Child obesity has become a significant health issue in Korea. Along with the adaptation to a Western diet over the past decades, the dietary patterns of the Korean population, especially Korean children and adolescents, have gradually been changing. The Western diet includes not only a diet high in processed or fast foods, but also physical inactivity¹. Nowadays, many children spend a great deal of time being inactive. According to statistics, the average child spends approximately 4 hours each day watching television². In addition, they spend most of their spare time playing video or computer games and cellphone. This can further increase the physical inactivity in children. The other characteristic of a Western diet is the high dietary lipid and sugar content present in processed foods³. Although the impact of specific macronutrients on health is highly debated, it is evident that diets rich in fat and carbohydrates, with or without exercise, result in obesity. Because of their palatability and higher energy density than that of other macronutrients, high fat diets may lead to excessive energy intake and cumulative positive fat balance. Moreover, because of the lower dietary induced thermogenesis, high fat diets may also decrease energy expenditure^4,5. There is no apparent autoregulation between fat intake and fat oxidation. Fat oxidation levels are changed by the intake of other macronutrients⁶. On the other hand, carbohydrate oxidation shows strong autoregulation with carbohydrate intake because the capacity of glycogen storage is limited. The diet-induced thermogenesis associated with high carbohydrate diets would be higher than that associated with high fat diets because of glycogen storage and glucose stimulation of the sympathetic nervous system^7-10. The types of carbohydrate may also influence metabolic responses such as thermogenesis and substrate utilization because the nutritional properties of carbohydrates depend on their rate and extent of digestion and absorption in the small intestine¹¹.

In this study, we used two types of carbohydrates, sucrose and starch. Sucrose, the scientific name for sugar, is a disaccharide, which is a combination of two monosaccharides¹². Sucrose is often combined with other sugars in foods such as sweets and beverages. It can also be hidden in many processed foods. Starches, sometimes called complex carbohydrates, are polysaccharides found in rice, the staple food of Koreans, as well as breads, potatoes, and corn. Hence, the body has to first break down the links between each sugar subunit in order to use the energy of the sugar molecules in the starch. This causes rapid increases in blood sugar levels, which stimulates the release of a large amount of insulin. Insulin draws sugar into the cells to be stored as glycogen, leading to a subsequent drop in blood sugar, which often leads to further consumption of carbohydrates. This phenomenon is called 'blood sugar roller-coaster'¹¹. This triggers insulin resistance, where the cells become fatigued from constant high insulin signaling and no longer respond adequately to its release. Poor blood sugar regulation also stimulates the release of stress hormones, such as cortisol, which can contribute to weight gain as well as hypoglycemia¹³. In this sense, sucrose and starch may also have different effects on energy metabolism.

Although there is increasing interest in understanding the relationships between different types of dietary carbohydrates and appetite regulation, energy metabolism, body weight, and body composition, little is known about the impact of the consumption of different types of carbohydrates on metabolic responses, at rest and during exercise, in childhood. Most previous studies have focused only on the effects of diet^14-17 and exercise^18-20 on the changes in energy metabolism at rest and during exercise. There is limited information on the effects of simultaneous consumption of high fat and different types of carbohydrates (sucrose or starch), with or without exercise, on body weight, fat accumulation, and energy metabolism. Hence, the purpose of this investigation was to study the change in the fuels oxidized at rest and during exercise on a high fat-high sucrose and high fat-high starch diet, with or without exercise, in growing mice as one possible method to address child obesity.

Along with the adaptation to a Western lifestyle over the past decades, the dietary patterns of Korean children and adolescents have been gradually changing. A Western lifestyle includes not only a diet rich in lipids and sugar, but also physical inactivity. Although there is strong evidence that the amount and type of fat in the diet can have strong effects on metabolism, the type of carbohydrates influencing metabolic responses in a diet is also of great interest. Hence, the long-term goal of this study was to understand the relationship between type of carbohydrates, weight, and energy metabolism, and to prevent childhood obesity and related diseases. To achieve this, we used an open circuit calorimetry system to investigate the influence of a high fat diet with different types of carbohydrates (sucrose or starch) with/without exercise on energy metabolism at rest and during 1 h of exercise in mice.

Our results showed that high sucrose did not cause greater obesity than starch. Unexpectedly, there were no significant differences between the groups in the final body weights. It is consistent with a previous study by Sakamoto et al., (2012) that demonstrated no change in body weight following similar, prolonged periods of high sucrose feeding²⁹. A previous study on energy balance by Dulloo et al. (1985) indicated that differences in energy balance between sucrose and starch are small³⁰. Indeed, several reviews suggest that there is no evidence that an ad libitum diet with sucrose causes more weight gain than a diet with starch when total carbohydrate intake is the same^31,32. On the other hand, some studies showed weight gain^7,33 and an overall systematic review suggested a positive influence of starch on weight loss³⁴. However, it is not clear what caused the differences between these two types of diets. Only a few studies have directly compared the effect of dietary intake of sucrose and starch on energy metabolism. According to the results of the study by Maekawaa et al., (2017), high sucrose-fed mice displayed decreased body weight compared with mice on a diet of normal chow or high starch after 14 weeks of feeding³⁵. They explained that this was due to increased energy expenditure. In addition, Raben et al. (1997) compared the effect of an ad libitum high sucrose, high starch, and high fat diet in lean or post-obese women on energy expenditure³⁶. After 14 days, the 24 h energy expenditure was significantly increased on the sucrose diet compared with that of the other diets.

