Obesity is a significant factor in the likelihood that someone will be severely affected by COVID-19, a major UK study has found.

A study of nearly seven million people found strong links between someone’s body mass index (BMI) and whether they ended up in hospital or died if they caught coronavirus in late winter or spring last year.

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The impact of a high BMI was worse if someone was in a younger age group or from an ethnically diverse community, according to the paper by the Universities of Oxford and Cambridge.

The study found that half as many people with a ‘healthy’ BMI of between 18.5 and 25 were admitted to intensive care, compared with the average, whereas nearly three times as many people with a BMI above 35 were admitted.

Among those who died, there were about a sixth fewer than average at the healthy range and nearly a third more among those with a BMI of 35 or more.

BMI is calculated by dividing an adult’s weight in kilograms by their height in metres squared.

Experts say that this adds significantly to evidence that a person’s weight is a major factor in how likely they are to be hit hard by COVID.

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Prof Naveed Sattar, professor of metabolic medicine at the University of Glasgow, said: “These data add to the overwhelming evidence from other sources (several hundred studies now in many parts of the world) that support excess body weight are an important, causal risk factor for more severe COVID-19 outcomes.

“The new findings support that such risks increase in a linear fashion from normal BMI upwards, and that excess weight may be a stronger risk factor for severe COVID-19 in younger people as we reported before.”

Each excess BMI unit above 23 kg per square metre (sqm) was associated with increased risk of hospital admission, ICU admission and death, the study found.

Models created from the study’s data also showed a moderately increased risk of hospital admission and death among people with a BMI of less than 23 kg/sqm, although the risk of ICU admission was directly related to BMI, and did not rise above 23kg/sqm.

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The prime minister, who has admitted he was “too fat” when he was taken to hospital as he struggled to overcome COVID last April, is among those who has stressed the need for the nation to get fit and lose weight as a way of protecting people from the impact of infection.

As he announced a national crackdown on obesity last July, Boris Johnson claimed he had been “way overweight” and planned to ban junk food adverts on TV and online before the watershed and stop shops from displaying sweets and chocolates at supermarket checkouts.

Weight is recognised as a growing problem in the UK, with the Health Survey for 2019 estimating that 28% of adults in England are obese, and a further 36% overweight but not obese.

Obesity rates have risen steadily since the early 1990s, from around a sixth of the population to approaching a third.

A study by the Social Market Foundation (SMF) published in December found the pandemic and Mr Johnson’s health drive had had little impact on people’s ability to lose weight, and said societal causes of obesity may need to be tackled rather than just relying on people’s “individual willpower” to reduce it.

The new study may provide further evidence for those trying to find out why the UK’s COVID death rate has been so high in comparison with other similar countries, some of which may have lower rates of obesity.

The US, which has seen similar death rates to the UK (175 per 100,000 people compared to 192 per 100,000), also has high levels of obesity. Developed countries like Japan and Taiwan, which have much lower obesity rates, have much lower COVID death rates.

But there is plenty of evidence that other factors are heavily influential in a country’s death rates, so studies about which have the most impact are ongoing.

Prof Sattar said it was still unclear as to why obesity appeared such a causal factor for severe COVID, but they may “include heavier people getting a bigger dose of the virus in the first place, to thicker blood, poorer metabolism (needed to help nourish immune cells and the brain) and an exaggerated immune response linked to excess adipose tissue”.

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