The idea you can be fat but fit is 'a myth' study argues

Behind the Headlines

Monday December 21 2015

Benefits of aerobic fitness were reduced in obese individuals

Obesity can trigger a range of complications

"No, you can't be fat and fit, say the experts," the Daily Mail reports. A major study involving more than a million men seems to contradict the idea that "obese individuals can fully compensate mortality risk by being physically fit".

The idea that you can be "fat but fit" is based on the idea that a high degree of aerobic fitness – how efficiently your heart and lungs can make use of oxygen – can compensate for the complications of obesity.

The study involved over one million Swedish men, with an average age of 18 on recruitment to the armed forces, who were followed for around 29 years.

Data was collected on their level of physical fitness, health conditions, socioeconomic status and causes of death. The study found that lower levels of aerobic fitness were associated with risk of early death, but that this risk was greater for those with a high body mass index (BMI), even if they had a high level of aerobic fitness.

The main limitation of this study was that it surveyed a population of young men, meaning the findings may not apply to women or older adults. Causes of premature death due to lack of fitness and obesity, such as heart disease or cancer, are more likely to occur in people over the age of 50, so may still occur after the end of this study.

The good news is that you can be both fit and lean by following the NHS Choices Weight Loss Plan.

 

Where did the story come from?

The study was carried out by researchers from Umea University, Sweden, and was funded by the Swedish Research Council.

The study was published in the peer-reviewed International Journal of Epidemiology.

This has been reported fairly accurately by the UK media, with numerous quotes from study researchers and health professionals on the importance of tackling obesity. However, there is no mention that this study cannot prove direct cause and effect, and its results may be limited to specific population groups.

 

What kind of research was this?

This is a very large prospective cohort study, which aimed to examine the association between aerobic fitness and mortality in later life, and the modifying effect of obesity, if any.

This type of study is useful for looking at trends over long periods of time; however, it is unable to prove direct cause and effect.

 

What did the research involve?

This study included men who attended mandatory military conscription in Sweden from 1969 to 1996.

Inclusion criteria were based on:

  • body weight – 40 to 160kg
  • height – 140 to 215cm

When participants were recruited, they were tested for a period of two days for intelligence, physical capacities and anthropometric measurements (a measurement of body size). 

Fitness testing was carried out by conducting a resting electrocardiography (a measurement of heart activity) and, if normal, an initial warm-up cycling session at a resistance set according to the participant's bodyweight. 

This was then followed by a main cycling exercise, where resistance was increased by 25 watts per minute until the participant was too tired to continue. The final resistance rate was noted. 

During the follow-up period, data was collected on:

  • diagnosed health conditions (using the International Classification of Diseases)
  • socioeconomic status (15 years after conscription, gathered from the Statistics Sweden database), which was based on annual income, highest achieved education and whether an individual drew disability pension
  • all deaths (up to December 31 2012 from the National Cause of Death Registry). Deaths were assigned to one of the following categories: trauma, cancer, cardiovascular disease, suicide, abuse (including abuse of alcohol and narcotics), diabetes, poisoning with unclear intention, epilepsy, infection, and other and/or unspecified causes

Study participants were followed until the date of death, emigration or December 31 2012, whichever came first.

 

What were the basic results?

The study included 1,317,713 men with an average age of 18 at the time of recruitment.  

Participants were followed for an average of 28.8 years, during which time there were 44,301 deaths.

The most common causes of death were:

  • trauma
  • cancer
  • cardiovascular conditions
  • suicide

The study found that participants who fell into the highest fifth of aerobic fitness had a 51% lower risk of death from any cause (hazard ratio (HR), 0.49; 95% confidence interval (CI), 0.47 to 0.51) in comparison to those in the lowest fifth. Similar associations were seen for deaths associated with cancer, cardiovascular or cerebrovascular disease and suicide, with the strongest link seen for death-related substance abuse, with 80% lower risk in the highest aerobic fitness group (HR, 0.20; 95% CI, 0.15 to 0.26) when comparing the group with the lowest level of fitness.

Comparisons were also drawn adjusting for the possible confounding effects of BMI, systolic and diastolic blood pressure, socioeconomic variables 15 years after conscription, and common diagnoses at baseline. Again, those with the highest level of fitness had significantly reduced risk of death from all causes, trauma, cardiovascular or cebrebrovascular disease, suicide and substance abuse.

The researchers also looked at the risk of death according to level of fitness and body weight. This saw a linear trend for risk of death from all causes, stratified by level of aerobic fitness across all BMI categories. When comparing the upper half of aerobic fitness compared to the lower half, analyses found that higher aerobic fitness was associated with a significantly reduced risk of death from any cause in normal-weight and overweight individuals. However, the benefit was not significant for obese individuals with a BMI of 35 or more.

 

How did the researchers interpret the results?

The researchers conclude, "Low aerobic fitness in late adolescence is associated with an increased risk of early death. Furthermore, the risk of early death was higher in fit obese individuals than in unfit normal-weight individuals."

 

Conclusion

This was a large prospective cohort study from Sweden, which was carried out in young adult males who were then followed for a period of around 29 years.

The study found that lower levels of aerobic fitness were associated with risk of early death, but that this risk of death was greater for those with a high BMI, even if they had a high level of aerobic fitness.

This study has a number of strengths and limitations. The limitations are that a large number of participants had missing smoking data and therefore it was not possible to control for the confounding effect of smoking. 

The lower fitness level group with a BMI of 35 or more had a much smaller population than the other groups in the analysis. As the confidence interval is very wide, we cannot be completely certain that they are at increased risk of death.

 The study population on the whole is not generalisable, as the study included only young men, meaning that the findings cannot be applied to women or older adults. 

The study is also unable to prove direct cause and effect. In addition, there was missing follow-up data, but this is often unavoidable for such study designs.

Strengths of the study are that they have a population and a long follow-up period. The researchers have attempted to control for the effects of important potential confounders, and follow-up data was collected from reliable sources.

The question of whether you can be fit and overweight has been a popular topic in recent years and this study has provided some evidence that, even if you are physically very fit, being obese increases risk of early death.

Obesity is a condition that leads to increased risk of type 2 diabetes, heart disease, some types of cancers and stroke, so it is important to treat, if possible.

The best way to treat obesity is to eat a healthy, reduced-calorie diet and to exercise regularly.

To do this you should:

  • eat a balanced, calorie-controlled diet as recommended by your GP or weight loss management health professional (such as a dietitian)
  • join a local weight loss group
  • take up activities such as fast walking, jogging, swimming or tennis for 150-300 minutes a week
  • eat slowly and avoid situations where you know you could be tempted to overeat

Read more about effective methods to lose weight

Analysis by Bazian. Edited by NHS Choices. Follow NHS Choices on TwitterJoin the Healthy Evidence forum.

Analysis by Bazian

Edited by NHS Choices

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