Obesity may be one of the most important predictors of severe coronavirus illness, new studies say. It’s an alarming finding for the United States, which has one of the highest obesity rates in the world.

Though people with obesity frequently have other medical problems, the new studies point to the condition in and of itself as the most significant risk factor, after only older age, for being hospitalized with Covid-19, the illness caused by the coronavirus. Young adults with obesity appear to be at particular risk, studies show.

The research is preliminary, and not peer reviewed, but it buttresses anecdotal reports from doctors who say they have been struck by how many seriously ill younger patients of theirs with obesity are otherwise healthy.

No one knows why obesity makes Covid-19 worse, but hypotheses abound.

Some coronavirus patients with obesity may already have compromised respiratory function that preceded the infection. Abdominal obesity, more prominent in men, can cause compression of the diaphragm, lungs and chest capacity. Obesity is known to cause chronic, low-grade inflammation and an increase in circulating, pro-inflammatory cytokines, which may play a role in the worst Covid-19 outcomes.