Causally or corollary? An innovatively random approach to the TMAO question

Cardiovascular disease remains one of the leading causes of death in the world, with an estimated 17.9 million deaths^[1] attributed to it in 2016. The most common risk factors^[2] for CVD are smoking, diabetes, high cholesterol, high blood pressure, lack of physical activity, and a poor diet. Therefore, CVD events are believed to be largely preventable^[3] through active lifestyle changes, and CVD itself is often labelled as a “lifestyle disease.”

Nutritional interventions have garnered a lot of interest from cardiovascular researchers. Two core recommendations that have come from this research are encouraging the consumption^[4] of plant foods, and limiting sugar-sweetened foods and beverages, added fats, and red meat. Red meat consumption, especially processed meat, has often been discourage^[5]d due to the epidemiologically established correlation with higher CVD mortality^[6], diabetes risk^[7], and increased colon cancer risk^[8]. While research on the precise underpinnings of this correlation is still ongoing, one mechanism that researchers believe make this correlation with CVD specifically plausible is the increased compound trimethylamine N-oxide (TMAO), which is produced by gut microbes^[9] from substrates that are particularly high in meat and animal foods, such as choline, betaine, and carnitine.

Higher levels of TMAO in the blood are believed to increase the number of scavenger receptors in macrophages^[10]. As shown in Figure 1, this eventually leads to these macrophages binding to low-density lipoprotein (LDL), which creates foam cells that stick to fatty deposits in the arteries. This process, along with several other hypothesized ones^[11], are believed to increase the risk of CVD.

High levels of TMAO have been correlated with a higher risk of cardiometabolic diseases, with three recent meta-analyses^[12][12][13] showing increases in CVD events and all-cause mortality. However, many of these studies are observational in nature and suffer from confounding^[14] and high measurement error^[15]. These types of studies have often conflicted with randomized controlled trials^[16], and other observational studies.

For example, fish is one of the highest-containing sources of TMAO^[17], yet observational studies^[18] find positive outcomes from its consumption, and randomized controlled trials^[19] that have investigated fish consumption as an intervention have found that it leads to a reduction in biomarkers that are risk factors for CVD.

Some researchers have hypothesized^[20] that it is not TMAO itself that increases the risk of CVD, but rather the presence of the gut bacteria that produce it increases the risk. Therefore, some believe that TMAO may simply be a confounder in the overall relationship, as it is a byproduct of a microbiome that increases the risk of cardiovascular disease through other means. One way to assess the causal relationship of TMAO and CVD would be to manipulate TMAO levels in a randomized controlled trial and compare CVD events. However, this sort of clinical trial is both impractical and unethical. The study under review assessed the causal relationship of TMAO on cardiometabolic diseases using a relatively new study design known as Mendelian randomization^[21], which can mimic the results achieved from a randomized controlled trial.

Consumption of animal products has often been discouraged due to associations with increased CVD risk and mortality. One mechanism that could contribute to CVD, in theory, is the production of the compound trimethylamine N-oxide (TMAO) from certain substrates found in high concentrations in animal products, which is believed to promote the atherosclerotic process. However, much of the research on TMAO has been inconsistent. Furthermore, most of these studies were observational in nature and suffered from problems of confounding and reverse causality. The study under review attempts to address some of these issues with a relatively new study design that mimics randomized controlled trials.