Notwithstanding the shortcomings of this research, the study, which followed nearly 2,000 Danish adults for around 12 years, found that subjects with thighs under around 60cm in circumference had a greater mortality risk, but the protective effect did not appear to increase beyond 60cm. Overall, this inverse association between thigh circumference and risk of death needs further study and exploration. It is currently unclear how doctors should use this information, or what this news means to the general public.
Where did the story come from?
Dr. Berit Heitmann and Peder Frederiksen from Copenhagen University Hospital and Glostrup University Hospital carried out this study. The research was funded by the Danish Medical Research Council, and published in the peer-reviewed British Medical Journal.
What kind of scientific study was this?
This was a prospective cohort study investigating death and cardiovascular outcomes in relation to physical measurements, physical activity and lifestyle. It followed a sample of 1,436 men and 1,380 women who were participating in the Danish MONICA project, a wider study assessing a number of health factors.
The participants were on average 50 years old at entry into the study, and were free from coronary heart disease, stroke or cancer. Their height, weight and body fat were measured, as well as thigh, hip and waist circumferences:
- Thigh circumference was measured just below the gluteal fold (the crease where the buttocks meet the thighs) of the right thigh.
- Waist circumference was measured at the mid-point between the lower margin of the ribs and the iliac crest (hip bone).
- Hip measurements were taken “at the point over the buttocks yielding the maximum circumference”.
Participants were followed up for between 10 years (for heart disease outcomes) and 12.5 years (for outcomes of death). Data was recorded on cardiovascular and coronary heart diseases and events, or death from any cause. Information on cause of death and new disease was taken by matching people’s personal identification numbers through Denmark’s National Registers of Hospital Discharge and Death Registry.
Factors that could be confounding the relationship between the anthropometric measures (body size and shape) and the outcomes were also measured. This included measures of physical activity, smoking, blood pressure, alcohol use, education and menopausal status. Participants self-categorised their activity levels as follows:
- Sedentary: sitting, reading, watching television, going to the cinema.
- Active at least four hours a week: building, sometimes walking or cycling, table tennis, bowling.
- Active in sports: running, swimming, tennis, etc. at least three hours a week, or doing heavy gardening or spare time work.
- An elite sportsperson: swimming, playing football, long distance running several times a week.
Because there were so few in the final activity group, groups three and four were merged for analysis.
Researchers then compared data on those people who survived the study period, those who died from any cause, and those who had a new diagnosis of cardiovascular or coronary heart disease. They specifically looked at anthropometric measures while taking into account potential confounders. Four different analyses were undertaken:
- Model 1 assessed the relationship between mortality and thigh circumference, smoking, education, physical activity and menopause.
- Model 2 also adjusted for percentage of body fat and height.
- Model 3 added BMI and waist circumference, and
- Model 4 also adjusted for systolic blood pressure, cholesterol and alcohol consumption.
What were the results of the study?
During the 12.5 years of follow-up, 257 men and 155 women died from any cause. In 10 years of follow-up, 263 men and 140 women had new cardiovascular disease and 103 men and 34 women were diagnosed with new coronary heart disease. Men who survived generally had lower BMI, body fat, hip and waist circumferences, age, blood pressure and cholesterol. They were also more active, smoked less and drank less than those who did not survive. At the start of the study, they also had greater fat-free mass, a greater thigh circumference and height.
Compared to those with an average thigh circumference in this study (55cm), all of those with smaller thigh circumferences (from the smallest 46.5cm circumference) were about twice as likely to die. People with a thigh circumference greater than the average were not at a greater risk of death, but there was no ‘dose effect’, i.e. their risk did not decrease as thigh circumference increased. For men, thigh circumference was also related to cardiovascular and coronary heart disease.
Thigh circumference was still significantly linked with cardiovascular disease in men, and with total deaths in both sexes using analysis model 4, which adjusted for all of the measured confounding factors (alcohol, blood pressure, total cholesterol and blood fats, BMI, waist circumference, percentage of body fat, height, smoking, physical activity and education level).
What interpretations did the researchers draw from these results?
The researchers say that they found independent associations between thigh circumference and mortality in men and women that “were particularly evident when thigh circumference was below a threshold of around 60cm”. They discuss possible reasons for this. This includes the hypotheses that insulin sensitivity may be lowered when the leg muscle is small, or that the metabolism of glucose and fat is negatively affected by a lack of subcutaneous fat.
What does the NHS Knowledge Service make of this study?
There are limitations to this research, some of which the researchers have acknowledged:
- Failure to measure the tissue composition in the thigh, e.g. proportions of fat or muscle. As the association between mortality and thigh circumference was independent of overall body fat and abdominal obesity, the researchers say that it could be due to insufficient muscle mass in the region. They did not take the measurements necessary to confirm this.
- The study may not have had enough statistical power to show any potentially significant age-related differences or to assess whether the association was stronger among smokers than among non-smokers.
- Although the researchers attempted to adjust for the effects of physical activity, it is possible that this was not fully achieved, and that some confounding may have remained. Low muscle mass is likely to be linked with less activity, which has a well-known link with increased risk of heart disease.
- Anthropometric measurements of height, weight, waist and thigh circumference, etc. were taken at the beginning of the study, but are unlikely to have stayed the same during the 12 years of follow-up.
In spite of these possible shortcomings, the researchers conclude that they have established an independent link between thigh circumference and risk of death and cardiovascular disease in some models. They say that there seems to be a ‘threshold’ effect, i.e. a circumference at which the risk seems most evident, but say that this needs further confirmation before the results can be generalised. The researchers are concerned that more than half of the men and women aged 35-65 in this study had thigh circumferences below the threshold.
People in this study with thighs under about 60cm in circumference had a higher mortality risk. However, the protective effect did not appear to increase beyond 60cm. Overall, the association between smaller thigh circumference and risk of death needs further study and exploration. It is unclear at this point how doctors should use this information or what significance it holds for the general public.