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About 20 years ago there was a discovery that would forever change our ideas about fiber: our gut bacteria eat the fiber we eat to produce important signaling compounds called short-chain fatty acids. But before I get carried away, a little science lesson to frame the discovery.
Cells are the fundamental unit of life. We’re composed of trillions of them. They communicate with each other through receptors on the surface of our cells. That’s how many hormones work; it’s like a lock and key. Hormones are signaling messengers, each with a unique shape. When released into the bloodstream, they circulate throughout the body until they find a receptor they can fit into. Once the key is in the lock, it can turn on or off a whole series of reactions in the target cell.
The largest family of cell receptors is known as G protein-coupled receptors (GPCRs). More than a third of the drugs currently on the market work by plugging into these receptors. That’s how drugs like antihistamines work. We’ve discovered hundreds of different GPCRs, but remarkably, we don’t yet know what many of them do. We have the lock; we just don’t know what key fits. Accordingly, they’re called “orphan” receptors.
Two of these mystery receptors, known only as GPR43 and GPR41, were found heavily expressed throughout the body in our immune and fat cells, and in our gut, muscles, and heart. We knew they must be vital, but we didn’t know what activated them until 2003, when they were “deorphanized” (that’s actually what scientists call it). The keys that fit into those important locks were the short-chain fatty acids that our gut bacteria make when we feed them fiber.
This is how our gut bacteria may communicate with us. Renamed free fatty acid receptors, their existence is now considered crucial insight into how fiber could play such an important role in so many of our chronic diseases.
Hormones are defined as signaling messengers produced in one organ that circulate through the bloodstream and have a regulatory effect on another organ. So, these short-chain fatty acids can be considered hormones. It’s just that the organ that produces them is our microbiome, the bacteria that populate our gut. They can’t make these hormones, though, without fiber.
What do the free fatty acid receptors on our fat cells do? Remember leptin, the hormone produced by fat cells that helps tell our brain how to regulate our body weight? Short-chain fatty acids stimulate the production of leptin. This can be demonstrated in a petri dish with individual fat cells or by using fatty tissue samples removed during surgery. Drip on some short-chain fatty acids, and leptin production almost doubles.
Leptin is an “anorectic” hormone, so-called because it generates a loss of appetite and weight, but it does so over the long term. Leptin levels slowly rise as the volume of body fat gradually increases. There are other anorectic hormones that instead work rapidly, signaling our brain on more of a meal-to-meal basis. Two of these short-term appetite suppressants are PYY and GLP-1, which are both secreted by specialized L cells that line our colon. Guess what receptors are crowded all over their surface? Free fatty acid receptors.
Drip some short-chain fatty acids onto L cells, and they start churning out PYY and GLP-1. You can do this in a petri dish or in a person, either by infusing their rectums with a short-chain fatty acid enema, or the old-fashioned way by feeding people fiber––or even better, fiber-rich foods, like this whole-rye kernel wheat bread. These hormones then get released into the bloodstream where they can shoot up into the appetite center of our brain and turn down our cravings.
The flipside is ghrelin, the so-called hunger hormone. Ghrelin levels rise in our blood before a meal to stimulate our appetite, and fall down again once we eat before slowly building back up to drive us back to the kitchen. But four hours after eating just 24 grams of fiber, people’s ghrelin levels are suppressed as much as if they had just had 500 calories worth of food. Longer studies show similar effects. For example, 11- and 12-year-old children with overweight or obesity who were randomized to increased fiber intake for 16 weeks ended up eating hundreds of fewer calories of a buffet meal compared to the placebo control group.
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