Does protein powder cause bowel cancer?

A concerned colleague recently shared an article with me from Ladbible - "Doctors shares urgent bowel cancer warning over protein powder ‘gym diet'". She was understandably worried that taking protein powder was doing her harm.

The article quotes a microbiome scientist says:

“These protein powders are bad news and really bad for your gut. When you break down and metabolise proteins, one of the byproducts is toxins to drive inflammation and cause bowel cancer. And the thing that is really bad for your gut are all of these high protein powders, like these creatine powders, they profoundly change your microbiome.”

It goes without saying that the article is sensationalist and I’m almost certain the expert quoted didn’t issue a “warning”. Journalists seem to love taking snippets from level-headed conversations and turning them into dire warnings. It’s certainly happened to me before.

Journalistic shenanigans aside, do protein powders:

• profoundly change your gut microbiome?
• get broken down by your microbiome into toxins that drive inflammation and cause bowel cancer?

Creatine is not a protein powder

First off, as any gym goer will tell you, “protein powders” is a very general term that can refer to lots of things. One thing it doesn’t usually refer to though is creatine. Creatine is composed of three amino acids (arginine, glycine and methionine) but is not a protein. It’s a naturally occurring substance in your body, found mainly in brain and muscle tissue, where it helps in recycling ATP, the energy currency of your cell.

I’m not a huge advocate of supplements (I think we get most of what we need from our diet), but if there’s one supplement I would recommend to nearly everyone (and that I take myself) it’s creatine. We have so much good evidence that creatine supports high intensity exercise capacity and there’s also possible benefits across other domains such as cognitive performance, particularly in older adults. When it comes to the microbiome and creatine there aren’t a lot of studies to go on. The only study I could find was done in Megalobrama amblycephala (or Wuchang bream - a freshwater fish found in the Yangtze basin in China). Probably not a lot we can draw from that then. However, researchers are actually interested in using creatine for therapeutic benefit in patients with inflammatory bowel disease as it may reduce intestinal permeability (“leakiness”) associated with inflammation¹.

Does protein powder change the human microbiome?

So, if creatine isn’t a protein powder, what was James referring to in his comments? Other protein powders include whey protein (derived from milk), plant-based protein powders (e.g. pea protein) and powders derived from muscle (e.g. beef hydrolysate). By far the most commonly used protein powder is whey protein.

For the sake of ease, let’s assume we’re talking about a pure whey protein with no additives. As you might expect, we don’t have many human studies looking at the effects of whey protein supplementation on the microbiome. A 2018 randomised study (n=90) concluded that the effects of 24g whey supplementation on the microbiome were limited to an increase in the diversity of the virome (the viruses present in your gut)². In contrast, a 2017 study (n=125) found no change in gut microbial composition after 12 weeks of whey protein supplementation³. Lastly, another 2018 study (n=36) found that combination of whey protein and beef hydrolysate supplementation for 10 weeks was associated with a decrease in the abundance of some health-associated bacteria⁴.

So, we have a few small studies with conflicting findings when it comes to whey protein supplementation and the microbiome. Nothing to get too excited about and I certainly don’t think we can say that whey protein supplementation “profoundly changes your microbiome”.

What about emulsifiers in protein powder?

One aspect of protein powders that wasn’t mentioned in the article was the use of emulsifiers. Many protein powders include emulsifiers to prevent clumping and make protein shakes easier to blend. Emulsifiers may affect the gut microbiome by disrupting the mucus layer separating the gut lining from the gut lumen (and the microbiome). Soy lecithin is one of the most commonly used emulsifiers but a 2021 in vitro study found it had no effect on the human gut microbiota⁵.

We also need to acknowledge that soy lecithin and other emulsifiers are also used in infant milk formula and many other foodstuffs. This is certainly an area that needs more study but I don’t think we have reason to be fearful just yet.

Are protein powders metabolised to toxins in your gut?

What about protein powders being metabolised by your microbiome into toxins that drive inflammation and cause bowel cancer? I suspect that James may be drawing this conclusion from data suggesting that bacterial fermentation of proteins found in meat can increase the production of potentially toxic nitrogenous and sulfur metabo­lites that could potentially drive bowel cancer risk. While it’s plausible that this could also apply to beef-hydrolysate, whey and pea protein have a different protein composition and I’m not aware of any studies demonstrating that any of them result in the production of toxic bacterial metabolites.

Epidemiologic data on protein consumption

Protein supplements have been used by millions of people for decades - do we have any epidemiologic evidence suggesting an association between protein supplementation and bowel cancer? Short answer, no. In fact, epidemiologic data looking at total protein consumption (i.e. not supplements specifically), found no association between protein consumption and bowel cancer risk⁶. Furthermore, dairy (from which whey protein is derived) consumption is actually associated with a lower risk of bowel cancer⁷. This has led to researchers suggesting that whey protein specifically may actually have anti-cancer properties⁸.

All-in-all, I don’t think we have convincing or even concerning data that protein supplementation is driving the development of bowel cancer. We should certainly be open-minded and I’d be very interested to see more studies done in the field particularly given the ubiquity of protein supplementation.

Let's be careful what we say in the media

As scientists commenting in the media we need to be cognisant that our comments may have unintended results. For example, a 2024 pilot study highlighted the potential for protein supplementation to increase muscle mass and physical function in patients with bowel cancer⁹. Furthermore, frailty is an increasing problem in our ageing population and strength and muscle mass are consistently shown to be associated with reductions in mortality risk and increased physical performance. For many, protein supplementation is the only feasible way to achieve adequate protein intake (around 2g per kg of body weight per day). Do we really want to be scaring people away from a beneficial intervention based on flimsy, contradictory science?

I, for one, won’t be ditching my daily protein shake any time soon, though I reserve the right to change my mind should persuasive evidence emerge!