“All disease begins in the gut” — Hippocrates
Trans fats — trans-unsaturated fatty acids — are formed artificially through a process called hydrogenation, and they appear overwhelmingly in food made in factories. They extend the stability and shelf life of processed food. There is now substantial evidence that they do considerable harm.
What hydrogenation actually is
In this industrial process, hydrogen is added to liquid vegetable oils, converting them to solid fats. The result is more stable and lasts longer, which makes it ideal for packaged goods. Trans fats have historically turned up in shop-bought baked goods, fried food, snack food and some margarines, where they were used to improve texture and prolong freshness.
Weight, metabolism and the liver
Trans fats are found disproportionately in food that is highly processed and calorie-dense, so untangling them from obesity in general is not straightforward. But there is more going on than calories. Evidence suggests trans fats interfere with metabolic processes directly, and that they affect insulin sensitivity.
The liver is not spared either. Reviewing the mechanisms of action of trans fatty acids, Oteng and Kersten note that industrial trans fatty acids promote fat storage in the liver at the expense of adipose tissue. That matters more than it might sound: the liver produces the bile that digests fat, and a fatty liver does that job poorly.
The brain
The effects are not confined to physical health. Higher trans fat intake is associated with systemic inflammation, which has consequences for neurological function. Trans fats may also influence mood and cognition through pathways linked to inflammation and insulin resistance. The mechanisms are not fully mapped, but the association is consistent enough that dietary fat belongs in any serious conversation about mental health.
The gut
Research into how trans fats affect the gut flora is still young, but the direction is clear enough. Trans fats appear to damage the gut flora, contributing to the increased intestinal permeability that lets undigested food escape into the bloodstream. Since the composition of the gut microbiome shapes immune function and metabolism, damage there does not stay local.
Where this leaves us
These synthetic fats, concentrated in processed and fried food, are associated with obesity, heart disease, effects on mental health, and changes to the gut microbiome. As the evidence has accumulated, so has the pressure to remove them from the food supply.
The practical conclusion is unglamorous and holds up: cook with the fats humans have always cooked with, and be suspicious of any fat that had to be manufactured to exist.