Last September, Vipul Shah spent 11 days in critical care battling Covid-19 in a hospital in Mumbai.

Mr Shah, who had no previous history of diabetes, was administered steroids which are used to treat the coronavirus infection.

Steroids reduce inflammation in the lungs for Covid-19 and appear to help stop some of the damage that can happen when the body's immune system goes into overdrive to fight off the virus.

But they also reduce immunity and push up blood sugar levels in both diabetic and non-diabetic Covid-19 patients.

Nearly a year after recovery, Mr Shah, 47, is still on medication to control his blood sugar.

"I know a lot of people like me who are taking medicines for diabetes after recovering from Covid-19," Mr Shah, a stock trader, said.

India accounts for one in six people in the world with diabetes. Home to an estimated 77 million diabetics, it is second only to China, which has 116 million people living with the disease.

Doctors reckon millions more remain undiagnosed for this chronic and progressive disease which occurs when the pancreas doesn't produce enough insulin or the body is unable to effectively use the insulin it produces.

This leads to increased concentration of glucose - a type of sugar - in the blood and poses serious health risks, including damage to the kidneys, eyes and heart.

Diabetes is among a group of underlying conditions which put people at increased risk of severe Covid-19 disease. The others are obesity, high blood pressure, and heart and lung diseases.

Now physicians fear a large number of patients who have recovered from Covid-19 are at risk of newly diagnosed, full-blown diabetes. With nearly 32 million recorded infections, India has the second-largest Covid-19 caseload in the world, behind the US.

"The worry is that Covid-19 could trigger a tsunami of diabetes in India after the pandemic is over," Dr Rahul Baxi, a Mumbai-based diabetologist, told me.

He said 8-10% of his patients with no history of diabetes who contracted Covid-19 continued to have high sugar levels months after recovery and were on medication.

"Some have borderline diabetes. Others are managing with medicines even a year after recovery," he said.

Doctors around the world are debating whether Covid-19 itself is causing diabetes in patients with no previous history.

This, they say, could be due to the use of steroids in treatment; the cytokine storm when the body's immune system goes into overdrive to fight off the coronavirus; and the virus itself injuring the cells in the pancreas which make insulin.

A recent peer-reviewed study by Indian doctors of patients who have recovered from mucormycosis or the deadly "black fungus" appears to suggest a link.

India has reported more than 45,000 cases of this fungal infection, which affects the nose, eyes and sometimes the brain, and usually strikes 12-18 days after recovery from Covid-19.

The study found that 13 of 127 patients - or 10% of the cases - had "new onset diabetes". Their average age was around 36 years. Significantly, seven of them were not even given steroids or supplemental oxygen when they were sick with Covid-19.

"Yet, these patients had elevated blood sugars. This makes us worry about an impending outbreak of diabetes in the coming years," said Dr Akshay Nair, an eye surgeon and one of the researchers.

Another study of 555 patients from two hospitals in Delhi and Chennai (Madras) found that those who had been diagnosed with diabetes after contracting Covid-19 had higher levels of blood sugar than those who had a previous history of diabetes.

Dr Anoop Misra, a diabetologist and co-author of the study, says the emerging evidence about the link between Covid-19 and diabetes presents a "complex" picture.

Patients have been newly diagnosed with diabetes - using the haemoglobin A1c level test, which gives a three-month average measuring blood sugar - during their treatment for Covid-19 in hospital.

These patients were likely to have been diabetic previously and never got tested.

Or they could have became diabetic after they were administered steroids during treatment.

After discharge from hospital, some patients have seen their blood sugar levels return to normal, while for others, like Mr Shah, they continued to remain above the range.

"Our assessment is that such patients were probably predisposed to diabetes because of obesity and family history," Dr Misra said.

A "rarer" group of patients was those who had become severely diabetic because coronavirus had damaged their pancreas. Such patients could have both Type 1 (bodies that cannot make insulin) and Type 2 (bodies that make too little insulin) diabetes.

The pancreas, including the part which makes insulin, is a target for the coronavirus, according to Prof Guy Rutter of Imperial College London.

"The virus seems to use different receptors in the pancreas than in other parts of the body. The degree to which the actions are directly on the insulin producing cells versus a cytokine storm is contested," Prof Rutter told me.

What is unclear is whether the "new-onset" diabetes in recovered Covid-19 patients is permanent.

"I suspect that the big problem from an Indian perspective is that with so many people who have diabetes, the probability of poor outcomes and death from Covid-19 is much higher than in countries with a lower burden of disease," Prof Rutter said.

Doctors believe India will inevitably face a higher burden of diabetes after the pandemic is over.

India's grinding lockdowns have kept a large number of people indoors, working from home, ordering takeaway food, and getting little exercise. Many are suffering from anxiety and depression.

"I am seeing a lot of new cases of diabetes in such people. This worries me a lot more than anything else," said Dr Misra.