In this study, the effect of high-fat diet and food ingredients on obesity-related metabolic stress was investigated. The research questions were addressed using various models, such as in vitro and in vivo models, and a clinical postprandial study was conducted.
Main findings of these studies according to each pre-set research question were the following:
9.1 RESULTS TO RESEARCH QUESTION 1
Studies I and II investigated if obesity-related adipose tissue inflammation can be affected by dietary modifications and specific food ingredients.
Study I utilized a human adipocyte stress model to test the effect of betaine on the inflammatory response of adipocytes in vitro. The low-oxygen condition imitated the inflamed obesity-associated adipose tissue. Results showed that hypoxic cell culture conditions induce tissue-level inflammation, as observed by increased mRNA expression of pro-inflammatory adipokines IL-6 and leptin. However, pre-treatment of the mature adipocytes with betaine (250 µmol/l) was able to invert this inflammatory response. Study I showed that betaine (250 µmol/l) may reduce inflammation in adipose tissue under hypoxia. Thus, betaine could potentially help to reduce systemic inflammation and the risk for other obesity-related conditions.
The effects of high-fat diet, polydextrose and betaine were observed in vivo with a DIO mouse model.
The most significant differences in adipokine gene expression were noted between the low-fat and high-fat control groups. Study IIconcluded that adipose tissue inflammation was present in animals fed the high-fat diet. The high-fat diet significantly increased the inflammatory state, expressed by the markers IL-6 and leptin derived from subcutaneous and visceral adipose tissues. In subcutaneous adipose tissue, 3.33% (w/v) polydextrose alone decreased adiponectin expression, and in combination with betaine it increased FIAF expression in animals fed the high-fat diet. In visceral adipose tissue of the high-fat-diet-fed mice, the two ingredients together increased leptin mRNA levels, but the administration of betaine or polydextrose alone did not have a similar effect. The effect of betaine was most notable in IL-6 mRNA expression, which was markedly lower in the betaine-supplemented animals in both adipose tissue depots compared with the high-fat control group. According to the alternative Hubert’s robust PCA approach, 1% (w/v) betaine supplementation correlated with lower inflammatory marker expression in both adipose tissue depots. This was in accordance with the in vitro findings of Study I.
9.2 RESULTS TO RESEARCH QUESTION 2
Studies I, II, III and IV investigated on how dietary modifications and supplementary food ingredients can affect the tissue and whole-body metabolism.
Study I reported the effect of betaine supplementation on specific adipokine protein secretion by human adipocytes undergoing hypoxic stress. The secretion of selected adipokines into the cell culture medium was measured. Hypoxic conditions clearly increased the release of leptin and IL-6 proteins. The highest dose of betaine (500 μmol/l) induced a slightly lower leptin secretion after the cells were kept under hypoxia for 20 hours.
Study II summarized the metabolic changes observed in subcutaneous and visceral adipose tissue in relation to high-fat feeding and supplemented food ingredients, betaine and polydextrose. The high-fat diet significantly altered the metabolite profile of several carnitine and lipid species by down-regulating them. Especially, the phosphatidylcholines and phosphatidylethanolamines were affected. Also, the occurrence of several other metabolites, such as those related to amino acid metabolism and biosynthesis of proteins, was decreased in animals fed only the high-fat diet. Most differences that were observed between the low-fat and high-fat diet groups were driven by the changes in lipid and carnitine syntheses pathways. 1% (w/v) betaine supplementation increased the markers of betaine synthesis—primarily
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betaine and butyrobetaine—found in adipose tissue, as reported similarly in liver and muscle tissue.
Some markers of carnitine metabolism were increased in adipose tissue due to betaine consumption but not to the same extent as seen in liver and muscle. The metabolic changes observed in betaine-receiving animals were driven by the altered levels of betaine, carnitine and lipid species. The observed changes in betaine synthesis were in accordance with other studies conducted in fat-induced animal models (Hanhineva et al., 2013, Han et al., 2017). The supplemented 3.33% (w/v) polydextrose did not induce any significant changes in the levels of betaine or its derivatives in adipose tissue. The effects of polydextrose on other identified metabolites, such as carnitines and lipids, were moderate in subcutaneous adipose tissue and virtually non-existing in visceral adipose tissue.
Study III was done to assess the metabolic effects of indigestible dietary carbohydrates, polydextrose and lactitol in rats fed the high-fat diet. The use of a combination of intestinal Bacteroides species, B.
thetaiotaomicron, and two structurally and metabolically different carbohydrates was a novel approach. In this postprandial feeding trial, Wistar rats were fed high- or low-fat diet and supplemented with an inoculum of B. thetaiotaomicron, 1010 bacteria/animal/day. In addition, the animals were orally administered with or without polydextrose (2 g/animal/day) or lactitol (1.6—2 g/animal/day) for 8 days.
