfunction and less favorable inflammatory biology. Those are reasonable hypotheses, not demonstrated pathways in this cohort. The Swedish study did not measure neutrophil function, cytokines, adipokines, or adult immune competence.

It also cannot distinguish two very different explanations. Adolescent BMI and CRF might have long-lasting biological effects, or they might mainly predict adult body composition and health behaviors. Body size and activity patterns often persist across life, and the study did not repeatedly measure participants in adulthood. Both explanations fit the observed association.

Fitness and BMI already matter for other outcomes

The infection finding sits beside a much larger literature on mortality, though that literature cannot confirm the infection result. A 2025 systematic review and meta-analysis by Singh et al. pooled 42 studies across 35 cohorts and 3.8 million observations. In adults, each 1-MET higher CRF corresponded to 13–15% lower all-cause and cardiovascular mortality. Those outcomes differ from bacterial infection, but they make it less surprising that objectively measured aerobic capacity carries information beyond a bathroom scale.

A separate systematic review by Weeldreyer et al. found that CRF and BMI each carried independent predictive value for all-cause and cardiovascular mortality. This supports the Swedish investigators' decision to analyze fitness and BMI together rather than treating one as a substitute for the other.

The same theme appears in the Prospective KIHD cohort study of 2,682 adult men in eastern Finland. Low maximal oxygen uptake combined with impaired glucose tolerance was associated with higher all-cause mortality than either factor alone. Again, no infection endpoint was measured, and the participants were adults rather than conscripts. The study supports a broader pattern: poor aerobic capacity and metabolic risk can compound one another.

What could distort the Swedish association

This remains one observational, male-only cohort. Sweden's conscription system supplied unusually standardized measurements and long follow-up, but it cannot randomize teenagers to different fitness levels or body sizes.

• Adult exposures were missing. Smoking, alcohol use, healthcare access, occupational infection exposure, later military service, antibiotic availability, and comorbidities developing over the next three decades could account for part of the association.
• Selection may matter. About 595,160 people were excluded because BMI or CRF data were missing—roughly 30% of the original cohort. If missingness tracked health status, the estimates could shift in either direction.
• The findings apply directly only to men. The cohort was male and of European-Swedish ancestry. It cannot establish the same risk pattern in women or other populations.
• The endpoint is narrow. Registry codes capture hospitalized infections. They do not reveal whether fitness or BMI predicts the much larger pool of outpatient bacterial infections.
• No independent replication exists yet. A million participants do not turn one cohort into several cohorts.

There is no direct null replication in the evidence reviewed here. That absence should not be mistaken for consensus; it mainly reflects how little research has tested this exact adolescent CRF/BMI-to-adult-infection question.

An indirectly relevant 2026 comparative cohort study of GLP-1 receptor agonists and SGLT2 inhibitors examined people with advanced chronic kidney disease and type 2 diabetes. It found that infection risk differed by metabolic drug class in that clinically ill population. This does not contradict the Swedish study, but it reinforces a complication: by middle age, established metabolic disease and its treatment may shape infection risk more strongly than a BMI measurement taken at 18.

What to do with the finding

For adolescents and young adults, the practical conclusion does not require certainty about the infection pathway. Aerobic fitness already has strong support as a predictor of longer-term health, and the Swedish data add severe bacterial infection to the list of outcomes associated with low CRF and higher BMI.

Exercise trials show that fitness can change. A meta-analysis of 29 randomized trials in young and middle-aged adults found that both high-intensity interval training and moderate-intensity continuous training improved cardiorespiratory fitness and body-composition measures, with HIIT producing larger gains in some comparisons. These trials did not test whether training prevents sepsis or bacterial pneumonia decades later. They establish modifiability of the exposures, not proof of an infection-prevention benefit.

The same restraint applies to midlife. Improving fitness, eating in a way that supports body composition, and treating metabolic disease remain sensible health goals. A 2025 meta-analysis of Mediterranean-diet trials in adults with overweight or obesity found improvements in cardiovascular risk factors and reported cardiovascular disease outcomes in the revised PREDIMED trial. It does not answer whether adult changes erase infection risk associated with adolescent fitness or BMI.

The Swedish study gives young people another reason to build aerobic capacity and avoid excess adiposity. It does not give anyone a validated forecast of future sepsis, nor evidence that adolescence permanently fixes immune fate.

The strongest conclusion is also the most useful: fitness and BMI at 18 tracked with severe bacterial infection risk up to 30 years later in Swedish men, independently of one another. Replication, adult follow-up measures, and mechanistic immune data must now determine whether that signal reflects lasting biology, long-lived lifestyle patterns, or both.