“People who eat diets rich in animal protein carry similar cancer risk to those who smoke 20 cigarettes each day,” reports The Daily Telegraph.

We have decades of very good evidence that smoking kills and – fortunately for meat lovers – this latest unhelpful comparison with high protein diets largely appears to be a triumph of PR spin.

The warning was raised in a press release about a large study which found that for people aged 50-65, eating a lot of protein was associated with an increased risk of dying.

However, the study, which assessed the diets of Americans in a single 24-hour period (rather than long-term), found in those aged over 65 that a high protein diet was actually associated with a reduced risk of death from any cause or from cancer. These differing findings meant that overall there was no increase in risk of death, or from dying of cancer with a high protein diet.

There are several reasons to be cautious when interpreting the results of this study, including that the researchers did not take into account important factors such as physical activity in their study.

The claim in much of the media, that a high protein diet in middle-aged people is “as dangerous as smoking” is unsupported.

We need to eat protein, we do not need to smoke.

Where did the story come from?

The study was carried out by researchers from the University of Southern California (USC) and other research centres in the US and Italy. It was funded by US National Institutes of Health, National Institute on Aging, and the USC Norris Cancer Center. The study was published in the peer-reviewed journal Cell Metabolism and has been made available on an open access basis to read for free.

In general, reporting of the results of the study was reasonable. However, the prominence given to the story (which featured as a front page lead in The Daily Telegraph and The Guardian) in the UK media seems disproportionate.

The headlines suggesting a high protein diet is “as harmful as smoking” was not a specific finding of the study and could be seen as unnecessary fear-mongering. This is particularly of note given that the effects of a high protein diet were found to differ dramatically by age.

To be fair to the UK’s journalists, this comparison was raised in a press release, issued by the University of Southern California. Unfortunately this PR hype appears to have been taken at face value.

What kind of research was this?

This study looked at the relationship between the amount of protein consumed and subsequent risk of death among middle aged and older adults. It used data collected in a previous cross-sectional study and information from a national register of deaths in the US.

While the data used allowed researchers to identify what happened to people over time, this wasn’t the original purpose of the data collection. This means that some information on what happened to people may be missing, as researchers had to rely on national records rather than keeping close track of the individuals as part of the study.

What did the research involve?

The researchers had data on protein consumption for 6,381 US adults aged 50 and over (average age 65). They then identified which of these people died over the following 18 years (up to 2006) using national records. The researchers carried out analyses to see whether people who ate more protein in their diets were more likely to die in this period than those who ate less protein.

The information on protein consumption was collected as part the third National Health and Nutrition Examination Survey (NHANES). These surveys are designed to assess the health and nutritional status of people in the US. The participants are selected to be representative of the general US population. As part of the survey they reported their food and drink intake over the past 24 hours using a computerised system. The system then calculated how much of different nutrients they consumed.

Each person’s level of protein consumption was calculated as the proportion of calories consumed from protein. Protein intake was classed as:

High – 20% or more of calories from protein (1,146 people)

Moderate – 10 to 19% of calories from protein (4,798 people)

Low – less than 10% of calories from protein (437 people)

The researchers used the US National Death Index to identify any of the survey participants who died up to 2006, and the recorded cause of death. The researchers looked at whether proportion of calories consumed from protein was related to risk of death overall, or from specific causes. As well as overall deaths, they were also interested in deaths specifically from cardiovascular disease, cancer, or diabetes. The researchers also looked at whether the relationship differed in people aged 50-65 years, and older individuals, and whether it was influenced by fat, carbohydrate or animal protein intake.

The analyses took into account factors (confounders) that could influence the results, including:

age

ethnicity

education

gender

"disease status"

smoking history

participants’ dietary changes in the last year

participants’ attempted weight loss in the last year

total calorie consumption

The researchers also carried out studies to look at the effect of protein and their building blocks (amino acids) in yeast and mice.

What were the basic results?

On average, the participants consumed 1,823 calories over the day:

51% from carbohydrates

33% from fat

16% from protein (11% from animal protein).

Over 18 years, 40% of participants died; 19% died from cardiovascular diseases, 10% died from cancer, and about 1% died from diabetes.

Overall, there was no association between protein intake and risk of death from any cause, or death from cardiovascular disease or cancer. However, moderate or high protein consumption was associated with an increased risk of death related to complications associated with diabetes. The authors noted that the number of people dying from diabetes-related causes was low, so larger studies were needed to confirm this finding.

The researchers found that results for death from any cause and from cancer seemed to vary with age. Among those aged 50-65, those who ate a high protein diet were 74% more likely to die during follow up than those who ate a low protein diet (hazard ratio (HR) 1.74, 95% confidence interval (CI) 1.02 to 2.97). People in this age group who ate a high protein diet were more than four times as likely to die from cancer during follow up than those who ate a low protein diet (HR 4.33, 95% CI 1.96 to 9.56).

The results were similar once the researchers took into account the proportion of calories consumed from fat and carbohydrates. Further analyses suggested that animal protein was responsible for a considerable part of this relationship, particularly for death from any cause.

However, the opposite effect of high protein intake was seen among those aged over 65. In this age group high protein intake was associated with:

a 28% reduction in the risk of death during follow up (HR 0.72, 95% CI 0.55 to 0.94)

a 60% reduction in the risk of death from cancer during follow up (HR 0.40, 95% CI 0.23 to 0.71)

How did the researchers interpret the results?

The researchers concluded that low protein intake during middle age followed by moderate to high protein consumption in older adults may optimise health and longevity.

Conclusion

This study has found a link between high protein intake and increased risk of death among people aged 50-65, but not older adults. There are some important points to bear in mind when thinking about these results:

The human data used was not specifically collected for the purpose of the current study. This meant that the researchers had to rely on the completeness of, for example, national data on deaths and causes of death. This may mean that deaths of some participants may have been missed.

Information on food intake was only collected for one 24-hour period, and this may not be representative of what people ate over time. Most people (93%) reported that it was typical of their diet at the time, but this may have changed over the 18 years of follow up.

The researchers took into account some factors that could affect results, but not others, such as physical activity.

Although the study was reasonably large, numbers in some comparisons were relatively low, for example, there were not many diabetes-related deaths and only 437 people overall ate a low protein diet. The broad confidence intervals for some of the results reflect this.

Many news sources have suggested that a high protein diet is “as bad for you” as smoking. This is not a comparison that is made in the research paper, therefore its basis is unclear. While we do need some protein in our diets, we don’t need to smoke, so this is not a helpful comparison.

While the authors suggested that people eat a low protein diet in middle age and switch to a high protein diet once they get older, it is not possible to say from the study whether this is what the older participants actually did, as their diets were only assessed once.

Ideally the findings need to be confirmed in other studies set up to specifically address the effects of higher protein diets, particularly the strikingly different results for different age groups.

While certain diet plans, such as the Atkins diet or the “caveman diet” have promoted the idea of eating a high-protein diet for weight loss, relying on a single type of energy source in your diet is probably not a good idea. Consumption of some high-protein foods such as red meat and processed meat is already known to be associated with increased risk of bowel cancer.

Analysis by Bazian

Edited by NHS Website