Short-chain fatty acids (SCFAs) are an important class of biologically active substances produced in the gut, specifically by the action of gut bacteria on plant-derived foods containing fiber that is otherwise resistant to digestion, such as artichokes and legumes. SCFAs are emerging as important contributors to body metabolism and weight regulation, immunity, and mental health. They play roles in mood, sleep, and stress resistance. But the full scope of their roles is very much a developing st
Functions of Short Chain Fatty Acids
The full scope of the SCFAs' activities is very much under active investigation. For example, there’s evidence that at least some SCFAs invigorate a tumor-specific T-cell response, boosting the efficacy of cancer immunotherapy. A diet that promotes the abundance of such bugs—largely plant-based—may, in the future, become a standard part of cancer care.
What is known for sure is that all three major SCFAs play important roles in maintaining the health and integrity of the intestines—but they also cross the blood-brain barrier and are active in the brain. A still-evolving list of their activities includes:
All three SCFAs, researchers have discovered, curb secretion of the stress hormone cortisol. Studies show that in both low and high doses, they shut off the cortisol response to acute psychosocial stress. The greater the levels of SCFAs, the lower the cortisol response and the lower stress reactivity.
They also activate a channel of biochemical signaling between the gastrointestinal tract and the brain, and they increase levels of BDNF, or brain-derived neurotrophic factor, which promotes mental flexibility by stimulating the growth of new neurons. That is likely why animals fed a diet enriched with fiber-containing supplements for 10 weeks resisted the animal equivalent of behavioral despair when subjected to specific kinds of stress. The researchers conclude that fiber-containing supplements function like an antidepressant .
What Special Roles Does Butyrate Play?
Butyrate, produced by members of the Firmicutes family (such as Faecal Bacteria and Roseburia) of gut bacteria, is a local source of energy powering cells lining the colon. But it does much more. It is critical for maintaining the integrity of the intestinal barrier and preventing a condition known as “leaky gut,” implicated as a source of inflammation of many organs, including the brain. Butyrate stimulates intestinal wall cells to produce mucus and maintains the integrity of the intestinal barrier, keeping intestinal wall cells tightly joined so that bacteria and toxins from the gut do not leak into the general circulation. The same actions protect the intestine itself against inflammation.
The local effects of butyrate on the gut have major repercussions for brain operations. A leaky gut, for example, allows bacteria and toxins to pass through the intestinal wall into the bloodstream and be transported to the brain. In the brain, they can set off an inflammatory response, disrupting normal operations of the nervous system to pose the risk of depression , anxiety , cognitive difficulties, and even paranoia , psychosis and dementia .
Butyrate also influences mental health in other ways. It is active as a neurosignaling agent in the gut-brain axis , and it activates the vagus nerve. The vagus, the longest nerve in the human body, originating in the brain stem and extending into the abdomen, is often called the main mind-body highway; it is the primary channel of communication between the gut and the brain. The SCFAs directly activate the vagus nerve , stimulating and facilitating gut-brain communication. Studies show that butyrate increases the rate at which vagal neurons can transmit signals up to the brain, particularly satiety-related signals.
Operating by way of the vagus nerve , butyrate is also thought to communicate nutritional preferences to the brain. Researchers find that butyrate shapes food preferences by altering receptors for umami taste. It has multiple effects on appetite regulation, affecting metabolism of fats in multiple ways that suppress feeding as well as influencing action of several hunger-related hormones .
Diet and Short Chain Fatty Acids
Short-chain fatty acids, by themselves, are not supplied in abundance by diet. Instead, they are manufactured in the lower intestine from fiber in food—carbohydrate-based nutrients derived largely from the cell walls of plants, substances largely impervious to the digestive enzymes that break down most other food in the small intestine. Once in the colon, digestion-resistant carbohydrates are met by several distinct species of fiber-degrading bacteria that act on them by fermentation. Short-chain fatty acids are among the substances produced by bacterial fermentation of complex carbohydrates in the gut.
The fermentation of complex carbohydrates in the gut and the resulting release of short-chain fatty acids have multiple effects not only on general health and mental health but also on the makeup of the microbiome . They shift the composition of the microbiome , increasing the diversity of bacterial species inhabiting the gut.
Microbial diversity is so much a marker of health for so many body systems that it is linked to lowered levels of all-cause mortality. The bacterial population of the gut is also so responsive to what you eat that shifts in bacterial composition have been detected within 24 hours. Independent of diet, the composition of the microbiome is also influenced by physical exercise.
The fermentation of resistant starches specifically increases the gut population of such bugs as Bifidobacteria and Lactobacilli while decreasing known pathogens such as E. coli. As a result of consumption of fiber-rich foods, the microbiome of older animals resembles that of young animals and displays souped-up metabolic efficiency. Unfortunately, the microbiome of people consuming an American-type diet is depleted of fiber-fermenting microbes.
What Are Important SCFA-Producing Prebiotics?
The digestion-resistant complex-carbohydrate nutrients that act as prebiotics bear a variety of names reflecting their chemical constitution. They are concentrated in fiber-rich foods like leeks, onions, bananas, artichokes, cabbage, and asparagus, but they can also be extracted from foods and made available as supplements.
Prebiotics are generally classified by their chemical structure, bearing names such as fructo-oligosaccharides (FOS) and galacto-oligosaccharides (GOS) that reflect the number of sugar molecules that make up their “backbone.” Collectively, they are often referred to as polysaccharides. More than one type of fiber can be found in a food.
Pectins—moderately fermentable fiber found in pears, apples, quince, plums, gooseberries, oranges and other citrus fruits
Beta glucans—moderately fermentable fiber found in barley, mushrooms, oats, algae, yeasts
Explore More About Short-Chain Fatty Acids
For a comprehensive understanding of short-chain fatty acids, read our complete guide: