Swapping animal protein for plant protein 'may improve health'

5 August 2016

plant protine

'Ditch sausages for a longer life,' The Telegraph advises after a new study found swapping animal sources of protein in favour of plant sources was linked to a longer lifespan.

Researchers looked at previously recorded data on health outcomes and diet for more than 130,000 US health professionals.

They found animal protein intake was weakly linked to an 8% higher risk of death, particularly from cardiovascular diseases, such as a heart attack, whereas plant protein was associated with a 10% lower risk of death.

However, an increased risk of death was only seen in people who also had at least one other unhealthy lifestyle factor, such as smoking, heavy alcohol intake, being overweight or obese, and physical inactivity.

This highlights an important limitation of studies like this – it's unable to prove that high animal protein intake has directly and independently caused the increased risk of death. It's not possible to rule out the role of other unhealthy lifestyle factors that also may have an influence.

Other limitations are the specific population group of health professionals, two-thirds of whom were women, which may not be representative of everyone.

As the current body of evidence stands, as well as considering this latest study, it would seem to be a good idea to stick to the existing recommendations about limiting your consumption of red or processed meat to no more than 70g a day.

Where did the story come from?

The study was carried out by researchers from a variety of institutions in the US and Italy, including the TH Chan School of Public Health, the Department of Medicine at Harvard University, the Broad Institute at Massachusetts Institute of Technology, and the University of Southern California, all in the US, and the Institute of Molecular Oncology in Italy.

It was funded by grants from the US National Institutes of Health.

This study was published in the peer-reviewed journal, JAMA Internal Medicine. It is available on an open access basis and is free to read online.

Generally, the media coverage around this topic was fairly accurate. However, the Daily Mail reported that, 'replacing red meat with vegetables, nuts and cereals saw the biggest drop in death rates', which is not quite the case, as vegetables as a specific group were not included in the food groups used to define plant protein.

Also, The Telegraph stated with confidence that 'switching 19g of animal protein – the equivalent of a sausage or a few slices of bacon – for nuts, vegetables or wholegrains significantly cut the risk of early death'.

Such precise predictions are arguably unwise given the notoriously complex interplay between diet, health and lifestyle.

BBC News did point out that if there is a benefit to eating plant protein, nobody really knows why this would be the case. A mystery worth investigating, perhaps?

What kind of research was this?

This was an analysis of two prospective cohort studies: the Nurses' Health Study and the Health Professionals Follow-up Study. It aimed to examine whether animal and plant protein intake was linked to mortality risk.

Studies like this are useful for assessing the influence of a specific exposure (in this case, diet) and outcome (mortality), but cannot confirm cause and effect.

We are unable to rule out confounding from many other health and lifestyle factors that may be involved in the link.

What did the research involve?

Researchers analysed data obtained from 131,342 participants (85,013 women [65%] and 46,329 men [35%]) from two long-standing ongoing studies in the US: the Nurses' Health Study and the Health Professionals Follow-up Study.

The Nurses' Health Study included 121,700 female nurses aged 30-55 recruited in 1976. This study used follow-up data collected between 1980 and 2012.

The Health Professionals Follow-up Study included 51,529 male healthcare professionals aged 40-75 in 1986. Follow-up data was collected up to 2012.

Data on dietary intake was collected through food frequency questionnaires, which were conducted every four years. The questionnaires asked participants on average how often they consumed a standardised portion of different foods in the previous year.

Within this, animal and plant protein intake was assessed. Animal protein was described as processed and unprocessed red meat, poultry, dairy products, fish and egg. Plant protein included mainly bread, cereals, pasta, nuts, beans and legumes.

Deaths were identified by linkage with the National Death Index. Cause of death was obtained from death certificates or medical records. All-cause mortality rates were calculated for deaths from cardiovascular disease, cancer and other causes.

The researchers then looked for links between animal and plant protein intake with different causes of death. The results were stratified by age and lifestyle factors. 

What were the basic results?

The average (median) protein intake among participants was 14% for animal protein and 4% for plant protein.

After adjusting for lifestyle and dietary risk factors, animal protein intake was weakly associated with higher mortality, particularly cardiovascular mortality (hazard ratio [HR] 1.08. 95% confidence interval [CI]: 1.01 to 1.16) – the association is described as weak as it just reaches the level of statistical significance.

Plant protein was associated with lower mortality (HR 0.90. 95% CI: 0.86 to 0.95).

However, these associations were observed only in participants with at least one other unhealthy lifestyle factor, and was not seen in those without any of these risk factors.

Replacing animal protein with plant protein resulted in lower mortality. For example, substituting 3% of energy from plant protein with an equivalent amount of protein from processed red meat was associated with a third lower all-cause mortality (HR 0.66. 95% CI: 0.59 to 0.75).

Slightly lower risk reductions were seen when substituting for unprocessed meat, poultry, fish, egg and dairy.

How did the researchers interpret the results?

The researchers concluded that, 'High animal protein intake was positively associated with mortality, and high plant protein intake was inversely associated with mortality, especially among individuals with at least one lifestyle risk factor.

'Substitution of plant protein for animal protein, especially that from processed red meat, was associated with lower mortality, suggesting the importance of protein source.' 

Conclusion

This analysis of two prospective cohort studies aimed to examine whether our intake of animal and plant protein is linked to our mortality risk.

It found evidence that animal protein intake was weakly associated with higher mortality, particularly cardiovascular mortality, whereas plant protein was associated with lower mortality.

However, the association with mortality was only seen in those with at least one other unhealthy lifestyle factor: smoking, heavy alcohol intake, being overweight or obese, and physical inactivity.

This suggests that it's not meat alone that has the effect – it seems to be more a compound effect when high meat intake is combined with other unhealthy lifestyle factors.

This reinforces the main inherent limitation of this study – it's an analysis of data from observational studies, which are unable to prove that animal protein intake has directly and independently led to an increased risk of death.

The authors attempted to control for various potential health confounders. As they've demonstrated, some of them also had an influence on risk. But it's not possible to fully take account of or rule out the influence of all unhealthy lifestyle factors.

The results may not be generalisable to the general population for several reasons. The two cohort studies only included health professionals, who may be more health conscious as a result of their jobs.

There was not an even gender representation, as roughly two-thirds of participants were women and one-third men. The results also do not represent children and younger adults. 

Although food frequency questionnaires are a validated tool for measuring dietary intake, results are not always representative of long-term eating and drinking habits.

Similarly, other self-reported data on smoking, alcohol or physical activity may not be completely accurate.

We may think that plant protein would include high amounts of vegetables. But the foods most commonly consumed in this group were actually carbohydrates and beans.

If you choose to eat a vegetarian or vegan diet, whether it's for health concerns, ethical reasons, or both, it is possible to get all the nutrients you need, provided you eat a wide range of foods.

Read more about vegetarian and vegan diets.

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