The science behind fullness and how fiber and gut health impact appetite. Food processing and low fiber have disarmed our ileo-colonic brake. some practical advice on increasing fiber is provided at the end.
The science behind fullness and how fiber and gut health impact appetite.
“Calories in, calories out!” is one mantra I often hear among my patients and friends that are trying to lose weight. We’re told by conventional wisdom to eat less and exercise more to achieve a calorie deficit. So maintaining a healthy weight is just a simple accounting problem, right?
Yet when we try to eat less, we all know what happens—we’re constantly hungry and that can lead to binging, even more weight gain and feelings of failure. In fact, as a physician and gastroenterologist, I very commonly hear, “I eat like a bird and still gain weight” and “I must have a slow metabolism!”
First, I am here to validate your experience. You are not crazy. Metabolism does in fact slow down when you try to lose weight, but contrary to popular wisdom, it is actually faster in folks that are overweight.[1,2] This response might make it harder to lose weight, but a slow metabolism doesn’t lead to weight gain. So what else is going on then?
How your diet impacts your gut health – and how your gut health impacts your appetite
The enlightened may know how important managing stress and sleep are in avoiding unhealthy snacking and maintaining a healthy weight. But those are pieces of a larger puzzle. It turns out that another huge piece of this puzzle is your gut health, specifically how efficiently the first part of your intestine absorbs nutrients and how this impacts your appetite![3,4]
Diets high in refined and processed simple carbohydrates (white bread, white rice, juice, soda, baked goods, and foods with added sugar) are missing natural structure with their fibers removed. They are so easily absorbed in the upper gut (duodenum and jejunum) that there is less trickle of these nutrients into the lower gut (ileum and colon) and fewer unabsorbed calories in the stool![5,6]
This disorients the natural state of our lower intestine, which expects there to be a certain amount of leftovers to feed the beneficial bacteria in the gut. In an ideal state, when the lower intestine senses these unabsorbed nutrients, it sends hormone signals (e.g. GLP-1 and PYY) to your stomach, upper intestine, and brain to feel full and slow down so you have time to digest and absorb them.
There’s actually a term for this. It’s called the ileo-colonic brake and it’s one way that gastric bypass surgeries and some therapeutics (e.g. acarbose and orlistat) are thought to work.[8–10] They essentially send more unabsorbed nutrients further down the tube where the ileo-colonic brake senses them and makes hormones to make you feel full and eat less.
Fiber makes for incredible leftovers for “good” gut microbes. Plus, these are missing in most processed foods.
In addition, fiber leftovers in foods are feasted on by the microbes in the lower gut (collectively referred to as the microbiome) and fermented to become molecules, including butyrate, that stimulate hormones like GLP-1 to tell us we’re full. They’re essentially telling the gut to pause while they take time to convert the leftovers into vital energy (e.g. fatty acids) and nutrients (e.g. B vitamins)!
Modern processed foods that are often missing their natural fibers are so quickly and completely absorbed that they negatively affect this natural process. So how do we turn on the ileo-colonic break to prevent needing medications or heroic measures like surgery? The answer lies not in how much we eat, but rather what we eat.
How can certain foods tell your body you’re full?
While eating simple carbohydrates can increase hunger and the efficiency of how well the gut takes up sugar calories by increasing transporters (e.g. SGLT1 and GLUT2), other foods can actually decrease the efficiency of uptake. This decreased efficiency is a good thing! It allows the nutrients to pass down to the ileo-colonic break and microbiome where hormones are produced to signal to us that we’re full so we have more time to digest. So what foods help you achieve this?
Whole foods and fiber to the rescue!
Diets based on whole foods are better at making it all the way to the ileo-colonic break and gut microbiome because they are slower to digest and absorb (which is good!).
But what’s in whole foods that makes them slower to digest? It turns out that important plant nutrients like fibers and polyphenols are at the top of the list. In fact, studies have shown that both fiber and polyphenol supplements alone can decrease the absorption efficiency of carbohydrates.[14,15]
Some special fibers like resistant starch and oat-beta glucan, for example, decrease the number of transporters (e.g. SGLT1) involved in glucose uptake so you absorb less carbohydrate calories in the upper intestine.[16,17] These fibers also pass through the upper gut and actually make it to the microbes in the lower gut where they’re fermented to butyrate, a molecule that feeds the cells that line the gut. When the gut feels full, it stimulates the natural hormone GLP-1 to slow your stomach’s emptying and signal to your brain that you feel full.
What’s super interesting is that a synthetic version of GLP-1 that has been used for some time for blood sugar control in diabetes was just recently approved by the FDA for weight loss, the first in over a decade. Indeed a landmark trial recently showed a synthetic GLP-1 combined with another hormone called GIP lead to an unheard of over 20% weight loss in obesity!
It’s exciting news, but these drugs are not without their issues including nausea in some people, requiring administration under the skin with a needle, not being covered by many insurance companies, and a high price tag. Still some ventures (e.g. Calibrate) are tapping into these fundamental metabolic reset mechanisms to help control appetite and weight. It turns out that both your body and innovative companies are on to something important!
So, next time you’re told “calories in, calories out” and think reflexively, “No that’s not true, I have a slow metabolism,” just remember — it may have as much to do with the types of calories you consume and how quickly and completely they’re absorbed by the gut as it does with how quickly they’re metabolized by the body. Indeed, adding fiber has a dual effect by decreasing calories in and increasing calories out the other end!
The bottom line
The pillars of healthy living remain: exercise routinely, sleep soundly, and eat in moderation. But a simple, important, and less appreciated addition is to give your gut and body an ileo-colonic brake! Reach for foods with high fiber content including whole grains, nuts, seeds, fruits and vegetables. In our busy lives, using supplements and convenient foods that have added fermentable fibers can also help you meet the goal set by the USDA of about 30 grams per day.
Your microbes and gut will thank you for the fiber leftovers by telling your stomach and brain you’re full and your body will thank you by achieving and maintaining a healthier metabolism and weight!
Stay tuned for Part 2 where weight loss and blood glucose control are discussed.
More Articles & Resources
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