In 2019, 37 leading scientists from across the globe attempted to design a diet that would balance the planet’s needs against our nutritional ones. Finding that “unhealthy and unsustainably produced food poses a global risk to people and the planet”, the Eat-Lancet Commission proposed a “planetary-health diet” involving “a greater than 50 per cent reduction in global consumption of unhealthy foods, such as red meat and sugar, and a greater than 100 per cent increase in consumption of healthy foods, such as nuts, fruits, vegetables and legumes”.

Picture half your plate piled high in fruits and vegetables, the other half primarily packed with whole grains, plant proteins (beans, lentils, pulses, nuts) and unsaturated plant oils. You are allowed a glass of milk every day, but your eggs are rationed to one and a half a week, while fish is limited to two portions a week, red meat to a weekly burger or a monthly steak.

In fact, the report concluded that “because intake of red meat is not essential and appears to be linearly related to total mortality and risks of other health outcomes… optimal intake might be 0g/day”. It argued that it was best all round – for you and for the planet – to keep protein plant-based.

Not everyone, however, agrees that plants are the perfect fix. “If you want to be a vegan, for personal or ethical reasons, then that’s quite rightly a matter of personal choice,” says James Goodwin, director of science and research impact at the Brain Health Network and author of Supercharge Your Brain. “But do look at the science.”

Cutting back on meat is a good idea for most people, says Goodwin, because too much of it can put oxidative stress on the body, leading to cell and tissue damage and poor cardiovascular health.

It is also beneficial if it helps you to diversify your diet, he points out, since around 75 per cent of the West’s food is now derived from just 12 plant and five animal species. In fact, he points out, the world’s longest living and healthiest communities eat meat only three or four times a week.

On the other hand, our dietary needs are embedded in an evolution that spans many millions of years of meat eating. Thus: “essential nutrients that are either not available in a vegan diet or are unavailable in physiologically sufficient amounts include four amino acids (lysine, threonine, methionine, tryptophan); three Omega-3 fatty acids (DHA, EPA, DPA), Vitamin D3 and Vitamin B12. All of these are found naturally in meat, animal products like eggs, cheese, milk, yoghurt or in fish.”

Children’s healthy development, Goodwin says, depends on adequate supplies of these nutrients, most easily available through eating meat. At the other end of our lives, meanwhile, our digestive systems become less efficient at extracting nutrients. So as we age, says Goodwin, meat may once again become a particularly valuable source of nutrition.

Evangeline Mantzioris, program director of nutrition and food sciences at the University of South Australia, points out that pre-menopausal women need particularly large quantities of iron in their diets (around 18 milligrams a day, compared with men’s 8mg). To get that quantity in while limiting your red meat to the 14g daily serving suggested in the planetary health diet, she suggests, you would need to consume 100g of brown lentils or 150g of red kidney beans each day.

Men, meanwhile, need more zinc (14mg a day, compared with women’s 8mg). Beef is high in zinc, but cut your meat back to the planetary diet ideal and you could get your daily requirement from “150g of red kidney beans, 30g of zinc-supplemented cereals, three slices of wholegrain bread, and a handful of mixed nuts”.

Giving up meat entirely may have unintended consequences for your carbon footprint, however, especially if you struggle to resist a cheese board. According to analysis by the US non-profit Environmental Working Group, lamb and beef are the highest emitting foods by a long way, but cheese ranks third, generating 13.5kg of carbon dioxide equivalent per kg consumed.

In fact, a study published last year by Johns Hopkins University found that those who eat two exclusively plant-based meals a day, but allow themselves meat and dairy in their third meal, might have a smaller footprint than those who give up meat entirely but replace it with dairy products. The average healthy “two-thirds vegan” diet was found to contribute the equivalent of 762.7kg of carbon dioxide emissions per person, compared with 1,265.2kg for a vegetarian diet that included dairy.

Meanwhile, cows are staging a comeback in environmental circles, too, with some now arguing that the carbon hoof-print of a grass-fed cow is significantly lighter than that of its intensively farmed cousin, since their grazing lands actually store carbon, helping to offset their impact.

Sound complicated? Perhaps it needn’t be. Prince Charles’s moderate approach may be just right. But Michael Pollan put it most pithily in his book In Defence of Food: “Eat food. Not too much. Mostly plants.”