Newspaper headlines are warning eating “a ham sandwich a day” increases a the risk of type 2 diabetes – but experts say it is not that simple.

A study of nearly two million people from 20 different countries found a link with both red and processed meats, such as steak, bacon and sausages.

But doctors say while the research has been done well, the findings are nuanced and should not cause fear or panic.

It is sensible to limit intake, in line with healthy-eating guidelines, they say, but the study should not wipe meat off the menu.

The research, in The Lancet Diabetes and Endocrinology journal, has an inevitable limitation – it cannot prove meat causes diabetes, because it is impossible to discount the other possible risk factors, such as other foods people in the study ate and the lifestyle they led.

Dr Duane Mellor, of the British Dietetic Association, said: “The authors did try to control for other risk factors associated with increased risk of developing type 2 diabetes, including having a higher body weight, smoking, alcohol, low vegetable intake.”

But missing data meant they could not account for the effect of family history, insulin resistance or waist circumference, which are more strongly associated with diabetes.

“So it is possible that the increased risk associated with processed and red meat intake could be a result of these other confounding factors,” Dr Mellor said.

Studies such as this also rely on participants telling the truth and accurately recalling what they eat.

Does that mean the findings should be entirely dismissed?

No. It adds to a growing picture of what foods – and how much – might be good or bad for us.

In the study, two thick slices of ham a day (50g; 1.7oz) or a small (100g) steak appeared to be a risk.

The findings around chicken and other white meat were less clear.

The NHS already advises people eating more than 90g (cooked weight) of red or processed meat a day to cut down to 70g, because experts believe eating too much processed meat can cause bowel cancer.

The International Agency for Research on Cancer puts it in the same category of risk as tobacco smoking and asbestos.

Processed meat has been modified, for taste or to extend shelf-life.

The chemicals and methods used, such as smoking, curing, or adding salt or preservatives, could be a factor.

And while meat is a good source of protein, vitamins and minerals, it sometimes also contains a lot of saturated fat, which has been linked to high blood-cholesterol levels, a risk factor for coronary heart disease.

Lead researcher Prof Nita Forouhi, from the University of Cambridge, said: “Our research provides the most comprehensive evidence to date of an association between eating processed meat and unprocessed red meat and a higher future risk of type 2 diabetes.

“It supports recommendations to limit the consumption of processed meat and unprocessed red meat to reduce type 2 diabetes cases in the population.”

Type 2 diabetes is high blood-sugar levels due to the body making too little of a hormone called insulin – or the insulin it makes not working properly.

Risk factors include:

• obesity or being overweight
• high blood pressure
• ethnicity
• family history

Dr Mellor warned people considering cutting down on meat to ensure they still got enough vital nutrients, such as iron and vitamin B12.

“It is important when considering reducing or taking a type of food out of the diet, that any replacement foods provide the same nutrients to maintain a healthy diet overall,” he said

Good sources of vitamin B12 include:

• milk
• cheese
• eggs
• fortified yeast extracts, such as Marmite
• fortified breakfast cereals
• fortified soya products