We then investigated the effect of the sucrose diet on energy metabolism using indirect calorimetry to compare each mouse after 10 weeks of experimental treatment. As expected, mice on a high fat-high sucrose diet consistently displayed a higher oxygen uptake and RER, indicating an increase in carbohydrate oxidation at rest compared with that of the Sta. diet. This implies both an initial switch to carbohydrates as the major energy source and continued adaptation to further increase carbohydrate use. However, in contrast to the results of the present study, Burchfield et al. (2018) showed that RER and oxygen uptake of mice on a high fat-high sucrose diet decreased with increasing time on the diet³⁷.

Fat oxidation in the Scu. diet was lower than that in the Sta. diet. However, there were no differences between the Suc. diet with exercise and the Sta. diet groups. This indicates that exercise helps to maintain the fat oxidation capacity in the Scu. diet. Although the differences did not result in significant weight reduction, total energy expenditure was increased in the Suc. diet with or without exercise. Small differences of just a few kcal/day can lead to substantial differences in body weight in the long term. Taken together, our findings indicate that energy metabolism over 24 h was more influenced by diet than by exercise.

The expression of some genes controlling energy homeostasis are regulated by epigenetic mechanisms that may play a role in body weight regulation. It is well recognized that sucrose stimulates the sympathetic nervous system and leads to an increase in brown adipose tissue (BAT) activity and mass^7,38,39. However, the pathway by which sucrose affects BAT activity and the effects of a chronic high sucrose diet on BAT, which plays an important role in energy expenditure, are not well understood. In addition, body weight and glucose metabolism is regulated by various hormones such as glucagon-like peptide 1 (GLP-1) and fibroblast growth factor 21 (FGF21), which contribute to the reduction in body weight by increasing energy expenditure^35,40,41. However, the expression and action of GLP-1 and FGF21 in mice chronically fed a high fat-high sucrose diet have not been fully investigated³⁵. Thus, further studies are required to confirm the optimal amount and type of dietary carbohydrates for energy metabolism.

During 1 h of exercise, we did not find any changes in oxygen uptake and energy expenditure in any of the experimental groups. In agreement with our results, most previous studies^42-44 have not found an effect of diet composition on total energy expenditure if energy intake was fixed. However, exercise had a significant effect on RER, fat oxidation, and carbohydrate oxidation. The average RER was lower in exercise groups; in particular, we found that the initial 20 min RER phase during the 1 h exercise period was significantly lower in exercised groups regardless of diet composition. Substrate utilization is largely driven by the availability of substrate. Excess carbohydrate intake promotes its own oxidation by stimulating the cellular uptake and oxidation of glucose during exercise, while decreasing the fat availability and oxidation⁴⁵. Exercise, however, had a positive effect on fat oxidation during the 1 h exercise period. This effect was seen for up to 40 min in the Sta. diet group. Taken together, these results indicate that energy metabolism during exercise was more influenced by exercise than diet. In our experiments, mice were made to exercise for approximately 70-75% of maximum oxygen uptake. However, high intensity exercise can also mediate stress related hormones, such as glucocorticoids, cortisol, and catecholamine¹³ that trigger the release of glucose stored as glycogen into the bloodstream, leading to high blood sugar levels, an insulin response, and therefore, metabolic diseases⁴⁶.

A limitation of this study is that the above results alone cannot prove that sucrose is not harmful. The greatest difference between sucrose and starch is probably the blood glucose and insulin response, and there may also be differences in some genes controlling energy homeostasis. Hence, future research should focus on the influence of high fat diets with different types of carbohydrates and/or exercise on more functional aspects, including glucose, insulin in blood, and UCP1, PGC1a, GLP-1, and FGF21 in the adipose tissue.

Our results showed that high sucrose did not cause greater obesity than starch. The weight of adipose tissue decreased markedly in exercise groups. Dietary sucrose increased energy expenditure during a 24 h period by increasing carbohydrate oxidation. Long-term exercise training increased fat oxidation during 1 h of exercise, regardless of diet. On the basis of these results, our study demonstrates that: (a) the type of carbohydrates included in the diet influence the metabolic responses for 24 h, (b) training had a larger effect on energy metabolism than diet during 1 h of exercise, (c) both abdominal adipose tissue weight and fat oxidation during exercise for 1 h showed the beneficial effects of moderate physical activity on weight maintenance.