Meat, under the right circumstances, is good for you. It is one of the best sources of protein, high in nutrients such as iodine, iron and zinc, and packed with B vitamins that can be difficult to get from a vegan diet. Even saturated fat has had its reputation rehabbed over recent years, with a meta-review of studies finding no association between the animal byproduct and heart disease or stroke. Enjoy grass-fed, free-range meat occasionally, and you are probably doing yourself more good than harm.

But what about processed meat such as succulent sausages, flavourful frankfurters and moreish mortadella? It is the stuff that spices up any carnivorous diet, but meat that has been cured, canned or otherwise smashed into a more palatable shape gets a much rougher ride from the medical establishment than slabs of unvarnished steak. But is it all created equal? And how much can you get away with?

Let’s start with the bad news. “The World Health Organization defines processed meat as carcinogenic to humans,” says Dr Keren Papier, a nutritional epidemiologist leading research into colorectal cancer at Oxford university. “Eating an extra 50 grams of processed meat a day – about two slices of ham – could increase the risk of colorectal cancer by 18%.” For reference, old age is the main factor in most forms of bowel cancer, with 7% of UK men and 6% of women likely to receive a diagnosis over the course of a lifetime – this would increase the risk to 8% for men and 7% for women.

Processed meats have also been linked with an increased risk of developing dementia, with one large 2021 study concluding that every additional 25 grams of processed meat in a person’s daily diet increases the chances of Alzheimer’s disease by 52%.

So what is causing that increased risk? “Several potential mechanisms have been proposed to explain the link between processed meat and cancer,” says Papier. “One of these is its high haem iron content, which may promote the formation of potentially carcinogenic N-nitroso compounds.” For dementia, less research has been done, but one possible culprit is inflammation: processed meat might contribute to it by altering the gut microbiome, leading to a range of unwanted effects.

But are some sorts of meat worse than others? Here, part of the problem is the way studies are conducted. For obvious reasons, most are observational, asking subjects to self-report how much meat they eat in a typical day or week, without making fine distinctions between hotdogs and bespoke Italian sausages. People who tend to eat one type of processed meat typically also eat the others, so it is difficult to find a cohort who eat pepperoni pizzas but not chicken nuggets. Are there any rules dictating what kinds are worst?

“There is some evidence that the way the meat is processed may have health implications,” says Papier. “For instance, curing meat with sodium nitrites, which gives some processed meats their recognisable colour, can increase the formation of N-nitroso compounds that may have carcinogenic potential.”

To unpack this a bit, nitrates and nitrites are two different compounds that occur in a lot of foods. Nitrates are typically found in whole plant foods such as beetroot and leafy greens – but are broken down into nitrites by bacteria in the mouth. What may make a difference is that nitrate-rich foods are also full of protective polyphenols and fibre; nitrites that occur in processed meat don’t have these, and one recent large-scale US study pointed to this as a key factor in their effects on health.

“Another potential mechanism is the method used to process or cook the meat,” says Papier. “For instance, when meat is smoked or cooked at very high temperatures, chemicals can form in the meat which may increase the risk of cancer.” That’s bad news for barbecue-lovers, especially if you tend to like your dinner on the charred side.

In the absence of further research about exactly which processed meats are the worst, the best advice is to lower your intake overall: the NHS recommends that you keep to under 70 grams a day. The odd bit of pepperoni on a pizza, or a slice of salami won’t cause your risk to skyrocket, so keep your diet mostly plant-based, with meat as an occasional treat. And try to keep it free-range where you can because it is probably better for you, as well as the animals.