A High-Fat Diet May Be Bad For Your Gut Bacteria, Live Science, 20 February 2019

Effects Of Dietary Fat On Gut Microbiota And Faecal Metabolites, And Their Relationship With Cardiometabolic Risk Factors: A 6-Month Randomised Controlled-Feeding Trial, BMJ Gut, January 2019

The concentration of total short-chain fatty acids was significantly decreased in the higher-fat diet group. … [Metabolites] known to be associated with host metabolic disorders, were decreased in the lower-fat diet group. … The higher-fat diet was associated with elevated plasma proinflammatory factors after the intervention.

Conclusion: Higher-fat consumption by healthy young adults whose diet is in a state of nutrition transition appeared to be associated with unfavourable changes in gut microbiota, faecal metabolomic profiles and plasma proinflammatory factors, which might confer adverse consequences for long-term health outcomes.

The low-fat diet in this study above increased levels of bacteria in the colon that produce beneficial short-chain fatty acids (SCFAs), notably butyrate. Lots of benefits from butyrate, as we’ve seen. More SCFAs are produced when we feed those good bacteria carbs.

In this next study, high-fat diets increased levels of bile acids in the colon. (Bile acids are made in the liver, stored in the gall bladder, and squirted into the intestine when we eat fat, in an amount proportionate to the fat we eat.) These researchers found that certain bile acids interfere with normal repair of our intestinal lining. That increases risk for colon cancer:

Salk Scientists Uncover How High-Fat Diet Drives Colorectal Cancer Growth, Eurekalert, 21 February 2019

FXR Regulates Intestinal Cancer Stem Cell Proliferation, Cell, 21 February 2019

A new study led by Salk Institute scientists suggests that high-fat diets fuel colorectal cancer growth by upsetting the balance of bile acids in the intestine and triggering a hormonal signal that lets potentially cancerous cells thrive. The findings … could explain why colorectal cancer, which can take decades to develop, is being seen in younger people growing up at a time when higher-fat diets are common.

This study provides a new way to lower inflammation, restore intestinal health and to dramatically reduced tumor progression [by reducing fat in the diet].

The researchers found that animals with an APC mutation, the most common genetic mutation found in humans with colorectal cancer, developed cancer faster when fed a high-fat diet. “It could be that when you’re genetically prone to get colon cancer, something like a high-fat diet is the second hit.”

High-fat diets (keto, Paleo, Atkins, low-carb) are popular right now. When you reduce the amount of carb, you naturally increase the amount of fat and protein (aka meat). Just because something is popular doesn’t mean it’s healthy.