Post-weaning metabolic programming by dietary monosaccharides

Nutrition in early life can have lasting effects on metabolism: nutritional programming. Pregnancy and lactation are well established critical periods of development impacting lifelong health, but development continues after these periods. Therefore, nutrition in later periods, such as the weaning and (early) post-weaning period, may also induce lasting metabolic effects. After lactation, when an infant relies solely on milk for nutrition, the weaning period commences. In this phase, solid foods are gradually introduced; it is a period of great diversification of the diet. The transition to solid foods introduces a major change in carbohydrate exposure. The main carbohydrate in the lactation period is lactose, a dimer of glucose and galactose. Gradually, a variety of di- and polysaccharides are introduced that are mainly glucose based, but can also contain fructose.

Whether or not exposure to different types of monosaccharides at weaning has lasting effects on metabolism and metabolic health is not known. The aim of this thesis was to establish whether the post-weaning period is a critical developmental period in which exposure to different types of monosaccharides can lead to programming of adult metabolic health. We were particularly interested in galactose and fructose. Our interest in galactose was because this sugar almost disappears from the diet after weaning. Our interest in fructose was because it appears in the diet at weaning, and is suggested to be more detrimental to health compared to glucose, which is always present and is taken as a reference. Thus we compared effects of the dietary monosaccharides fructose and galactose, to those of glucose. Analyses were focussed on body composition and metabolic health. A mouse model for nutritional programming was used. In this model, newly weaned C57BL/6JRccHsd mice were fed with diets differing in monosaccharide content for three weeks. Afterwards, all mice were fed an obesogenic high-fat diet (HFD) for nine weeks.

Chapter 2 describes a mouse study where the effects of fructose in the post-weaning diet on later life health were compared to the effects of glucose alone. Body weight, body composition, and organ weights were similar in both groups after the nine-week HFD period. Indirect calorimetry analyses indicated that respiratory exchange ratio’s (RER’s), energy expenditure, activity, and metabolic flexibility were not different between fructose- and glucose fed animals, neither directly, nor when on the HFD. Serum insulin concentrations were significantly lower in females fed fructose post weaning, than in females fed glucose post weaning, while serum insulin concentrations were not significantly affected in males. From these data we concluded that fructose and glucose are comparable in their direct effect, and there is no adverse programming of fructose compared to glucose in the post-weaning period.

For additional insight in metabolic effects of fructose, the direct effects of fructose, fructose and glucose in a 1:1 ratio, and glucose, were studied in adult male and female mice on a moderate HFD (Chapter 3). Mice on the HFD with fructose had slightly lower energy intakes overall. Body weight was not affected by the monosaccharide composition of the diet, and nor were plasma insulin concentrations. Hepatic gene expression analyses showed minor upregulation of hexokinase expression in fructose fed compared to glucose fed males, without significant alterations in sugar transporters, or glycolysis- or de novo lipogenesis-related enzymes. Gene expression in the liver and intestine of female mice showed no consistent differences. Overall, our physiological data indicate that isocaloric dietary fructose does not result in more adverse physiological effects than a diet containing glucose-fructose or glucose.

Next, we examined the effects of galactose. The programming effect of post-weaning galactose and glucose in a 1:1 ratio mimicking the milk sugar lactose was compared to the effect of glucose alone (Chapter 4). In females, body weight and fat mass gain on the HFD were significantly lower in animals fed galactose post weaning. These females had lower circulating serum insulin concentrations, lower adipose depots weights, with a tendency towards smaller adipocytes in gonadal white adipose tissue, and altered insulin-signalling-related gene expression. Although food intake was significantly higher in the post-weaning period, and significantly lower in the HFD period, no effects in hypothalamic gene expression on food-intake related neuropeptides or leptin signalling were found. In males, fat mass development was not affected by post-weaning monosaccharides. Concluding, this study showed that replacing glucose with galactose in a post-weaning diet, in a 1:1 ratio (mimicking lactose), had beneficial metabolic programming effects in female mice, over glucose alone.

Chapter 5 shows the direct effect of post-weaning galactose and glucose in a 1:1 ratio compared to glucose alone in females. Females on the galactose diet had a higher food intake and a two-fold higher drink intake than females on the glucose diet. High-performance anion-exchange chromatography analyses indicated galactose presence in the urine of females on the galactose diet. Indirect calorimetry measurements showed no significant effect on energy expenditure or average RER, but maximal RER in the dark phase was lower in females on the galactose diet. Serum insulin concentrations and hepatic triglyceride levels were lower in females on the galactose diet. Transcriptomic analysis of the liver indicated that the gene expression profiles in metabolic pathways were not significantly affected by the diet, but inflammation-related gene expression profiles were slightly downregulated in galactose-fed females. Concluding, replacing part of glucose with galactose in the post-weaning diet reduces hepatic TG content and hepatic inflammation, implying immediate beneficial effects.
In a second mouse study examining the metabolic programming effects of post-weaning galactose in females, presented in Chapter 6, no differences in body weight gain and fat mass gain were seen in the HFD period. The oral glucose tolerance test showed no difference in glucose tolerance, but indicated that circulating plasma insulin concentrations were relatively more increased in females fed galactose as analysed by the insulin incremental area under the curve. At the end of the study, no significant differences were found in circulating insulin concentrations, AKT phosphorylation and insulin-related gene expression in gonadal white adipose tissue. Concluding, replacing part of glucose with galactose in the post-weaning diet did not beneficially affect body composition or insulin signalling in adult female mice in an obesogenic environment in this study. Differences between the results of Chapter 4 and Chapter 6 may be due to differences in the experimental conditions.

In Chapter 7 the findings of this thesis are discussed. Concluding, this thesis shows that the post-weaning period may be susceptible for nutritional programming by dietary monosaccharides, in particular by galactose. The effect is modest, and inconclusive as two studies yielded different outcomes. Although there are some clues that insulin signalling is involved, it is so far not clear what the main mechanism is. Females seem to be more susceptible to programming by monosaccharides in the post-weaning period than males. Our results suggest that there are no major differences in metabolic programming by fructose and glucose in the post-weaning period. Also no major differences were seen between fructose and glucose in adult mice. Extending the period of galactose intake after weaning may be beneficial as it seems to protect against liver inflammation.