It was the diet du jour of the noughties, as popularised by the Paleo diet, the broscience diet, Atkins v2.0 and the Dukan diet.

Carbs were out of fashion so we needed something to replace them with. Besides, high protein diets left people feeling full for longer and, some studies showed, they led to greater weight loss.

Protein is important but too much has a negative impact on health.Credit:Alamy

But, then a wave of research revealed that the weight loss came at a cost; high protein diets were linked with higher rates of heart disease and also to shorter lifespans.

Now, for the first time, researchers understand why a high protein diet can reduce both our healthspan and lifespan.

The new study, published in Current Biology, explored how protein levels in our diet affect our bodies at a molecular level.

When we eat and digest protein, it breaks down into amino acids, which are used for protein synthesis, that is to make new proteins, and these become the “building blocks” for virtually every process in our body including tissue growth, energy production, immune function and nutrient absorption.

The more protein we eat, the faster our bodies synthesise protein, which sounds good on paper, right? But, the researchers found, this sped-up process is not good in practice.

“When cells have more of these amino acids they make protein faster but they also make more mistakes,” said lead author Professor Christopher Proud, Nutrition and Metabolism Theme Leader at the South Australian Health and Medical Research Institute (SAHMRI). “It’s a bit like typing. If you try to type too fast, you’ll make more mistakes.”

Additionally, Proud explained, when the process is sped up the body may not be able to remove protein waste fast enough. “It always has to be kept in balance,” he said.

Researchers believe it is the build-up of “mistakes” that contributes both to ill health and a shorter life. In fact, protein “mistakes” are responsible for an array of health issues including degenerative diseases like Alzheimer’s, Parkinson's Disease and diabetes.

Proud says his colleagues plan to use this new research as a platform to explore whether low protein diets can delay the onset of Alzheimer’s. He and his team also plan to look at whether the type of protein, and whether it is from plant, dairy or meat, makes a difference to our health.

While most of the science is based on animal studies, Proud says the same mechanisms they were looking at exist in humans

Animal studies have also shown that low protein diets – about 60 grams a day is all we need to fulfil our body’s requirements, Proud says – result in healthier lives and about a 15 to 20 per cent life extension.

Proud adds that these findings are “relevant” for people in early adulthood through to their mid 60s. “Kids and people over 65 need about 50 per cent more protein,” he said, explaining that children are growing and making muscle while older people are losing it.

For those looking to lower their protein intake he recommends adding in more carbs but stresses "it has to be the right type of carb". That means high-fibre carbohydrates like those found in fruit, vegetables and unprocessed grains and seeds.

“This research fits into a much bigger picture about health and diet,” Proud said. “Far too much information about what diet is right and what diet is wrong is not based on research. High protein diets shorten lifespan and shorten healthspan; that is the amount of time you spend healthy…. according to quite a lot of research.”

How to eat 60 grams of protein

200 grams of chicken or steak contains 50 to 60 grams; 90 grams of salmon or tuna contain 22 grams; a small tub of Greek yoghurt contains 10 to 20 grams; 30 grams of pumpkin seeds contains 9 grams; and half a cup of cooked lentils, edamame or Adzuki beans contain 9 grams; a tablespoon of peanut butter contains about 8 grams and one large egg contains about 6 grams.

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