When exposed to cold temperatures, you begin shivering to compensate for the sudden temperature decrease. Biologically, exposure to cold environment can induce brown adipose tissue differentiation and catabolism of lipids to produce heat. The gastrointestinal tract is an important site for the regulation of our energy metabolism, and as we know, harbors a diverse population of gut bacteria. A team of researchers sought to investigate what role the microbiome of mice plays in induction of physiological changes to cold exposure.

The researchers exposed different groups of mice to cold temperatures for 31 days, and the initial observation was attenuation of energy expenditure and white fat. Importantly, food intake was consistent throughout the trial, pointing to a temperature-driven effect. Fecal collection and post-mortem intestinal analysis revealed marked shifts in microbiota compositions of cold-exposed mice as compared to those kept at room temperature. Notably, families within Actinobacteria, Verrucomicrobia, and Tenericutes were less abundant in the cold sample mice, and Akkermansia muciniphila species were observed to be significantly decreased in the cold mice. The researchers also transplanted microbiota from cold-group mice to naïve, germ-free mice. Upon cold temperature exposure, germ-free mice demonstrated heightened insulin sensitivity, browning of white fat, increased energy expenditure, and white fat loss, indicative of a tolerance to the cold. Prolonged cold exposure in both cold-exposed and cold-transplanted mice also induced adaptive biological mechanisms in intestinal villi and microvilli to maximize caloric uptake, ultimately attenuating weight loss. This increase in intestinal absorption was observed concomitant to altered intestinal gene expression, as genes that promote tissue remodeling where upregulated while apoptotic genes were suppressed. In a final experiment, the aforementioned effect was reduced when Akkermansia muciniphila (which were originally observed to decrease in cold-induced mice) were transplanted to cold-exposed mice, supporting a microbiota-driven relationship.

This was a very interesting study that sheds lights on organismal adaptive mechanisms during energy scarcity brought on by factors such as cold temperatures. Again we observe heavy involvement of the microbiome in these processes, a finding that calls for more investigation.

The microbiome has long been associated with cardiovascular disease, especially after studies showing differences between the gut microbiomes of obese and slim individuals. The mechanisms by which the microbiome may be influencing heart disease are still unknown, but there are a few mechanisms that have been identified. For example, as has been previously discussed on this blog, trimethylamine N-oxide (TMAO) in the blood is an independent risk factor for atherosclerosis, and is produced by gut bacteria from choline and carnitine. In addition, systemic, chronic inflammation is associated with heart disease, and our avid readers will know that the microbiome can cause chronic inflammation in the vagina, gut, and mouth. Overall though, a direct relationship between specific bacteria and heart disease has not been shown. A recent epidemiological study though, did just that. The researchers, mostly from the Netherlands, were able to identify specific species that were associated with higher BMIs, as well as those that were directly correlated with HDL cholesterol levels. They published their results in the journal Circulation Research.

The scientists measured the genomes, microbiomes, BMI, and blood lipids of 1500 adults. Their results showed that higher overall diversity and richness of the gut microbiome was associated with a lower a lower BMI (healthier state), lower triglycerides (healthier state), and higher level of HDL cholesterol (healthier state). The diversity was not, however, associated with total cholesterol nor LDL levels. The researchers then identified specific bacteria associated with these health indicators. There are too many to list in this blog, so we encourage interested readers to take a look at the article. Some examples though: Akkermansia, Christensenellaceae, and Tenericutes were each associated with low BMI, low triglycerides, and high HDL (all healthy states), while Eggerthella was associated with high BMI and high triglycerides, and Butyricimonas was associated with high BMI, high triglycerides, and low HDL (all unhealthy states). Finally, the researchers sought to determine just how important the microbiome was to overall BMI, triglyceride levels, and HDL levels by incorporating the host genetics, age, and gender into their calculations. They showed the 4.5% of the variance in BMI, 6% of the variance in triglycerides, and 4% of the variance in HDL is directly attributable to the microbiome.

These study results reaffirm the importance of the microbiome to our overall health, and even quantitatively show its influence on specific health indicators. The authors do not attempt to explain why specific bacteria would cause variation in these metrics, although as previously mentioned some mechanisms have already been demonstrated. To check to see which other diseases these bacteria have been associated with, use the search tool, or click the tags below to see all the blog articles that mention them.

Exposure to the cold affects our microbiome and modulates our energy homeostasis

New study suggests gut microbiome directly influences BMI, triglyceride, and HDL levels