A major study involving more than 260,000 people suggests that where you carry fat may be more important for your heart than your overall weight – a finding that could be particularly relevant as we get older.
For decades, stepping on the scales and calculating your body mass index (BMI) has been one of the standard ways of assessing whether someone is carrying too much weight.
But new research suggests that BMI may not tell the whole story.
A study published in the Journal of the American College of Cardiology has found that measures of abdominal fat – particularly waist circumference and waist-to-hip ratio – can provide important information about cardiovascular risk that BMI alone can miss.
The researchers looked at more than 260,000 people who were followed for an average of 20 years and examined their risk of a range of cardiovascular problems, including heart attack, stroke, heart failure, atrial fibrillation and cardiovascular death.
Their findings suggest that people who appear to be a healthy weight according to BMI can still have an elevated cardiovascular risk if they carry more fat around their middle.
For older Australians, that could be an important reminder that health is about more than the number on the bathroom scales.
BMI is calculated using your weight and height.
It is useful as a broad population measure, but it has a major limitation: it doesn’t tell us where the fat is stored.
Two people can have exactly the same BMI but very different body shapes and body compositions.
One might carry more fat around their hips and thighs, while another carries more around their abdomen.
That distinction matters because visceral fat – the fat stored deep inside the abdomen around organs – is associated with metabolic problems including inflammation, insulin resistance and other factors that can contribute to cardiovascular disease.
The new research found that among people classified as having a normal weight by BMI, 5 per cent had a high waist circumference and 18 per cent had a high waist-to-hip ratio.
Among people classified as overweight, 39 per cent had a high waist circumference and 40 per cent had a high waist-to-hip ratio.
And these weren’t simply differences on a tape measure.
People with normal weight or overweight who also had clinically defined high waist circumference or waist-to-hip ratios had a 15 to 50 per cent greater risk for most of the cardiovascular outcomes studied.
That is why the researchers say relying solely on BMI could result in some people having their cardiovascular risk underestimated.
Getting older naturally brings changes in body composition.
We tend to lose muscle mass and gain fat as we age, and the way fat is distributed can change too.
That means an older person’s weight may not tell the same story it did when they were 30 or 40.
A person can weigh roughly the same for years while their proportion of muscle and fat changes.
This is one reason the new findings could be particularly relevant to people in their 60s, 70s and beyond.
It doesn’t mean BMI is useless.
Rather, it suggests it should be considered alongside other information when assessing someone’s health.
Your blood pressure, cholesterol, blood sugar, physical activity, diet, smoking history, family history and other medical factors all contribute to cardiovascular risk.
Your waist measurement may provide another piece of the puzzle.
It’s important to distinguish between the fat you can pinch underneath the skin and visceral fat deeper inside the abdomen.
Visceral fat surrounds internal organs and is metabolically active.
Research has linked higher levels of visceral fat with conditions such as type 2 diabetes and cardiovascular disease.
That doesn’t mean that having a larger tummy automatically means someone has heart disease.
Nor does it mean that people with a smaller waist are automatically healthy.
It simply means that where fat is stored can provide information that body weight alone cannot.
The researchers’ findings reinforce this idea.
They found that some people classified as obese by BMI but with relatively low measures of central adiposity did not have significantly different risks for most of the outcomes compared with people with normal BMI and low central adiposity.
That doesn’t mean obesity is harmless.
It highlights the limitations of using one measurement to describe something as complex as body composition and cardiovascular risk.
The study isn’t a reason to throw away your bathroom scales or start measuring your waist every morning.
Instead, it is a reminder to have a more complete conversation about your health.
If you’re having a routine check-up, you could ask your doctor whether your cardiovascular risk is being assessed using more than your weight and BMI.
A waist measurement is quick and inexpensive, and your doctor may consider it alongside your other risk factors.
It’s particularly worth discussing if you’ve noticed that your waistline has increased even though your overall weight hasn’t changed much.
And if you’re concerned about your weight or abdominal fat, don’t assume that an aggressive diet is the answer.
As we get older, maintaining muscle and staying physically active are important parts of healthy ageing.
A sustainable eating pattern, regular physical activity and appropriate strength training can all form part of a heart-healthy lifestyle.
Your doctor or an appropriately qualified health professional can help you work out what is suitable for you, particularly if you have existing health conditions or take medications.
Perhaps the most reassuring message from this research is that it moves the conversation away from simply asking, “How much do I weigh?”
A person’s health can’t be reduced to a single number.
You might have a BMI that looks perfectly healthy but carry more abdominal fat than is ideal.
Or you might have a higher BMI without that telling the complete story about your cardiovascular health.
The researchers found that adding waist circumference and waist-to-hip ratio to BMI helped identify differences in cardiovascular risk across the BMI spectrum.
Senior author Michael J. Blaha, from the Johns Hopkins Ciccarone Center for the Prevention of Cardiovascular Disease, said the findings showed that waist measurements could “reclassify” risk identified through traditional BMI categories.
Lead author Zeina A. Dardari said clinicians should consider the distribution of abdominal fat when assessing cardiovascular risk, including among people whose BMI falls within the normal or overweight ranges.
The study does have limitations. Researchers only had one measurement of waist circumference or waist-to-hip ratio for participants, so they couldn’t fully assess how changes in abdominal fat over time affected cardiovascular risk. They also didn’t have information on factors including physical activity, diet and genetic obesity risk.
So this isn’t a case of BMI being declared obsolete overnight.
But it is another reminder that your body is more complicated than the number on the scales.
For older Australians, that’s a useful thing to remember.
When it comes to protecting your heart, the goal isn’t necessarily to chase a particular number on the bathroom scales.
It’s to understand your overall risk – and give your doctor as much useful information as possible to help you stay healthy for longer.
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