Forget BMI? A new study has revealed the one thing you need for a longer, healthier life

We're often told that maintaining a healthy weight is key to a longer, healthier life. There is merit to this, of course, but it's not the whole picture, according to a new study.

If you like to get your 10,000 steps in and enjoy hiking, running, swimming, or cycling every day to work, you're in luck. Keeping fit is more important for a long and healthy life than being slim, a study published in the British Journal of Sports Medicine has found.

The study was the largest and most comprehensive review of earlier research on the link between aerobic fitness, body mass index, and longevity. It's also one of the more inclusive - involving almost 400,000 people in middle age or older from various countries, of which 33% were women.

From these studies, researchers divided participants into two categories: unfit (with stress test results in the bottom 20%) and fit (with results in the top 80%). They also gathered data on who had died during follow-up periods across 20 years, and compared BMI, fitness, and death.

They found that being obese was strongly linked to an early death - with unfit and obese men and women three times more likely to have died prematurely than those who had a normal BMI and were fit. However, those in the 'normal' weight category who were in the bottom 20% for fitness were twice as likely to have died prematurely as those who were obese but had a higher level of fitness.

Researchers concluded that aerobic fitness was more important than body mass index (BMI) when looking at long-term health, finding that if someone is obese but aerobically fit, they are about half as likely to die prematurely as someone at a 'healthy' weight but with low aerobic fitness.

Aerobic fitness relates to exercise that uses the body's larger muscle groups in rhythmic and repetitive movements. Typically, we think of sports like running, walking, cycling, and swimming. These raise the heart rate and increase how much oxygen you use.

"It's much more important, all things considered, to focus on the fitness aspect [of health for longevity] rather than the fatness aspect," Siddhartha Angadi, the study's senior author and an exercise physiologist at the University of Virginia, told The Washington Post.

The review, published recently, adds further evidence to the idea we can live long and healthy lives at almost any weight - so long as we stay active and fit throughout our lives.

Is BMI outdated?

Body mass index (BMI) is not an outdated way of measuring health and predicting longevity, as the study shows that it's still a serious indicator of health issues - but it does have limitations. Not only was the calculation created based on the scores of European white men, but it can be inaccurate as it takes total weight into account. Muscle mass weighs more than fat mass, so someone who has lots of muscle (i.e. a weightlifter or rugby player) may be classed as obese, possibly incorrectly.

"BMI is still important to provide some insight into potential health risks of individuals but it shouldn't be used on its own when trying to assess the detailed health picture of a person," says GP Dr Alexis Missick, who works with UK Meds. "The individual and their lifestyle and diet need to be included when considering the detailed picture."

How to boost your aerobic fitness

Small amounts of exercise can improve your health but the heart needs to be worked like any other muscle, says Dr Missick. "In the same way that people exercise to strengthen muscle in the body, the heart gets stronger and therefore more effective at its job, particularly as we get older."

This means you have to push yourself and continually make exercise harder once you're able to do the activity without trying. Here's how to do it:

• Change the incline: Walking uphill will be more of a challenge than walking on flat pavements.
• Walking vs running: Keen walkers and hikers might like to try running or power walking to boost their cardiovascular fitness. It will boost the heart rate more than steady-state strolling.
• Increase the intensity: Try brisk walking, sprinting in a treadmill workout, cycling faster over a 30-minute session, or working for a faster lap time in the pool.
• Go for longer: If you can go running for 20 minutes a day, why not try going running for 30 minutes a day instead?
• Cover more distance: Walking or running over a further will challenge your fitness in much the same way as exercising for a longer amount of time will. It just gives you something different to aim for.

When it comes to heart health, exercise also burns up excess calories from sugar and fats. "This is great because it's protective to the heart, reducing a build-up of cholesterol [plaque formation] and sugar [diabetes development], which poses risks to the heart," says the doctor. However, she also warns that diet and exercise go hand in hand, so it's important to also have a balanced diet as well as do regular exercise.

More important than the activity you choose though is your commitment to doing it. So, don't set yourself the task of training for a marathon or buying an indoor bike if you don't enjoy running or cycling. Instead, find cardio exercises for beginners that you do enjoy - that way, you're more likely to stick to a routine.