A high-fiber, minimally processed diet reduces calorie absorption and promotes weight loss Original paper

In this randomized controlled crossover trial, a high-fiber diet based around minimally processed foods altered the gut microbiome, reduced calorie absorption, and resulted in weight loss, compared to a Western-style diet.

The effect of a microbiome-supporting diet — a diet designed to deliver more dietary substrates to the colon and therefore modulate the gut microbiome — on energy (calorie) balance.

The microbiome-supporting diet was high in fiber (approximately 26 grams per 1,000 calories), high in resistant starch, had large food particle sizes, and was low in processed foods. A highly digestible Western-style diet was used as a control. This control diet was low in fiber (approximately 6.4 grams per 1,000 calories), low in resistant starch, had small food particle sizes, and was high in highly processed foods. The two diets contained similar amounts of calories, protein, fat, and carbohydrates.

17 weight-stable adults with a BMI of 30 or below (average age of 31; 9 men, 8 women).

The researchers conducted a randomized controlled crossover trial with several phases. Over the course of 9 days, the investigators determined the participants’ daily calorie needs. The participants were assigned to follow either a microbiome-supporting diet or a Western-style diet for 11 days in an outpatient setting (free living), then for 12 days in an inpatient setting (in a metabolic ward), with all meals provided and personally designed to keep the participants in calorie balance (i.e., calories burned were equal to calories ingested). The participants then repeated this process with the other diet after a minimum 14-day washout period.

The primary outcome was metabolizable energy, or estimated energy absorption. This involved measuring energy intake and subtracting the energy lost in feces. Other outcomes assessed were digestive tract hormone levels (GLP-1, leptin, and pancreatic polypeptide), self-reported hunger and satiety, food intake, and body composition. The investigators also assessed microbiome changes, including fecal bacterial biomass (using 16S rRNA gene copy number), microbiome diversity (both beta-diversity and alpha-diversity), and microbial species abundance. They also measured levels of short-chain fatty acids (SCFAs) in participant feces and serum samples.

The participants absorbed fewer ingested calories on the microbiome-supporting diet (89.5% absorbed) compared to the Western-style diet (95.4% absorbed). This represented a difference of 116 calories lost in feces between the two diets.

Compared to the Western-style diet, the microbiome-supporting diet resulted in a decrease in body weight (–626 grams vs. –134 grams) and body fat (–290 grams vs. –65 grams) during the inpatient period, although the difference in body fat did not reach statistical significance (p=0.06).

The microbiome-supporting diet increased fecal microbiome biomass, increased beta-diversity, altered the abundance of various bacterial species (including 10 species whose levels changed with a large effect size), and resulted in higher levels of SCFAs in feces and serum, indicating increased intestinal SCFA synthesis and absorption.

Food intake, gut hormones, and hunger/satiety appeared similar between groups.

Percentage of calories absorbed

One area of discussion and debate in nutrition is whether “a calorie is a calorie.” In other words, can different diets contain the same number of calories but differ in their effects on body fat loss or gain? This debate often includes the question of whether avoiding carbohydrates can lead to greater fat loss due to the effect of carbohydrates on the hormone insulin. This hypothesis, generally termed the carbohydrate-insulin model (CIM), contends that carbohydrates stimulate insulin, which causes more calories to be stored as fat. As a result, consuming a low-carbohydrate diet will produce a “metabolic advantage”, resulting in greater fat loss even when the same number of calories are consumed.^[1]

However, tests of the CIM suggest it doesn’t stand up to scrutiny. Specifically, tightly controlled clinical trials conducted in labs called metabolic wards have found that carbohydrates and fat do not have a substantially different effect on energy balance independent of calories. Ultimately, calories from fat and carbohydrate appear to be roughly identical in their effects on body fat.^{[2][3][4][5][6][7]}

Although carbohydrates and fat seem to have minimal difference when it comes to body fat changes when calories are equal, this may not be the case for high-fiber and low-fiber diets. It has long been known that fiber can increase calorie excretion, with research noting that adding fiber to the diet can decrease macronutrient (i.e., fat, carbohydrate, and protein) absorption from the diet.^[8] Because this is a feasible way of producing a negative energy balance,^[9] a high-fiber diet could therefore promote weight loss compared to a low-fiber diet, even when calories are the same.

This does assume that the anti-nutritive effect of fiber is not counterbalanced by some other factor. For example, fiber can be metabolized by gut microbes into short-chain fatty acids (SCFAs), a unique category of fats that are between 1 and 6 carbons long (unlike most dietary fats, which are at least 12 carbons long). Because SCFAs provide calories, estimates that take SCFAs into consideration have produced widely different estimates for the effect of fiber on calorie balance, ranging from negative (i.e., more calories out) to positive (i.e., more calories in), depending on the type of fiber.^[10] With that in mind, the results of the current study suggest that a high-fiber diet based on minimally processed foods can ultimately result in fewer calories being absorbed by the body, compared to a more processed, lower-fiber diet, even when accounting for SCFAs. Furthermore, the current study suggests this is significant enough to result in weight loss and perhaps fat loss as well.

Also keep in mind that the participants’ calorie intakes were intentionally kept steady in this study. Unfortunately, this removes perhaps the most important factor determining body weight changes: calorie intake, itself determined by a complex array of biological signals and psychosocial factors that are well beyond the scope of this review. However, current evidence supports the idea that a greater intake of fiber from fiber-rich foods or supplements generally promotes weight loss,^{[11][12][13][14][15]} meaning that the phenomenon of lower calorie absorption confirmed in the current study could have real-world relevance.

It’s also worth delving into how the gut microbiome could play a role in these associations. The idea that the gut microbiome may influence body weight has been the subject of considerable scientific research. Initially, studies in mice^[16][17] seemed to support the idea that the microbiome can promote either obesity or leanness depending on the microbes present, possibly in part by altering how many calories the animals absorbed. However, this once promising notion has been strongly undermined by clinical trials in humans involving what is called a fecal microbiota transplant (FMT). These trials tested whether “transplanting” the microbiome from a lean person to people with obesity can cause weight loss in the receivers, with most trials finding no clear effect. These FMT studies create considerable doubt as to whether the microbiome is actually an important regulator of body weight by itself.^[18][19][20] This doesn’t necessarily shut the door on the relationship, though — it’s also possible effects occur only in certain contexts.

In the current study, estimated calorie absorption displayed limited variability on the Western-type diet (from 94.1% to 97%), whereas there was a decent amount of variability on the microbiome-supporting diet (84.2%–96.1%). It’s hard to say why this was the case, although the investigators explored whether this was due to differences in gut microbes. They found that levels of several bacterial species predicted calorie absorption on the microbiome-supporting diet. This introduces an interesting, if currently speculative, possibility: perhaps the gut microbiome influences calorie absorption in the context of a high-fiber diet but has little-to-no effect in the context of a low-fiber, Western diet. Future research is needed to shed light on this topic.

Only the primary outcome — metabolizable energy — was a preregistered outcome. As a result, the other findings should be interpreted with caution.