In injecting TMAO ( trimethylamine-N-Oxide) into mice makes their arteries look like those of older mice. Blocking TMAO makes the arteries of mice look like those of younger mice:

… treatment with 3,3-dimethyl-1-butanol for 8 to 10 weeks to suppress trimethylamine-N-oxide selectively improved endothelium-dependent dilation in old mice to young levels …

The quotation is taken from the abstract of the Hypertension article “Trimethylamine-N-Oxide Promotes Age-Related Vascular Oxidative Stress and Endothelial Dysfunction in Mice and Healthy Humans” whose abstract is shown at the top of this post.

Lisa Marshall’s CU Boulder Today article “What makes arteries age? Study explores new link to gut bacteria, diet” on this finding, goes on to make the point that two amino acids common in animal protein, L-carnitine and choline, are converted into TMAO by gut bacteria:

Eat a slab of steak or a plate of scrambled eggs, and your resident gut bacteria get to work immediately to break it down. As they metabolize the amino acids L-carnitine and choline, they churn out a metabolic byproduct called trimethylamine, which the liver converts to trimethylamine-N-Oxide (TMAO) and sends coursing through your bloodstream.

At least in mice, these bacteria that produce TMAO from L-carnitine and choline are more common in older individuals:

(This falls in line with a previous study in mice, showing the gut microbiome—or your collection of intestinal bacteria—changes with age, breeding more bacteria that help produce TMAO).

The dangers from TMAO add to the reasons to worry about animal protein.

• In “Meat Is Amazingly Nutritious—But Is It Amazingly Nutritious for Cancer Cells, Too?” I worry about the cancer-promoting powers of animal protein.

• In “Forget Calorie Counting; It's the Insulin Index, Stupid,” I worry about the insulin spike induced by the protein in certain types of meat, including certain types of fish.

In general, I am more worried about animal protein than I am about saturated fat. But I am becoming convinced by Peter Attia that in a minority of people (he estimates between 5% and 20%), saturated fat raises high-quality measures of the number of low-density-lipoprotein (LDL) particles (sometimes called “bad blood cholesterol”).

I think protein has a better reputation in our culture than it deserves. We need some protein, but few of us in rich countries have any serious danger of having too little protein. When lowering carbs, if you care about health, it is dietary fat that should be increased, not protein.

On potential trouble from protein, see also:

• The Surprising Genetic Correlation Between Protein-Heavy Diets and Obesity

For annotated links to other posts on diet and health, see:

• Miles Kimball on Diet and Health: A Reader's Guide