I’ve written quite a lot on this blog about why a high protein diet is the most effective way to rapidly lose weight and achieve an optimal body composition. I’ve also written about how it’s been shown to prevent fractures and muscle loss in elderly people.
So it comes as a bit of a fly in the ointment when you hear that “ah yes, it may help you lose weight and improve bone density and muscle mass, but it will also cause you to die sooner.”
The claim that a high protein diet shortens lifespan actually comes from the same people who discovered that protein is the central factor controlling appetite and calorie intake, David Raubenheimer and Stephen Simpson at the University of Sydney.
Unfortunately, there is at present no human experimental evidence that can show whether or not a high protein diet has a life shortening effect, so everyone on either side of this debate is forced to rely on experiments in animals. In particular, on mice. Mice, like humans, are omnivores – they generally eat whatever they can get a hold off, and at a fundamental physiological level, they’re quite similar to us. But they’re small and only live two years, which makes big diet experiments feasible.
A decade or so back, Raubenheimer and Simpson conducted a large experiment on mice, in which the mice were randomised to follow diets with varying proportions of protein over the course of their entire life span. In their book “Eat Like the Animals”, they state that this resulted in a dramatic difference in the longevity of the mice. They use the following graph to prove it:
Yes, it is a bit difficult to understand, I agree. Basically, they’ve converted every individual mouse life span in to pin point and coloured it a shade of grey, with darker representing a longer lifespan and lighter representing a shorter lifespan, and then put that colour at the junction on the graph representing the relative protein and carbohydrate intake for that individual mouse. All clear? Thought so.
When I look at the graph, my spontaneous interpretation is that the mice following a very high carb low protein diet lived a median of 117 weeks, while the mice on a very low carb high protein diet lived a median of 105 weeks. In human terms, that would represent a difference of five or six years between those following the highest protein diet and those following the lowest protein diet for their entire lifespan (two weeks for a lab mouse is roughly equivalent to a year for a human).
However, if you avoid the fancy statistics and just go and look at the overall median lifespans instead (which is intuitively easier to understand and more in line with how studies normally try to determine if there is a difference in longevity between groups), a different picture emerges:
Although the mice that were getting 60% of their macronutrients as protein lived noticeably shorter than the rest, below that extreme level there is no clear pattern. In fact, the mice on the 42% protein diet were the second longest lived, and only lived two weeks shorter than the group that was on the 5% protein diet.
Ok, so the graph is showing the protein to carb ratio, and the table is showing the total intake of each of the three macronutrients, so they’re not showing the exact same thing, but it’s odd that the picture they seem to paint is so divergent. While the graph seems to show that a high protein diet noticeably shortens life, the table suggests that it doesn’t. How do we explain the big apparent difference?
I corresponded with Stephen Simpson a few months back and was able to get an answer which finally made some sense to me. The explanation is offered by looking at another somewhat complicated graph, which was recently published in a review in iScience.
The graph shows average mortality for each age group over the course of the lifespan, with blue meaning lowest, green-yellow meaning in-between, and red meaning highest. What we see is that during the first 2/3 of the normal mouse life span, the mice are least likely to die on a relatively low protein diet, with a 0.3 protein to carb ratio appearing optimal.
During the final 1/3 of the normal life span, things switch over however, and the mice become much less likely to die if they dramatically increase their protein intake, and start to consume 1.3 grams of protein for every gram of carbohydrate.
In other words, while the mice on the high protein diet were more likely to die during their early years, they made up for this by living longer once they reached middle age.
The reason there is no noticeable effect on the overall median life expectancy is that death is uncommon during the first two thirds of the life span, regardless of diet chosen, but common during the final third of the life span. Most deaths happen in the final third of the normal life span regardless of what you do to the mice – just as most humans subsisting entirely on junk food and cigarettes will still live long enough to start collecting a pension. So although the period in life where a low protein diet is optimal for longevity is roughly twice as long as the period where a high protein diet is optimal, there is little difference at the end of the day.
I know we’re just talking about mice here, but in the absence of any good human data, it’s the best we have to work with, and it’s reasonable to think that a similar pattern would be seen in humans. As mentioned earlier, the benefits of a high protein diet in elderly people are indisputable – it prevents loss of muscle and bone mass, which should allow them to not just live, but to live independently, for longer.
So what can we conclude from this?
If you just look at overall averages, then there is no noticeable difference in life expectancy between those following a low protein and a high protein diet. If you dig in to the details however, the most optimal diet to follow for longevity appears to be a relatively low protein diet up to middle age, followed by a relatively high protein diet during the final decades of life. That is true if you’re a mouse, and might be true if you’re a human.
If you are eating a low protein diet though, you need to make sure to get lots of fibre – if you eat a low protein diet without plenty of fibre, you will absolutely get fat. The only way to avoid obesity on a low protein diet is to pare it with a high fibre intake.
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