Eating too much protein could be as dangerous as smoking for middle-aged people, a scientific study has found.
Research which tracked thousands of adults for nearly 20 years found that people who eat a diet rich in animal protein are four times more likely to die of cancer than someone with a low protein diet.
The risk is nearly as high as the danger of developing cancer by smoking 20 cigarettes each day.
Previous studies have shown a link between cancer and red meat, but it is the first time research has measured the risk of death caused by regularly eating too much protein.
Nutritional advice has traditionally focused on cutting down on fat, sugar and salt. The World Health Organisation will announce a consultation today suggesting that guidelines on sugar consumption should be lowered, but there have few warnings about excess protein.
The US study found that people with a high protein diet were 74 per cent more likely to die of any cause within the study period than their low-protein counterparts. They were also several times more likely to die of diabetes. But this trend appeared to reverse for those aged over 65, researchers found.
High-protein food plans, such as the Atkins Diet, have become popular in recent years because of their dramatic weight-loss results.
The new research from the University of Southern California suggests that such dieters may harm themselves in the long run.
“We provide convincing evidence that a high-protein diet – particularly if the proteins are derived from animals – is nearly as bad as smoking for your health,” said Dr Valter Longo, of the university.
The researchers define a “high-protein” diet as deriving at least 20 per cent of daily calories from protein. They recommend consuming about 0.8g (0.03oz) of protein per kilogram of body weight every day in middle age. It means a person weighing nine stone should eat about 45-50g (1.6-1.7oz) of protein a day. A 300g (10.5oz) steak contains 77g (2.7oz) of protein.
As well as red meat, dairy products high in protein are also dangerous, the researchers said. A 200ml (7fl oz) glass of milk represents 12 per cent of the recommended daily allowance, while a 40g (1.4oz) slice of cheese contains 20 per cent.
Chicken, fish, pulses, vegetables, nuts and grain are healthier sources of protein. However, a chicken breast or salmon fillet still accounts for about 40 per cent of recommended daily protein intake.
“The research shows that a low-protein diet in middle age is useful for preventing cancer and overall mortality,” said Dr Eileen Crimmins, a co-author of the study.
“However, we also propose that at older ages, it may be important to avoid a low-protein diet to allow the maintenance of healthy weight and protection from frailty.”
British experts agreed that cutting down on red meat had been proven to lower the risk of cancer but said a balanced diet was still the best option.
Dr Gunter Kuhnle, a food nutrition scientist at the University of Reading, said: “While this study raises some interesting perspectives on links between protein intake and mortality… It is wrong, and potentially even dangerous, to compare the effects of smoking with the effect of meat and cheese.”
He claimed that sending out such statements “can damage the effectiveness of important public health messages”, adding: “The smoker thinks: 'why bother quitting smoking if my cheese and ham sandwich is just as bad for me?’ ”
Prof Naveed Saattar, an expert in metabolic medicine at Glasgow University, said the low-protein effect in older people could be due to “survival bias”, where those who have lived longer are already generally healthier.
Protein controls the growth hormone IGF-I, which helps bodies grow but has been linked to cancer susceptibility.
Prof Tim Key, a Cancer Research UK epidemiologist, said: “Further research is needed to establish whether there is any link between eating a high protein diet and an increased risk of middle aged people dying from cancer.”
The study was published in the journal Cell: Metabolism allow the maintenance of healthy weight and protection from frailty.”
British experts agreed that cutting down on red meat had been proven to lower the risk of cancer but said a balanced diet was still the best option.
