Feeding Your 'Good Gut Bacteria' May Help With Appetite Control

There are 10 times more bacterial cells in the body than there are human cells, with the greatest concentration in the intestinal tract. The gut microbiome is complex and diverse, but has a fundamental physiological role. Prebiotics are functional foods that ‘feed the good bacteria’ to maintain a healthy digestive system and modulate immunity. However, from a recent randomised control trial conducted by Hume et al. it was proposed that prebiotic supplementation may also aid appetite regulation in obese individuals [1]. From discussing their results within the context of wider research, including both animal and human studies, it has been concluded that there are inconsistencies in observations from human trials regarding the role of prebiotics in appetite control and obesity prevention, but that it may still be beneficial to frequently consume prebiotic containing foods to promote a favourable microbial composition, and, simultaneously, increase the fibre content of the diet.

The human gut contains trillions of microbes [2], comprising over 1kg of body weight [3]. The symbiotic relationship that exists with these organisms is essential for metabolic, immunologic and endocrine functioning [4] as gut microbiota are responsible for energy extraction from indigestible dietary components, synthesis and absorption of micronutrients, metabolism of toxins [3], protection against pathogens, and immune system regulation [2].

The gut of a newborn is sterile but is immediately colonised by bacteria [5]. The microbiome then increases in diversity and complexity within the first few years of life to a level similar to that of an adult [2], with environmental influences causing vast inter-individual variation in intestinal flora composition [6]. In general, there are over 1000 microbial species [2], with most bacteria being classified as one of two phyla, Bacteroidetes or Firmicutes [7]. However, gut microflora is susceptible to dietary changes throughout the life course [5]. Most notably, energy dense, high fat, high sugar and low fibre diets may result in an increase in intestinal pH, a greater number of Firmicutes and a decrease in the presence of Bacteroidetes [8]. Such disruption of microbial communities in the gut is termed dysbiosis, a state that has been associated with adverse health. It has been reported from animal studies that obese mice display dysbiosis [7], therefore it is hypothesised that  manipulation of the gut microbiome could alter energy intake and metabolism, and consequently reduce adiposity. The three main methods used to alter gut microflora are prebiotics, probiotics and synbiotics.

Prebiotics are described as non-digestible carbohydrates that change the composition and/or activity of gut microflora [9] in a way that benefits host health [4]. They should be resistant to gastric acid, hydrolysis by host enzymes and absorption in the upper gastrointestinal (GI) tract [9], but should be a selective substrate for a limited number of colonic bacteria [2]. The main prebiotics used are inulin type fructans, which are fructose based oligomers and polymers, indigestible to endogenous enzymes due to their β(2→1) glycosidic linkages [10]. The two forms are inulin, which is extracted from chicory root [11] and has an average degree of polymerisation of 10-12, and oligofructose (FOS), which is synthetically produced or obtained from partial hydrolysis of inulin, and has an average degree of polymerisation of 4 [10]. In addition to their use as prebiotics, these fructans may have a role in food reformulation as inulin can act as a fat replacer due to its ability to stabilise water into a creamy structure and replicate the mouthfeel of fat, and FOS can be used as a natural sweetener. They can therefore be used to produce low fat and low sugar products, that are also low in energy due to their calorific values of only 1kcal/g and 1.5kcal/g respectively [11].

A double-blind randomised controlled trial conducted by Hume et al. investigated the effect of prebiotic supplementation in overweight and obese children on appetite regulation and energy intake [1]. This review will discuss their conclusions within wider research to determine whether regular consumption of prebiotics should be promoted as a weight loss strategy, and if the use of inulin and FOS in ‘healthier’ alternatives to current high energy density foods could confer benefits that extend beyond simply reducing their calorie, fat or sugar content.

Method

Study population

Overweight or obese (BMI ≥85^th percentile) 7-12 year olds were recruited to a 16 week double blind randomised control trial.

Study design

Subjects randomly assigned to the prebiotic, consuming 8g/day of FOS-enriched inulin or the placebo group, consuming 3.3g/day maltodextrin. The powders were prepared in 250ml water and taken 15-30 minutes before dinner. Dosage was gradually increased for the first 2 weeks.

