10 GENERAL DISCUSSION
10.3 LIMITATIONS AND PRACTICAL IMPLICATIONS OF THIS WORK AND RECOMMENDATIONS FOR FURTHER RESEARCH RECOMMENDATIONS FOR FURTHER RESEARCH
Even though the two studies included in this thesis showed inflammation-diminishing potential after betaine supplementation, further studies to assess the mechanism and efficacy of supplemental betaine to reduce obesity-related tissue inflammation in vivo are warranted. The future findings may bring new insight into the potential effect of betaine in obese individuals or those with MetS and might confirm these preclinical findings of reduced inflammatory state after betaine consumption.
The fact that the preadipocytes were obtained from only one single donor can be considered as a limitation in Study I. Nevertheless, this approach was knowingly chosen to minimize the number of variables in the study and to focus on the effects of betaine alone. The BMI range 25.0—29.9 kg/m2 was initially selected, and the company (Zen-Bio, Inc.) provided the specific preadipocyte batch from a donor matching the selection criteria.
While conducting the Study II, it was clearly noticed that the proper dose of polydextrose should be pre-tested before running a full-scale mouse trial, since the tolerability of polydextrose can vary in different animal models. In Study II, the initial polydextrose dose of 6.66% (w/v) needed to be adjusted two days after the initiation of the trial due to the adverse events (loose stools) reported in the GI functions of the C57BL/6J mice receiving polydextrose in drinking water. After halving the dose to 3.33% (w/v), the animals were no longer symptomatic. In fact, diarrhea has been reported after the administration of high doses of polydextrose in chronic animal studies (Burdock & Flamm, 1999).
One limitation of Study III was the use of inoculum containing only one strain of Bacteroides spp. The Bacteroidetes phylum includes a varied range of bacterial strains, which contain diverse properties.
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Therefore, the administration of only one single strain of Bacteroides spp. over a relatively short time proved not to be enough to counteract the effects induced by high-fat feeding.
If postprandial effects of dietary fibers, or other supplementary food ingredients, are to be studied in obese population, certain consideration needs to be taken. When the satiating effect of polydextrose was recently studied in normal and overweight individuals, it was noticed that the ability of overweight individuals to suppress appetite and react reliably to normal food stimuli was deranged, and this affected the variance in the results (Ibarra et al., 2017). According to this knowledge, it is now recommended to recruit as homogenous a population as possible and preferably not to include severely obese individuals in a study to investigate individual sensations of appetite.
Even though polydextrose has demonstrated to reduce energy intake during a subsequent meal and this effect was shown to be dose-dependent (Ibarra et al., 2015), its full mechanism of action is not yet completely understood. This may, however, be related to the potential ability of polydextrose to provide sustained energy during the post-absorptive phase when offered at breakfast. This was demonstrated recently in a randomized, placebo-controlled clinical study involving normal and overweight participants (Ibarra et al., 2017). In the same set-up, polydextrose also increased the postprandial GLP-1 levels, similarly as seen in Study IV. This would imply that while providing more sustained energy after the meal, polydextrose could potentially hinder further energy consumption by reducing appetite and hunger sensation through GLP-1 signaling. In Study IV, the subjective feelings of hunger were significantly decreased by polydextrose during the satiety period, i.e., between meals. The method that was used in Study IV to evaluate the subjective feelings of appetite—i.e., dividing the appetite ratings into ‘satiation’ and ‘satiety’ periods—was developed while reporting the clinical study results. This particular method has been used ever since when reporting changes in appetite sensations (Ibarra et al., 2016, Ibarra et al., 2017). In fact, the ability of polydextrose to reduce the sensation of hunger during the post-meal satiety period was also looked at in a meta-analysis (Ibarra et al., 2016), but it was showing only a trend.
A recent clinical study conducted in overweight and obese population showed promising effect with polydextrose in combination with a probiotic, Bifidobacterium animalis spp. lactis 420, in reducing the energy intake during a 6-month intervention (Stenman et al., 2016). However, in this combination group, the body weight and body fat mass did not differ statistically from baseline, suggesting that polydextrose, in combination with a probiotic, could normalize the energy balance by reducing energy intake to a level that maintains, rather than increases, body fat mass (Stenman et al., 2016). Those results imply that polydextrose has an appetite-suppressive effect, even though the study did not include a postprandial evaluation of appetite sensation. Future clinical studies may confirm the tendency of polydextrose to reduce hunger in the satiety period.
