One of the main reasons why the microbes living in our digestive system, our microbiome, is so important, is because of the fermentation end-products they produce: short chain fatty acids (SCFA). There are three main SCFA our gut friends make: Propionate, Butyrate, and Acetate. Recently, more research is revealing itself on the importance of these fermentation products, and this study, done in 2016 by Claire Byrne and colleagues at the Imperial College in London, is a great example. 

As a whole, SCFA have been shown to reduce acute energy intake, and previous research has shown that a we need quite a large dose of non-digestible carbohydrates to stimulate appetite (>35g NDC/d; Pederson et al. 2013). In the current study, Byrne and colleagues wanted to see the effect of SCFA on anticipatory reward responses to high energy foods. Specifically, researchers looked at a non-digestible compound ester, inulin-propionate (IPE: a prebiotic food PLUS extra added SCFA) versus a control: inulin alone (solely a prebiotic food), to see whether the added propionate would reduce signaling in the reward processing regions of the brain to high energy foods (they examined the caudate, nucleus accumbens, amygdala, anterior insula, and orbitofrontal cortex). 

Scientific Question: Does the added SCFA Propionate have an appetitive effect (does it change overall food intake) and does it have any signaling effects to high energy food images in reward regions of the brain? 

Study Design: Twenty men participated in a randomized, placebo-controlled, crossover study where they attended 2 separate study visits, 6 days apart. Breath hydrogen (a measure of colonic bacterial fermentation) and appetite scores (via visual analog scales) were collected to assess fermentation and subjective appetite and mood ratings. Standard breakfast was administered with 10g inulin (chocolate milkshake and snack bar; 574 kcal, 86g carbs, 18g fat, 14g protein, 3.2g fiber). Lunch, at 180 min, was a cheese sandwich and snack bar (558 kcal, 62g carbs, 24.9g fat, 21.7g protein, and 2.8 g fiber). A functional magnetic resonance imaging test (fMRI) was then administered to look at brain signaling to different food and non-food images, followed by dinner of tomato and mozzarella pasta bake, for which subjects were instructed to eat until they were comfortably full (per 100 g: 129 kcal, 17g carbs, 3.9g fat, 4.8g protein, 3.4g fiber). 

fMRI brain scan images consisted of:

1) 60 high energy (HE) foods: for example pizza, chocolate, & cake.

2) 60 low energy (LE) foods: for example salads, vegetables, & fish.

3) 60 non-food related: for example furniture or clothing.

4) 180 blurred images

Subjects then had to rate how appealing each food was on a scale of 1 ---> 5  (1 = not at all appealing, 5 = a lot/very appealing).

Results:

1. IPE treatment significantly reduced energy intake by 9.5% +/- 5.3% (IPE treatment 711 +/- 79.9 kcal; compared to Control: 810.4 +/- 83.4 kcal; P= 0.03).

2. IPE decreased brain signaling to HE foods in caudate (measure via BOLD= blood-oxygen-level dependent imaging), but not nucleus accumbens, anterior insula, hypothalamus, and amygdala (Effect size: -0.078 +/- 0.032 P= 0.029).  

3. Subjects receiving IPE also rated HE foods as less appealing compared to control (P= 0.046).

4. Researchers could not show a direct correlation between changes in BOLD signal and energy intake.

Why is this research important?

Past research has shown that increases in non-digestible carbohydrates and colonic short chain fatty acids (SCFA) can improve body composition and decrease energy intake independent of changes in gut hormone concentration (So P-W, et al 2007, Frost et al 2014, Anastasovska et al 2012), so the importance of eating fiber, resistant starch, indigestible material within foods, and hence producing more fermentation products is therefore gaining traction.

SCFA produced via colonic fermentation have a widespread health effect. Byrne and colleagues specifically show that SCFA are the crucial factor here (as seen by the difference in decreased anticipatory reward and decreased food intake in IPA (a prebiotic plus the SCFA) versus control (prebiotic alone). 

What are some food sources that will provide microbiome fermentation products?

1. Resistant starches: plantains, cooked and cooled foods, grains, beans & legumes.

   • There are 4 main categories of resistant starches labeled as RS 1-4 ranging from completely indigestible (whole grains, and uncooked) to manufactured with more resistant starch (corn for example).

2. Prebiotic foods: foods that impart a health benefit through increasing beneficial microbiota- asparagus, leeks, onion, garlic, artichoke, chicory, apples, dandelion greens, and whole grains & legumes.