Own meals were prepared throughout the study and usual amount of physical activity was maintained, being assessed at baseline, week 8 and week 16.

Assessment of outcomes

3 day weighed food records were completed at baseline, week 8 week 16. Energy intake was measured at an ad libitum breakfast buffet at baseline and week 16.  A visual analogue scale (VAS) was used at baseline and week 16 before and after the ad libitum breakfast buffet to assess appetite. The Children’s Eating Behaviour Questionnaire (CEBQ) was completed at baseline, week 8 and week 16.

Fasting blood samples taken at baseline and week 16 were analysed for GLP-1, PYY, ghrelin, GIP, leptin, adipokines, adiponectin and resistin. Body weight changes were assessed using BMI z score.

Statistical analysis

Data analysis was conducted on an intent-to-treat basis and per-protocol basis. Differences between groups at baseline were assessed, as were changes in measures of appetite and BMI z score, with adjustment for age, sex and any baseline scores significantly different between groups.

Results

Study population

38 participants completed the study; 20 were in the prebiotic group and 18 in the placebo group. Blinding was reported as 50% of the prebiotic and 72% of the placebo guessing their group. There was no statistical difference in compliance between the two groups.

70% of participants in the prebiotic and 61% in the placebo groups reported no GI side effects. 61% of participants indicated that the prebiotic powder was very acceptable to consume on a day-to-day basis and 39% rated it moderately acceptable.

Results of statistical analysis

There was no treatment effect on energy intake at 8 or 16 weeks from the 3-day weighed food records. Prebiotic intake reduced energy intake in older participants (11-12 years) at the week 16 breakfast buffet compared to placebo (-113kcal vs +137kcal) but in younger participants energy intake increased in both groups compared to baseline.

There was no difference between groups in prospective food consumption before breakfast at week 16 nor prebreakfast fullness, but participants in the prebiotic group reported feeling fuller after breakfast at week 16 than baseline compared to placebo. The increase in the Satiety Responsiveness subscale of the CEBQ was similar between groups.

Prebiotic supplementation increased fasting ghrelin by 28% from baseline and adiponectin, but not GLP-1, PYY, GIP, insulin, leptin or resistin. A significant reduction in BMI z score of 3.8% in the prebiotic group was found in the per-protocol analysis. 

Discussion

The study by Hume et al. found that subjective appetite ratings were improved in overweight and obese children in response to prebiotic supplementation, as reflected in greater ‘fullness’ rating and lower prospective food consumption. This was translated into decreased energy intake in older but not younger participants [1]. There are few published studies that investigate the effect of prebiotics in relation to these factors, with the scope of the literature tending to be mostly limited to animal studies where potential mechanisms have been explored. Therefore, these will firstly be discussed.

Indigestible carbohydrates are fermented by gut bacteria to short chain fatty acids (SCFA) [6], which account for 10% of total energy intake in humans [4]. As a result, bacteria facilitate maximum extraction of calories from foods. Some of the SCFA, particularly butyrate, provide energy for colonocytes, yet those not used are transported across the basolateral membrane for ATP synthesis in muscle tissue [12] or cholesterol synthesis and lipogenesis in the liver [13]. This suggests negative effects of enhancing microflora activity by prebiotics.

However, SCFA are also more favourably associated with regulation of gut hormone release. They act as ligands for G-protein coupled receptors expressed by colonic epithelial cells [3]. Acetate activates GPR43 which increases differentiation of adipocytes and inhibits adipose lipolysis. In obese mice GPR43 is overexpressed, promoting adiposity, however the prebiotics have been demonstrated to blunt the overexpression, increasing lipolysis [5]. Additionally, activation of GPR41 by butyrate stimulates release of PYY [3], increasing intestinal transit time to promote nutrient absorption and acting as an anorexigenic peptide to induce satiety [14]. The SCFA profile in the gut reflects both the microbial composition and the substrate, with fermentation of inulin-type fructans primarily yielding butyrate [13], implying the potential for significant increases in PYY. Further to this, it has been shown that SCFA from prebiotics trigger GLP-1 release and decrease ghrelin [15], modulating appetite [16]. This could be described as the microbiota-gut-brain axis, where SCFA act as signalling molecules to the hypothalamus for energy regulation.

