We already know that patients with obesity-related conditions such as cardiovascular disease are at higher risk of diabetes, which can mean a higher risk of developing complications if you catch COVID-19.

But is obesity itself a risk factor for developing complications and, if so, why?

Writing about obesity is always difficult. People are overweight for different reasons, and often these are far-reaching and complex in nature. It is also unfair to group all overweight people into the same category, but for the purposes of this article, I will be looking at obesity as an individual risk factor for complications of COVID-19 from a strictly medical point of view.

How big of a risk factor is obesity?

Obesity impacts millions of people around the world. According to the World Health Organization (WHO), in 2016, more than 1.9 billion adults were overweight, of whom 650 million were classed as obese.

As time has gone on, it has become increasingly evident that obesity is linked to poorer outcomes for those who are infected with the coronavirus.

A study in New York looking at more than 4,000 people concluded that obesity was the single biggest factor, after age, in whether those with COVID-19 had to be admitted to a hospital.

This was particularly true for young people, who by and large have been told they are likely to develop only mild symptoms should they contract the virus.

However, young (under 60) patients with a Body Mass Index (BMI) - a calculation taking into account height and weight - of between 30 and 34 were almost twice as likely to be admitted to ICU compared to patients with a BMI of less than 30. This likelihood increased to 3.6 times in those patients with a BMI of 35 or greater. ("Normal" BMI is 24.9 or below.)

It is not just in the US that studies are showing obesity as a risk factor for developing complications of COVID-19. A study in China looking at 383 patients showed that people who were overweight or obese were more than twice as likely to develop complications, such as severe pneumonia, with the coronavirus than those who were of a healthy weight. This was particularly true for overweight men.

Similar findings have been shown to be true in French and British studies, with an NHS audit showing two thirds of patients who fall seriously ill from coronavirus are obese and nearly 40 percent are under the age of 60. It is a sobering statistic for those who thought that only the elderly are dying from COVID-19.

Why is obesity a big risk factor?

There are multiple reasons why obesity is such a big factor.

Overweight or obese patients often have underlying health conditions related to their weight, such as Type 2 diabetes or cardiovascular disease.

Their immune systems may be impaired - being overweight may mean someone is in a chronic state of inflammation. As a result, their immune system is always "switched on" and may not have enough in reserve to fight a serious illness effectively. You can imagine it as being a bit like leaving a car engine running while stationary and burning the fuel needed for an upcoming long distance drive.

Excess body weight around the abdomen can also prevent the lungs expanding fully and filling with air, so when they need to work harder, for example with a pneumonia, they are restricted.

A patient who is obese is also physically more difficult to intubate - a procedure where an airway tube is fed down the throat to allow air to pass into the lungs - due to the fact that their neck tends to be thicker, putting pressure on the airways, especially when the patient is flat on his or her back. Patients in ICU with COVID-19 do better when they are lying on their stomachs as more air can get into the lungs. This is harder to do for obese patients.

People with obesity are also harder to transport. Specialist beds and equipment are often needed for larger patients, equipment that is not widely available.

It is fair to say that most healthcare systems are not well set up to manage patients with obesity, and this pandemic may well highlight that fact and expose its limitations further.

Obesity and general health concerns

Obesity is defined as a disruption of energy balance that leads to weight gain and metabolic disturbances that cause tissue stress and dysfunction. This means that obesity is not just being overweight but also the adverse effect that being overweight has on your body's health.

One of these adverse effects is a disruption in a function of the immune system called the lymphoid and neutrophil line. These are cells that can tell infection from normal tissue and that need to be kept in careful balance in order to maintain a healthy immune system.

Obesity can disrupt this balance and lead to a low level of chronic inflammation of normal tissue, caused by these cells. This places the body under a constant low level of physical stress on a microscopic scale.

This imbalance in the immune system and chronic inflammation of normal tissue has put people with obesity at increased risk of complications of infections and higher rates of chronic diseases generally.

During the influenza A (H1N1) - or swine flu - viral pandemic of 2009, studies showed that obesity was an independent risk factor for increased morbidity and mortality following infection.

Those studies found that people with a BMI of 30 or above fared much worse after being infected by the flu virus than those with a "normal" BMI of 24.9 or below, with more cases of pneumonia reported in this cohort.

It comes as no surprise, then, that if people who are obese are struck down with the COVID-19 virus, they will also be at increased risk of its complications. As hard as it might be for some people to hear, obesity looks to be the second biggest risk factor after age when it comes to developing serious complications of COVID-19.