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 can obesity itself be 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.

More Doctor's Notes:

Last week, Diederik Gommers, chairman of the Dutch intensive care association, told TV reporters in the Netherlands that 66 percent to 80 percent of COVID-19 patients in intensive care wards that he had seen were overweight.

On the same TV news programme, Peter van der Voort, of Groningen University's teaching hospital, said of the higher numbers of overweight COVID-19 patients in ICU beds: "We don't know why, but it is very noticeable."

Is obesity a risk factor for disease?

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.

A greater understanding of obesity's associated physiological disturbances is needed to address this key public health concern.

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 Body Mass Index (BMI) - a calculation taking into account height and weight - 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.

But is the same true for coronavirus?

A Chinese study published last month looked at 112 COVID-19 patients and their outcomes. Sadly, 17 of these patients died. Fifteen of the deceased (88 percent) were classed as being overweight or more on the BMI scale. By comparison, only 18 of the 95 survivors (19 percent) were classed as overweight or above on the BMI scale.

This study prompted experts to question what it was about obesity that might make it a risk factor when it comes to coronavirus.

What problems can obesity cause?

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.

This is true of common infections such as skin or wound infections. Their immune systems are already working hard, even at rest. 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, including breathing difficulties and pneumonia.

There are significant challenges when it comes to managing patients with obesity in ICUs. They are 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.

People with obesity are 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.