In addition to the role of SCFA, an increase in gut microbiota suppresses angiopoietin-like 4 expression, promoting fatty acid uptake and re-esterification in adipocytes due to an increase in lipoprotein lipase activity. Consequently, conventional mice have been shown to have 40% more body fat than germ free mice [17]. Nonetheless, this is generally associated with dysbiosis [18], with prebiotics having the potential to regulate the effect. Alterations in gut microbiota also influence the endocannabinoid system; activation in adipose tissue promotes angiogenesis [7], yet prebiotics in obese mice cause its inhibition [5]. Moreover, prebiotics enhance activity of AMP-activated protein kinase in the liver and muscle, upregulating fatty acid oxidation [4]. Similarly, microbiota composition affects cholesterol metabolism, with an increase in the deconjugation of bile acids, forming more readily excretable free bile acids [3]. This stimulates further bile acid synthesis in the liver, for which cholesterol is a precursor. Consequently, there is a reduction in hyperlipidemia, a major risk factor for obesity associated comorbidities.

Finally, obesity is often classed as a low-grade inflammatory condition, with an increase in oxidative stress predisposing an individual to features of the metabolic syndrome, particularly insulin resistance. It has been suggested that prebiotics may decrease secretion of pro-inflammatory cytokines such as IL-6, CRP and TNF-α [4]. The proposed mechanism involves the release of endotoxins in the form of lipopolysaccharides (LPS) by Gram -ve bacteria. The LPS can be transported by chylomicrons from the intestines into circulation where they induce systemic inflammation by binding to CD14 and TLR4 on immune cells, stimulating the release of cytokines [3]. Colonocytes act as a barrier to LPS release, with its integrity maintained by tight junctions. This regulates the passage of nutrients, antigens and microbes into the bloodstream [2]. However, a high fat diet is likely to alter the distribution and expression of ZO-1 and occludin tight junction proteins [5], increasing intestinal permeability, at the same time as increasing the ratio of Gram -ve to Gram +ve microbes in the colon [3]. Prebiotics frequently increase Bifidobacteria [19], which improves gut barrier function, and stimulate GLP-2 release by SCFA, improving the integrity of tight junctions [3]. This reduces the potential for LPS translocation, metabolic endotoxemia and the resulting inflammatory response.

Despite the reported biological effects of prebiotics, there is less evidence that exemplifies their role in reducing obesity and appetite within human trials. As with the study by Hume et al., lower post prandial prospective food consumption has been observed in subjects consuming 6g of inulin yoghurt for breakfast compared to the control, yet these changes did not translate to a decrease in energy intake [10]. However, this trial was short, at only 8 days duration, whereas a meta-analysis reported studies featuring reductions in energy intake to last longer than 2 weeks [20]. For example, Bosscher reported an overall daily reduction in energy intake in those consuming FOS at breakfast and dinner for 2 weeks compared to those the placebo [11], although the reliability of the conclusions from this study are limited due to only 10 subjects being included.

The reduction in BMI irrespective of changes in energy intake at the ad libitum lunch in the study by Hume et al. is a further conclusion inconsistently reflected in the wider literature. A trial in similar aged subjects observed no difference in BMI for age z score after 12 weeks of FOS consumption [21], yet another found greater weight loss, particularly in fat mass, in overweight or obese subjects consuming 21g of FOS per day compared to the placebo [22]. This result may be attributed to the 13% increase in PYY concentrations and lower ghrelin concentrations [22], a combined effect that is likely to induce satiety and reduce food intake. In contrast to the lack of change in gut hormone level observed by Hume et al., one study found an increase in PYY and GLP-1 concentrations, with the GLP-1 increase correlating with breath hydrogen excretion, suggesting it to be a consequence of enhanced fermentation in response to the prebiotic [23]. It has been said that a longer duration of study is more likely to observe reductions in body weight [20], therefore many of the differences in the literature may be attributable to heterogeneity in design, with great variation in length, age group and timing of consumption. In addition, dose may be a critical factor influencing the extent of the benefits experienced as it has been shown for 16g/day of FOS to increase PYY, GLP-1 and reduce energy intake greater than a dose of 10g/day [24].