Care needs to be taken when extrapolating results from preclinical animal trials to humans. There are various factors that can dictate how well the results can be translated between species. Among these factors affecting the translatability are, for example, the type of intervention, suitability of the chosen animal model, investigational product’s administration schemes and the complexity of the modelled disease. It is widely known that the data generated from preclinical animal studies often appear to be associated with high risk of bias. Therefore, multiple animal studies—preferably with different species and utilizing several models—are usually needed to reproduce the results, especially before any firm claims of the effectiveness of the study product are made.
10.4 CONCLUSIONS
Various models, such as in vitro model mimicking obese conditions, two separate DIO animal models and a post-prandial clinical trial with a high-fat diet, were utilized in this thesis to study the metabolic changes and satiety effect induced by the consumption of specific food ingredients. In addition, several novel analysis methods were used to study the relevant metabolic parameters.
The results of this thesis show that the impact of high-fat feeding and specific food ingredients is noticeable at the metabolic and genetic level. Tissue hypoxia, mimicking the conditions inside enlarged adipose tissue, caused notable inflammation in adipocytes—a phenomenon that further assists in the development of other obesity-related chronic disturbances. High-fat feeding led to increased body weight
and changes in postprandial glucose concentration in a rodent model. However, the consumption of supplementary food ingredients may assist in reversing obesity-related metabolic changes. This was seen in cultured adipocytes, in which betaine—a small amino acid derivative and natural osmolyte—
diminished the inflammatory status. In the white adipose tissue of DIO mice, betaine alleviated the inflammatory state, increased tissue carnitine levels and improved the metabolic flexibility. The supplementation of polydextrose, a dietary fiber, and another soluble carbohydrate, lactitol, resulted in modified satiety hormone release in humans and animals, respectively. Polydextrose showed hunger-reducing effects in a clinical study by increasing the postprandial plasma concentration of satiety hormone GLP-1 and by reducing the post-meal hunger sensation. In a high-fat-enriched rat model, lactitol increased the release of gastrointestinal satiety hormone peptide YY. Polydextrose and lactitol both reduced the mean body weight, plasma insulin and triglyceride levels in rats fed the high-fat diet.
This study confirms the previously reported harmful metabolic effects of the Western diet rich in fat and suggests that specific bioactive food ingredients, such as polydextrose and lactitol, may provide beneficial means to regulate postprandial metabolism and weight management. Polydextrose and lactitol improved postprandial triglyceride metabolism, which is considered as an important risk factor for chronic disease development, especially in the obese population. In addition, both polydextrose and lactitol positively influenced on the sensation of satiety and the overall postprandial health, and thus, they may function as potential supplements for weight management purposes. This study also shows that betaine supplementation ameliorated obesity-related adipose tissue inflammation in vivo and increased the production of carnitine metabolites in mouse white adipose tissue. Mechanistically, it was suggested that changes in intracellular levels of carnitines and lipids may be driving the metabolic improvements observed with betaine. Thus, betaine may be a potential ingredient to ameliorate systemic inflammation in obesity-related conditions.
The results presented in this thesis give new scientific information on the mechanisms behind the metabolic consequences of high-fat feeding and the role of specific food ingredients in overcoming the obesity-related metabolic stress. The results of this thesis may benefit in the development of efficient and safe food ingredients or dietary supplements for weight management in the future.
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Based on the research questions, these are the most significant findings from the Studies I to IV:
I: Betaine reduced the human adipose tissue inflammation in vitro.
II: The animals that were fed the high-fat and low-fat diet differed in their adipose tissue metabolite profile. Betaine supplementation increased the carnitine metabolite expression and correlated with lower inflammatory marker expression in the mouse adipose tissue.
III: Polydextrose and lactitol affected the body weight gain, plasma triglycerides and insulin secretion in rats fed the high-fat diet. Lactitol increased the satiety hormone PYY secretion.
Lactitol and polydextrose may both provide additional means to regulate postprandial metabolism and weight management.
IV: Polydextrose increased the postprandial secretion of the satiety hormone GLP-1 and reduced the sensation of hunger after a high-fat meal in a clinical trial. Polydextrose ameliorated the postprandially elevated lactic acid levels in human plasma.
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