“Muscular young men may live longer” reports BBC News, while conversely, stating those with weaker muscles ‘were at increased risk of early death'.
This headline comes from a long-term, Swedish study of over a million male adolescents (aged 16-19 years). It found an association between decreased muscular strength in adolescence and increased risk of premature mortality from any cause. The same link was also found for deaths from cardiovascular disease and suicide, but not for cancer.
While the link between low levels of physical fitness and increased risk of cancer and cardiovascular disease may seem self-evident, the increased risk of suicide is not.
The researchers offer a number of unsubstantiated theories about the association between muscle strength and suicide risk. Teenagers with low muscle strength may have poor self-esteem, which could impact on their mental health. This was just a theory and was not verified in the study.
The researchers point out that this could be a case of ‘reverse causation’ (rather than ‘cause and effect’ this may be ‘effect and cause’). In this case, the explanation could be that adolescents with mental health problems which make them more likely to commit suicide in later life, may be less likely to be physical active and so have weaker muscles.
A significant limitation of the study was that it did not account for physical activity levels, diet, lifestyle factors (such as smoking), or measure general health and wellbeing. Consequently, the measure of physical strength may have been an indirect indicator of general physical fitness.
Where did the story come from?
The study was carried out by researchers from academic institutions in Sweden and Spain and was funded by the Swedish Research Council and the Spanish Ministry of Science and Innovation.
The study was published in the peer-reviewed British Medical Journal.
The BBC coverage was reasonably balanced in its account of this study. It included a statement from “experts” that “the findings do not mean muscle building makes you live longer” which is a valid counter point to the headline that “muscular young men may live longer”. However, the BBC did not make clear that the researchers were unsure exactly why there was an association between muscle weakness and increased risk of suicide.
What kind of research was this?
This was a prospective cohort study looking at the extent to which muscular strength in adolescence (16-19 years) was associated with premature mortality. That is any causes of death under the age of 55.
A prospective cohort study is appropriate to answer this type of question, as it involves following a group of people over time while measuring various aspects of their health until they develop the outcome of interest, in this case until they die. Researchers then look at how the people who died early were different to those who lived longer.
What did the research involve?
The study followed 1,142,599 Swedish male adolescents, aged 16-19 years, who were eligible for military conscription, over a period of 24 years. Conscription examinations are mandatory by law for all young male Swedish citizens and predate active military service, so they are done for all boys, irrespective of whether they go on to enter military service.
At the start of the study (baseline), the adolescents were examined to test their strength in three ways:
- knee extension strength – how powerfully the knee and calf muscles can push upwards
- handgrip strength – how powerfully the hand muscles can grip an object
- elbow flexion strength – how powerfully the elbow and upper arm muscles can push upwards
Body mass index (BMI) and blood pressure were also measured, and information was gathered on parental socioeconomic position and the adolescent’s highest educational level.
They excluded men with “extreme values” (outliers) for:
- height (included range: 150 to 210cm)
- weight (included range: 40 to 150kg)
- BMI (included range: 15 to 60)
- blood pressure (included those with lower BP figure 40-100mmHg and upper BP figure 100-180mmHg)
After the baseline measurements, adolescents were not followed up until the research team was notified of their death or the study ended.
Age at death and underlying cause of death was then obtained from a death register.
At the end of the study, the researchers looked for links between the three different measures of strength and premature death from all causes. They also looked for a link between strength and cause of death specifically from cardiovascular disease, cancer and suicide.
Strength measures were divided into 10 strength categories for the analysis and all groups were compared against the weakest group for statistical differences.
The statistics were adjusted for variation in well-known factors associated with premature mortality such as BMI and high blood pressure.
What were the basic results?
During an average (median) follow-up period of 24 years (range 1.0 to 37.3 years), 26,145 participants died (2.3%). Information on cause of death was available for 22,883 participants (87.5%).
Among the 22,883 deaths with information available:
- 1,254 (5.5%) deaths were caused by coronary heart disease
- 526 (2.3%) by stroke
- 3,425 (14.9%) by any type of cancer
- 5,100 (22.3%) by suicide
- 5,921 (25.9%) by non-intentional accidents
- the remaining deaths, 6,657 (29.1%), were categorised as “other causes of mortality”
In summary, greater muscular strength in adolescents was found to be significantly associated with lower risk of premature death from any cause. Also, there was a lower risk of death from cardiovascular disease and death from suicide, independent of the effect of body mass and blood pressure.
However, no association was found linking muscle strength to risk of premature death from cancer.
The associations were stronger for handgrip and knee measures of strength, compared to elbow flexion.
Adolescents in the lowest tenth of muscular strength showed the highest risk of mortality for different causes. All cause mortality (death from any cause) rates (per 100,000 person years) ranged between 122.3 and 86.9 for the weakest and strongest adolescents, respectively.
Corresponding figures were 9.5 and 5.6 for mortality due to cardiovascular diseases and 24.6 and 16.9 for mortality due to suicide.
In the analysis that adjusted for BMI and blood pressure, higher levels of muscular strength (as assessed by knee extension and handgrip) were significantly associated with lower risk of all cause mortality. This means that for all BMI categories (underweight, normal weight, overweight and obese) and blood pressure groups, higher levels of strength were associated with lower risk of dying before 55 years of age from any causes, compared to the lowest strength group.
High muscular strength in adolescence, as assessed by knee extension and handgrip tests, was associated with a 20-35% lower risk of premature mortality due to any cause or cardiovascular disease, independently of BMI or blood pressure.
No such association was observed with mortality due to cancer.
Finally, stronger adolescents had a 20-30% lower risk of death from suicide and were 15-65% less likely to have any psychiatric diagnosis (such as schizophrenia and mood disorders).
How did the researchers interpret the results?
The authors concluded that “low muscular strength in adolescents is an emerging risk factor for major causes of death in young adulthood, such as suicide and cardiovascular diseases. The effect size observed for all cause mortality was equivalent to that for well established risk factors such as elevated body mass index or blood pressure.”
This large cohort study of over a million male Swedish adolescents found associations between greater muscular strength in adolescence (16-19 years) and reduced risk of premature mortality from any cause (death before 55 years of age) as well as specifically from cardiovascular disease and suicide. There was no association found for deaths from cancer.
The study has many strengths, including a very large study size and relatively low drop-out rates over the follow up period. However, it also has limitations to bear in mind.
The main limitation is that the analysis did not adjust for general physical fitness including physical activity levels, diet, lifestyle factors (such as smoking or alcohol), or other medical illnesses (including mental illness).
All these are known to heavily influence risk of chronic disease and subsequently risk of premature death, so there is a risk that the measure of strength was simply an indication of general health and fitness.
If this is the case, then the study has told us that those people who are more unfit tend to die prematurely more often than their fitter counterparts. Most people would consider this common knowledge and not to be particularly surprising.
Leaving aside the issue of reverse causation, there is a wide body of evidence suggesting that increased physical fitness and regular exercise can boost mood. So a similar link between physical strength and reduced risk of suicide may explain the findings of this study.
Also, the study only included men, so the effects on women were not directly assessed and may be different. The authors highlight other literature which suggests that any mortality and strength link may be stronger in men than women.
Finally, strength was only measured once, at the start of the study, hence, it takes no account of strength changes from early adulthood onwards that may also influence death rates and risk of illness.
Future research would need to take proper account of fitness or physical activity levels to assess whether strength is an important indicator of risk of premature mortality. This study did not do this and so only limited conclusions can be drawn.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
BBC News, 21 November 2012
Links to the science
BMJ. Published online November 20 2012