Finally, regardless of physiological benefits, use of prebiotics is likely to be influenced by experience of adverse side effects. Many studies report no significant difference in GI symptoms such as diarrhoea, bloating and flatulence following regular prebiotic consumption [25][10], as was also the case in the study by Hume et al.. However, Parnell and Reimer found that subjects in the FOS group had negative side effects 45% of the time that would deter them from continuing the treatment [22]. It would therefore be essential to consider this when making recommendations for prebiotic use.

 

Impacts

It has been discussed that there is a significant amount of evidence and a biological plausibility for the role of prebiotics in appetite regulation and obesity, particularly because of the increase in anorexigenic hormone secretion stimulated by SCFA. However, due to the lack of stool analysis in human studies for changes in gut microbiota [20], it may be difficult to assess whether such results are due to the prebiotic or solely result from its high fibre content [2], where an increase in SCFA production results from a greater availability of substrates for bacterial fermentation [4]. Nonetheless, it is likely to be  beneficial to frequently consume foods containing prebiotic components such as chicory, onion, garlic, asparagus, leek, bananas and tomatoes [26] to promote a favourable gut microbial composition. This would also be likely to increase the quantity of dietary fibre consumed, which is proven to have a role in appetite regulation.

In addition, it could also be concluded that inulin-type fructans should not be included in ‘healthier’ food products purely for appetite suppressing functions, but that there may be potential for the effects that result from the reduction in calorie, fat and sugar content to be augmented by greater induction of satiety, whether this be as a direct consequence of its prebiotic nature or purely the higher fibre content of the product. When choosing the appropriate replacement ingredient, it would be essential to minimise risk of adverse side effects. Although shorter chain length inulin-type fructans are fermented more rapidly in the colon, longer chain inulin is thought to be better tolerated [10]. Consequently, a compromise is likely to be required.

As a final note, it is important to appreciate that any recommendation to include prebiotics or prebiotic-containing foods as a regular part of the diet, may not be applicable to all individuals. Those with increased susceptibility to irritation within the GI tract are more likely to experience discomfort from their intake, therefore avoidance may in fact be suggested.

---

[1] Hume, M. P., Nicolucci, A. C. & Reimer, R. A. (2017). Prebiotic supplementation improves appetite control in children with overweight and obesity: a randomized controlled trial.

[2] Brahe, L. K., Astrup, A. & Larsen, L. H. (2016). Can We Prevent Obesity-Related Metabolic Diseases by Dietary Modulation of the Gut Microbiota? Advances in Nutrition, 7 (1), 90-101.

[3] Gerard, P. (2016). Gut microbiota and obesity. Cellular and Molecular Life Sciences, 73 (1), 147-162.

[4] Nicolucci, A. C. & Reimer, R. A. (2017). Prebiotics as a modulator of gut microbiota in paediatric obesity. Pediatric Obesity, 12 (4), 265-273.

[5] Delzenne, N. M., Neyrinck, A. M., Backhed, F. & Cani, P. D. (2011). Targeting gut microbiota in obesity: effects of prebiotics and probiotics. Nature Reviews Endocrinology, 7 (11), 639-646.

[6] Scarpellini, E., Campanale, M., Leone, D., Purchiaroni, F., Vitale, G., Lauritano, E. C. & Gasbarrini, A. (2010). Gut microbiota and obesity. Intern Emerg Med, 5 Suppl 1, S53-6.

[7] Li, D. T., Wang, P., Wang, P. P., Hu, X. S. & Chen, F. (2016). The gut microbiota: A treasure for human health. Biotechnology Advances, 34 (7), 1210-1224.

[8] Barczynska, R., Bandurska, K., Slizewska, K., Litwin, M., Szalecki, M., Libudzisz, Z. & Kapusniak, J. (2015). Intestinal Microbiota, Obesity and Prebiotics. Polish Journal of Microbiology, 64 (2), 93-100

[9] Yoo, J. Y. & Kim, S. S. (2016). Probiotics and Prebiotics: Present Status and Future Perspectives on Metabolic Disorders. Nutrients, 8 (3).

[10] Heap, S., Ingram, J., Law, M., Tucker, A. J. & Wright, A. J. (2016). Eight-day consumption of inulin added to a yogurt breakfast lowers postprandial appetite ratings but not energy intakes in young healthy females: a randomised controlled trial. British Journal of Nutrition, 115 (2), 262-270.