The high-fat diet had detrimental effects to glucose metabolism and body weight. Polydextrose and lactitol, in conjunction with B. thetaiotaomicron, decreased the body weight gain and plasma triglyceride concentrations in respective rat populations. However, even though reduced levels of Bacteroidetes have previously been linked with increased risk of obesity (Turnbaugh et al., 2008), in this study, the microbial supplementation alone did not counteract the detrimental effect of high-fat diet on body weight or plasma triglycerides. Supplemented B. thetaiotaomicron affected only the fecal energy value, which was increased, indicating that more food-borne energy was transported through the gut without being utilized by the body. Study III suggests that the two supplemental indigestible carbohydrates may provide additional means to regulate postprandial metabolism, especially in weight management and attenuation of postprandial triglyceridemia.
The effect of dietary polydextrose on the postprandial metabolic parameters in obese individuals was evaluated in Study IV. An acute, postprandial human clinical study including a high-fat challenge reported significant effects of 15 g of polydextrose on plasma lactate concentration. The supplemental polydextrose lowered the postprandial plasma lactate levels, which tend to be high in obese subjects due to the increased lactate production within the enlarged adipose tissue (DiGirolamo et al., 1992). This attenuation of lactate with polydextrose could be associated with a lower post-meal inflammatory status;
however, the inflammatory response was not measured in this study to support this conclusion. High-fat meals tend to induce postprandial inflammation, while dietary fiber intake in general is associated with a reduced, low-grade inflammation (Calder et al., 2011). Altogether, Study IV brought new insight into the obesity-linked postprandial metabolism and the potential preventive use of dietary fibers.
9.3 RESULTS TO RESEARCH QUESTION 3
In order to study the ability of dietary modifications—such as high-fat feeding and supplemental food ingredients—to affect satiety hormone responses in animals and humans, Studies III and IV were conducted.
Study III showed that high-fat feeding of Wistar rats led to increased weight gain and elevated blood glucose levels. The oral administration of B. thetaiotaomicron (1010 bacteria/animal/day) alone did not counteract the effects of the high-fat diet, even though reduced levels of Bacteroidetes have been linked with increased risk of obesity (Turnbaugh et al., 2008) and high abundance of Bacteroides has been associated with lean body weight (Turnbaugh et al., 2006). However, lactitol supplementation increased the satiety hormone PYY response in high-fat-diet-fed rats when administered alone or in combination with B. thetaiotaomicron. In addition, the acute insulin response decreased significantly in groups receiving supplemental polydextrose or lactitol.
The effect of 15 g of polydextrose on the postprandial satiety hormone release in obese individuals was evaluated in Study IV. This study demonstrated that polydextrose can enhance satiety hormone GLP-1 response after the simultaneous ingestion of a high-fat meal. Since it has been proposed that GLP-GLP-1 could abolish the postprandial rise in triglyceride concentrations (Meier et al., 2006), the effect of
polydextrose to increase the level of GLP-1 in obese individuals could be considered beneficial also in terms of postprandial dyslipidemia. At the time of the conduct of this study, there was not much clinical evidence on the GLP-1 effect after the use of polydextrose, although preclinical evidence was plentiful.
Since then, a similar result was repeated in a more recent acute clinical study with obese and normal-weight individuals (Ibarra et al., 2017). In Study IV, an additional observation was made regarding the age of the participants: it seemed that age could influence the postprandial 1 response. Lower GLP-1 concentrations were reported in participants older than 40 years old when compared to those aged 40 years or less. Reportedly, aging has been shown to reduce insulin and incretin (GLP-1) production in aged diabetics (Geloneze et al., 2014); however, also contradictory results have been presented.
9.4 RESULTS TO RESEARCH QUESTION 4
Finally, it was examined whether the subjective feelings of appetite can be affected by supplementary food ingredients, such as polydextrose. This effect was investigated in Study IV, in which the participants consumed a high-fat meal with or without 15 g of polydextrose. The Study IV hypothesized also that the supplemental polydextrose could have a beneficial effect on the postprandial metabolic parameters in obese participants. This was based on earlier observations found in a preliminary study conducted with obese participants (Tiihonen et al., 2010).
As a result, polydextrose significantly decreased the subjective feelings of hunger assessed during the post-meal satiety period. This was in accordance with previously reported results regarding the hunger sensation due to polydextrose supplementation (Hull et al., 2012). In addition, a marginal increase in the subjective feelings of satisfaction during the satiety period was observed in Study IV after polydextrose consumption when compared with the placebo. Although not confirmed in this study, it has been shown that polydextrose can help to maintain low postprandial blood glucose levels (Jie et al., 2000). As stated previously, food ingredients that help to moderate blood glucose levels are also likely to assist in suppressing hunger sensation. Even though polydextrose has demonstrated to reduce energy intake during a subsequent meal, its full mechanism of action is not yet completely revealed. Thus, polydextrose may offer an additional way of regulating inter-meal satiety and improving postprandial metabolism in obese subjects.
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