The most important health function of prebiotic fiber is to increase the formation of short-chain fatty acids (SCFAs) in the colon.
Not only do these fatty acids benefit colon health, but they also play a role in regulating appetite.
Recently, a team of researchers examined the effects of elevated propionate, which is one of the most common SCFAs, on calorie intake and brain signals involved with food reward-driven eating behavior.
Today’s review provides a detailed summary of their findings.
Fiber is a key component of a healthy diet.
This especially applies to prebiotic fiber, which sustains the beneficial bacteria living in your digestive system.
When reaching the colon, prebiotic fiber is fermented by intestinal bacteria, producing short-chain fatty acids (SCFAs), mainly acetate, butyrate and propionate (1).
One study in rodents showed that elevated amounts of acetate in the blood suppressed appetite by affecting brain function (4).
Another human study found that increasing the formation of propionate in the colon by supplementing with inulin-propionate ester was associated with a reduced food intake and protected against weight gain (5).
However, until now, no studies have examined the potential effects of colon-derived SCFAs on the human brain.
A team of researchers examined the effects of elevated short-chain fatty acid formation in the colon on reward-driven eating behavior in humans.
This randomized, crossover trial examined the effects of increasing propionate formation in the colon on eating behavior.
A total of 20 healthy men, aged 18 to 65, participated in the study. Their body mass indexes ranged from 20 to 35.
They were assigned to receive two supplements in a random order:
- Inulin: A soluble fiber that promotes the formation of SCFAs (acetate, butyrate and propionate) in the colon.
- Inulin-propionate ester (IPE): Previous studies suggest that IPE provides similar amounts of acetate and butyrate as inulin but higher amounts of propionate (5).
After an overnight fast, the supplements were taken with breakfast when the participants visited the lab. The two lab visits were separated by at least 6 days.
The breakfast consisted of a standard chocolate milk shake and snack bar with 10 grams of either IPE (treatment) or inulin (control).
During each visit, the researchers measured the following:
- Brain activity: 5 hours after breakfast the researchers measured brain activity using functional magnetic resonance imaging (fMRI) while the participants looked at pictures of low- and high-calorie foods.
- Food appeal: While the participants were looking at the food images, they were asked to rate how appealing they found each of the foods, on a scale of 1 to 5.
- Blood values: Blood samples were collected to measure circulating levels of insulin, glucose and two appetite-suppressing hormones: glucagon-like peptide 1 (GLP-1) and peptide YY (PYY).
- Self-rated appetite: The participants were asked to rate their appetite and mood using visual analog scale questionnaires.
- Breath hydrogen: The concentration of breath hydrogen was measured to assess the level of fermentation in the colon.
- Food intake: At the end of each lab visit, the participants’ food intake was assessed by serving them a large meal consisting of a savory tomato and mozzarella pasta bake.
Bottom Line: This was a randomized, controlled trial examining the effects of elevated colonic propionate formation on calorie intake, appetite and food reward-related brain activity.
Finding 1: Colonic Propionate Reduced Food Intake
Supplementing with inulin-propionate ester (IPE) reduced food intake at an experimental meal by 9.5%, compared to supplementing with inulin.
These results are shown in the chart below:
The findings are supported by one previous human study showing that supplementing with IPE reduced food intake significantly (5).
The researchers concluded that these differences were likely due to elevated levels of propionate in the colon after the IPE supplement.
Changes in levels of insulin, glucose and the appetite-suppressing hormones GLP-1 and peptide YY were not significantly different between supplements, suggesting that other mechanisms may be responsible.
Bottom Line: Supplementing with inulin-propionate ester (IPE), which increases propionate levels in the colon, reduced calorie intake at an experimental meal, compared to supplementing with inulin.
Finding 2: Colonic Propionate Suppressed Reward Signals in the Brain
The researchers measured brain activity – the blood oxygen level–dependent (BOLD) signal – in brain regions that have previously been associated with food reward processing (6).
Supplementing with IPE suppressed food reward-related brain signals when the participants looked at images of food more than supplementing with inulin did.
Suppression of these brain signals was detected in two brain regions — the nucleus accumbens and caudate. It was greater when the participants looked at images of high-calorie foods, compared to low-calorie foods.
In fact, brain signals in the caudate were only suppressed by images of high-calorie foods, but not low-calorie foods.
The findings are presented in the chart below.
These results indicate that propionate suppressed food intake by affecting reward-driven eating behavior, although the researchers didn’t find any significant association between changes in brain signals and food intake.
However, this is the first study to suggest that propionate may affect these signals in humans.
Bottom Line: Supplementing with IPE suppressed brain activity in brain regions involved with food reward processing.
Finding 3: Colonic Propionate Made High-Calorie Foods Less Appealing
After the participants supplemented with IPE, they found high-calorie foods less appealing, according to ratings of food images.
However, self-rated appetite, as evaluated by visual analog scale questionnaires, was not significantly different between supplements.
Bottom Line: Supplementing with IPE made images of high-calorie foods less appealing, compared to supplementing with inulin.
Finding 4: Prebiotic Fiber Increased Fermentation in the Colon
Hydrogen gas is a by-product of fiber or carb fermentation in the colon. A large proportion of it is absorbed into your blood and released into your breath.
Since there are no other major sources of hydrogen in the human body, breath hydrogen levels are used to assess how much fermentation is taking place in the colon (13).
In the present study, breath hydrogen levels had increased significantly 3.5 hours after taking the supplements and stayed high until the end of the study visit.
Supplementing with inulin led to significantly greater breath hydrogen levels than IPE. This is because inulin contains higher amounts of fermentable fiber, or 10 grams, compared to 7.3 grams in the IPE supplement.
Circulating levels of butyrate also increased after supplementing with inulin and IPE, but the increase was not significantly different between supplements.
Bottom Line: Both supplements led to increased fiber fermentation in the colon, according to hydrogen breath tests.
The study’s design appears excellent. However, there are a few limitations to the interpretation of its findings.
First, all of the participants were men, and none of them severely obese. The findings might not apply to women or severely obese people and need to be confirmed in these groups.
Second, the study examined the appetite and brain responses after a single dose of inulin or IPE. The long-term effects of these supplements on brain function need to be investigated.
Summary and Real-Life Application
The study showed that increased colonic formation of short-chain fatty acids (SCFAs), specifically propionate, may temporarily reduce calorie intake and the appeal of high-calorie foods.
This is supported by previous studies in both humans and animals, but this is the first study to show that propionate affects brain regions involved with food reward-driven behavior in humans.
Simply put, prebiotic fiber may play an important role in appetite regulation through its effects on SCFA formation in the colon.