[11] Bosscher, D. (2006). Limit food intake by inducing satiety. Agro Food Industry Hi-Tech, 17 (5), VI-VII.

[12] Rastall, R. A. & Gibson, G. R. (2015). Recent developments in prebiotics to selectively impact beneficial microbes and promote intestinal health. Current Opinion in Biotechnology, 32, 42-46.

[13] Verbeke, K. A., Boobis, A. R., Chiodini, A., Edwards, C. A., Franck, A., Kleerebezem, M., Nauta, A., Raes, J., van Tol, E. A. F., Tuohy, K. M. & Force, I. E. P. T. (2015). Towards microbial fermentation metabolites as markers for health benefits of prebiotics. Nutrition Research Reviews, 28 (1), 42-66.

[14] Sanz, Y. & Santacruz, A. (2010). Probiotics and Prebiotics in Metabolic Disorders and Obesity. Bioactive Foods in Promoting Health: Probiotics and Prebiotics, 237-258.

[15] Sanchez, M., Panahi, S. & Tremblay, A. (2015). Childhood Obesity: A Role for Gut Microbiota? International Journal of Environmental Research and Public Health, 12 (1), 162-175.

[16] Kovatcheva-Datchary, P. & Arora, T. (2013). Nutrition, the gut microbiome and the metabolic syndrome. Best Practice & Research in Clinical Gastroenterology, 27 (1), 59-72.

[17] Delzenne, N. M. & Cani, P. D. (2010). Nutritional modulation of gut microbiota in the context of obesity and insulin resistance: Potential interest of prebiotics. International Dairy Journal, 20 (4), 277-280.

[18] Shen, J., Obin, M. S. & Zhao, L. P. (2013). The gut microbiota, obesity and insulin resistance. Molecular Aspects of Medicine, 34 (1), 39-58

[19] Bindels, L. B., Delzenne, N. M., Cani, P. D. & Walter, J. (2015). Towards a more comprehensive concept for prebiotics. Nature Reviews Gastroenterology & Hepatology, 12 (5), 303-310.

[20] Kellow, N. J., Coughlan, M. T. & Reid, C. M. (2014). Metabolic benefits of dietary prebiotics in human subjects: a systematic review of randomised controlled trials. British Journal of Nutrition, 111 (7), 1147-1161.

[21] Liber, A. & Szajewska, H. (2014). Effect of oligofructose supplementation on body weight in overweight and obese children: a randomised, double-blind, placebo-controlled trial. British Journal of Nutrition, 112 (12), 2068-2074.

[22] Parnell, J. A. & Reimer, R. A. (2009). Weight loss during oligofructose supplementation is associated with decreased ghrelin and increased peptide YY in overweight and obese adults. Am J Clin Nutr, 89 (6), 1751-9.

[23] Cani, P. D., Lecourt, E., Dewulf, E. M., Sohet, F. M., Pachikian, B. D., Naslain, D., De Backer, F., Neyrinck, A. M. & Delzenne, N. M. (2009). Gut microbiota fermentation of prebiotics increases satietogenic and incretin gut peptide production with consequences for appetite sensation and glucose response after a meal. American Journal of Clinical Nutrition, 90 (5), 1236-1243.

[24] Verhoef, S. P. M., Meyer, D. & Westerterp, K. R. (2011). Effects of oligofructose on appetite profile, glucagon-like peptide 1 and peptide YY3-36 concentrations and energy intake. British Journal of Nutrition, 106 (11), 1757-1762.

[25] Beserra, B. T. S., Fernandes, R., do Rosario, V. A., Mocellin, M. C., Kuntz, M. G. F. & Trindade, E. (2015). A systematic review and meta-analysis of the prebiotics and synbiotics effects on glycaemia, insulin concentrations and lipid parameters in adult patients with overweight or obesity. Clinical Nutrition, 34 (5), 845-858.

[26] Brown, L., Poudyal, H. & Panchal, S. K. (2015). Functional foods as potential therapeutic options for metabolic syndrome. Obesity Reviews, 16 (11), 914-941.