Importantly, it showed that it was not just the saturated fat in meat and dairy that could cause health problems. It was the protein itself: meaning that lean meat and skimmed milk wouldn’t cut it.
He has a particular fixation with how eating a lot of animal protein can increase our risk of cancer. Over years in the laboratory, he observed that the molecules in high levels of animal protein can help cancerous cells to form in the first place, and once they’ve been created, to multiply more quickly.
In The Future of Nutrition, he points to his own research that finds cancer development begins to markedly increase when more than a tenth of your calorie intake comes from animal protein. He argues that below this threshold, animal protein is used for essential maintenance work in the body, and anything beyond that is surplus. However, he says, the link between high protein intake and cancer doesn’t exist for vegan foods such as soy or wheat.
In practice, Campbell advocates that we forget the 10 per cent threshold and cut out animal products entirely. We should go “cold turkey on cold turkey”, he says, because removing something from our diet entirely is psychologically easier than reducing how much we eat.
Beyond that, he points to studies linking the consumption of meat and dairy to cancer, which show that countries whose populations eat the least animal products also have the lowest rates of several types of the disease. “That’s suggestive, pretty powerfully, that we should get down to zero animal protein consumption,” he says.
He is backed up by other research, too: a 2014 study found that midlife adults on high-protein diets had a fourfold increase in their risk of dying from cancer. However, the risk was mitigated if the protein came from plant sources instead of meat, eggs or dairy.
And it’s not just cancer – Campbell links high animal protein consumption to several other conditions, ranging from diabetes to severe menopause symptoms. A study of 50,000 men released this month found that swapping red meat for beans or lentils could reduce risk of heart disease by up to 20 per cent.
These problems are not necessarily just caused from eating too much protein, adds Campbell. There is also an indirect problem: if you’re having beef for dinner, you will be missing out on all the vitamins and minerals that you could have got from, say, lentils.
Some may counter that plant proteins are “low-quality” compared with meat, meaning they don’t include every amino acid – the building blocks of protein – we require. Campbell has a problem with this argument, that meat protein is better for us because it is the most similar to the protein found in our own bodies. “If we were to take this measure to its logical conclusion, we would have to conclude that the ‘highest-quality’ protein is derived from human flesh – try serving that for Thanksgiving,” he writes.
But what about people who report significant…