Obesity still poses a risk for women who are otherwise healthy

"Women who are overweight or obese but otherwise healthy are still at an increased risk of cardiovascular disease," BBC News reports. Cardiovascular disease is a general term for conditions affecting the heart or blood vessels.

Researchers in the US tracked the health of more than 90,000 women over 30 years in a study looking at risk of having a heart attack or stroke. Women were categorised by weight – normal, overweight or obese – and whether they were "metabolically healthy".

There is no universally agreed definition of what constitutes being metabolically healthy, but most studies, including this one, use the term to describe women who have not been diagnosed with diabetes, high blood pressure or high cholesterol.

The study found that women who were a normal weight and metabolically healthy had the lowest risk of having a heart attack or stroke. In comparison with these women, those who were obese and metabolically healthy had a 39% increased risk of heart attack or stroke.

But importantly, cardiovascular risk was much higher in all women who were metabolically unhealthy, regardless of weight. Normal-weight, metabolically unhealthy women had a more-than-doubled risk, while the risk was tripled for women who were both obese and metabolically unhealthy.

The good news is that the same methods you can use to lose weight will also improve your metabolic health. These include taking regular exercise and eating a healthy, balanced diet.

Where did the story come from?

The researchers who carried out the study were from the German Institute of Nutrition in Germany and the Harvard TH Chan School of Public Health in the US.

It was funded by the US National Institutes of Health and the German Federal Ministry of Education and Research, and published in the peer-reviewed medical journal The Lancet Diabetes and Endocrinology.

The reporting from BBC News and the Mail Online, while accurate, focused mainly on the risk of heart attack or stroke for women who are metabolically healthy but overweight or obese. However, their reports did not make it clear that, in this study, metabolic ill health was more strongly associated with heart attack and stroke than weight was.

What kind of research was this?

This was a long-running cohort study. Researchers wanted to see how metabolic health and weight affected cardiovascular health over time.

Cohort studies can identify links between factors, such as weight and cardiovascular health, but cannot prove that one factor directly causes another because unmeasured factors, such as physical fitness, may be involved.

What did the research involve?

Researchers recruited more than 100,000 US women to the Nurse's Health Study in 1976. Every 2 years, women were sent questionnaires about their weight, height, lifestyle and medical history. The present study looked at the questionnaires from 1980 onwards, as these had more detailed questions.

The answers were used to track women's body mass index (BMI), metabolic health – determined by a confirmed diagnosis of high blood pressure, high cholesterol or diabetes – and whether they had developed cardiovascular disease.

None of the women had cardiovascular disease at the start of the study, and they were followed up for an average of 24 years.

After adjusting their figures to take account of potential confounding factors, the researchers looked to see how weight and metabolic health affected women's chances of getting cardiovascular disease.

Confounding factors included:

• age
• ethnic background
• education
• alcohol consumption
• smoking
• menopausal status
• hormone use
• screening examinations
• aspirin use
• family history of heart attack and diabetes
• physical activity

The researchers largely relied on women to accurately report diagnoses of heart attacks, strokes, diabetes, high blood pressure and high cholesterol. However, they did check a subgroup of women's reports against their medical records and found a high degree of accuracy.

What were the basic results?

During the 30-year study, 3,304 women had a heart attack and 3,080 had a stroke. Women who were metabolically unhealthy – having diabetes, high blood pressure or high cholesterol – were at highest risk of having a heart attack or stroke.

In comparison with normal-weight women who were metabolically healthy:

• normal-weight women who were metabolically unhealthy had more than double the risk of heart attack or stroke (hazard ratio [HR] 2.43, 95% confidence interval [CI] 2.19 to 2.68)
• overweight women who were metabolically unhealthy had more than double the risk heart attack or stroke (HR 2.61, 95% CI 2.36 to 2.89)
• obese women who were metabolically unhealthy had triple the risk heart attack or stroke (HR 3.15, 95% CI 2.83 to 3.50)

Also, women who were obese but metabolically healthy still had a 39% increased risk of heart attack and stroke in comparison with metabolically healthy women of normal weight (HR 1.39, 95% CI 1.15 to 1.68).

When looking at individual metabolic risk factors, researchers found that developing diabetes or high blood pressure during the study increased the risk of heart attack or stroke when compared with women who stayed metabolically healthy. Developing high cholesterol made little difference.

Most women who were metabolically healthy at the start of the study became metabolically unhealthy over a period of 20 years, including 68% of normal-weight women and 84% of those who were obese.

This suggests that maintaining metabolic health is a challenge for all women but especially for those who are obese.

How did the researchers interpret the results?

The researchers said the findings showed that "metabolically healthy women had a substantially lower risk of cardiovascular disease than women with pre-existing metabolic conditions across all BMI groups", but that women who were metabolically healthy but overweight or obese "were at an increased risk compared with women with metabolically healthy normal weight".

They added: "Most women with metabolic health are likely to convert to a metabolically unhealthy phenotype [develop high blood pressure, cholesterol or diabetes] over time, which is associated with an increased cardiovascular risk."

Conclusion

The media focused on the finding that women who are obese still have an increased risk of heart attack or stroke, even if they are metabolically healthy. However, two other findings are possibly more important:

• being metabolically unhealthy was most strongly linked to women's chances of having heart attack or stroke
• most women who began as metabolically healthy (at an average age 45 to 50 years) went on to develop high blood pressure, high cholesterol or diabetes over the next 20 years

For women who are overweight or obese, the main point is that, over time, avoiding developing these cardiovascular risk factors is harder than if you are a healthy weight. The study's results suggest that high blood pressure and diabetes are most strongly associated with heart attack or stroke.

The study was unusually large and had a particularly long follow-up period, which increases confidence in the results. However, there were some limitations:

• the results relied on women self-reporting their health conditions
• not all women returned every questionnaire, meaning some data were missing
• no information about physical fitness was collected, and fitness may directly influence weight, metabolic health and risk of cardiovascular disease
• the women in the study were all nurses, so they may have had different health behaviours from other groups of people, meaning the results might not apply to everyone

The take-home message of the study seems to be that women who are a normal weight, with normal blood pressure, normal cholesterol levels and no diabetes are at the lowest risk of having a heart attack or stroke.

Find out more about maintaining a healthy body.