'Meat intolerance': Can our bodies forget how to digest flesh?

Some people who stop eating meat have reported unpleasant effects when they return to it. Can your body forget how to digest flesh?

Eating less meat is one of the simplest ways to reduce your carbon footprint.

If everyone in the UK moved to a low-meat diet, eating less than 50g, or one Cumberland sausage’s worth of meat a day, it would save as much carbon as if eight million cars were parked for good, researchers have calculated. Data from the UK government shows that meat consumption is dropping – between 1980 and 2022, consumption of beef, pork, and lamb fell by 62% – and though the reasons cited vary, and may have more to do with rising costs than an environmental conscientiousness, more and more people are clearly experimenting with saying no to meat.

But if you go a long time without eating it, does that change your body's ability to digest it? Vegetarians and vegans sometimes post on social media, asking whether going back to meat could cause stomach pain, bloating, and other symptoms. Others weigh in with their own experiences, and a great, late-night whirlwind of curiosity and cramps is born.

However, there isn’t much research on whether consuming meat after a long break can trigger an upset stomach, says Sander Kersten, a professor of nutrition at Cornell University in the US. "A lack of evidence doesn't mean it doesn't exist, it's just that people haven't studied it," he reflects. "That's not always a satisfying situation or answer, but it's just what you have to deal with sometimes."

It is possible – though it is very rare – to be allergic to meat. Alpha-gal syndrome, in which the immune system recognises animal proteins as invaders, can lead to anaphylaxis and death. But this allergy, which can crop up after a lifetime of cheerful meat eating, is not related to switching to a low-meat diet. You can, for example, develop the condition after a tick bite.

For many who are avoiding meat, discovering that they have unknowingly consumed it can be emotionally painful – it can feel like a personal violation, says Kersten, who is a vegetarian himself. "That would make some people extremely sad," he explains. " I don't know if that would lead to physical symptoms. It certainly could trigger a lot of anger."

But when you consider the biology of digestion, it is not as plausible as you might imagine that the body could, over a long period of time, lose the ability to digest meat. Meat is generally very easily digested, unlike the fibre in fruits, vegetables, and legumes. To break that up, our bodies require help from our microbiome, whose microbes do possess the enzymes necessary for digesting it.

Moreover, the enzymes used to digest plant proteins are the same as those used on meat proteins. These enzymes recognise and sever particular chemical bonds in proteins. Whether they come from plants or animals, the proteins are made of building blocks called amino acids. Enzymes can generally break them up no matter where they came from.

This process is different from the case of, say, animal milk sugars like lactose. To digest lactose, your body requires a specific enzyme called lactase, and people who do not produce enough of the enzyme, making them lactose intolerants, can suffer a tummy upset after eating dairy products.

But with meat proteins, it does not make sense to think of the body somehow ceasing to make the enzymes necessary to digest a hamburger comfortably – they're always there, breaking down any protein that comes through, be it from pea, soybeans, or steaks, says Kersten.

The human gut microbiome does morph and change depending on what its host eats, though. Sometimes this means that the specific types of bacteria there change; sometimes it's just that the microbes make other enzymes. While there are differences between the microbiomes of omnivores and vegetarians and vegans, they do not seem to be radically divergent, so long as the omnivores consume a variety of plants, research has found.

Microbiomes can shift quickly as a result of dietary changes, however – one study in which people switched to a fully animal-based diet showed that a shift away from their baseline microbiome was visible within a day (it swiftly reverted back to normal once the diet ended). Subjects were encouraged to report any discomfort they felt, but nothing made it into the paper.

More like this:

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If anything, it's the sudden consumption of large amounts of fibre after a long hiatus that could cause digestive problems. It's better to ease into such dietary changes. "Depending on the fibre, you can have some pretty strong reactions to it," says Kersten.

In short, worrying about your body somehow losing the ability to digest meat shouldn't impair any plans you have to extend Veganuary into the spring. If you're among those who've had an upset stomach after eating meat following a long hiatus, a loss of enzymes is not likely to be the culprit, though this phenomenon remains understudied, Kersten says.

"The body is quite adaptable," he continues. "It can do more than you think."

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