In animals, similar to humans, osteogenesis ability becomes weakened with age [1]. Obesity is associated with a large number of serious complications; therefore, it can cause serious lifestyle-related diseases. The risk of increasing body fat is determined by lifestyle factors (e.g., high-energy food intake and inactivity). The accumulation of body fat impacts the health of bones [2]. The relationship between lean body mass and bone is well known [3], but the ability of bones to adapt to changes in body composition caused by changes in the diet is currently not well understood. In animals, the impact of a high-fat diet on bone health is not yet determined [4]. It is well accepted that dietary calcium intake is important for bone health [5]. The main source of calcium components is dairy. Dairy products are rich in calcium, which tends to be insufficient in our daily lives, and the absorptivity is good [6,7]. Skim milk is low in fat, yet is rich in protein, calcium, and lactose, meaning its nutritional value is high. Fried et al. [8] reported that milk protein is a combination of proteins with high-speed and low-speed absorptivity; therefore, the effects of ingesting skim milk, whey, and casein may be different.

A low level of physical activity is assumed to be a risk factor for early death along with a smoking habit, arterial hypertension, and dyslipidemia. Regular and medium-strength training is known to improve insulin sensitivity, increase HDL cholesterol level, decrease triglyceride level, and maintain a normal blood pressure, leading to weight loss [9]. In addition, regular exercise induces positive effects on bones, and is able to attenuate the bone loss induced by weight loss [10,11]. In recent years, Shah K et al. concluded that if exercise is added to the weight loss therapy provided to obese seniors, it prevents bone turnover increase and controls bone density (bone mineral density: BMD) loss in the hip induced by weight loss [12]. However, there is no study on the effect of exercise on bones using senior female rats. In addition, the mechanism of bone response during weight loss therapy and the impact of exercise are still unclear. Especially, there is little research on the combination of a skim milk diet and exercise. We examined the combined dietary effects of protein source, age, and energy density on bone microarchitecture and strength (trabecular bone structure, cortical bone structure, and cortical bone mechanical properties). We hypothesized that senior female rats fed SMP would have higher bone structural and strength parameters than rats fed a normal diet. Therefore, in this research, we aimed to examine whether exercise inclusion has an impact on obesity prevention and bone metabolism in senior rats with dietary obesity induced through skim milk intake.

There are very few studies on the effectiveness of a skim milk diet and exercise on the bones of middle-aged or old rats. Weight loss is linked to accelerated bone loss. Furthermore, exercise is known to have beneficial effects on bone. Beyer et al. (1985) reported that running training increased the femoral and tibial weights in 26-month-old male rats 12). In 19-month-old rats, training increased the skeletal calcium content 13). Thus, physical activity and fitness are important determinants of bone density in normal populations 14). In humans, increasing physical activity from a normal status of mobility produces little new bone, whereas a lot of bone is produced when the basal condition is immobility 15). We investigated the impact of a skim milk diet (SMP) and exercise on bone tissue in obese rats. The experiment results showed that body weight and fat weight at the end of the experiment were significantly lower in the normal diet group compared to the SMP groups due to the high fat intake (p < 0.05). The inclusion of exercise resulted in significantly lower values in only Ex groups with a normal diet (p < 0.05). Significantly lower values were noted in the Non-Ex and Ex groups with a normal diet compared to the Non-Ex group with SMP (p < 0.05). In addition, compared to the Ex group with SMP, the Ex group with a normal diet showed significantly lower values (p < 0.05). In the exercise groups, only the Ex group with a normal diet had significantly lower values (p < 0.05). Glucose and insulin concentrations at the time of dissection are shown in Table 3. There was no significant difference between any of the groups for glucose and insulin concentrations at the time of dissection. The Ex groups with both normal and SMP diets had significantly lower values (p < 0.05). Plasma total cholesterol, triglyceride, free fatty acid, HDL cholesterol, adiponectin and leptin concentrations at the time of dissection are shown in Table 4. Plasma free fatty acid, HDL cholesterol, and adiponectin concentrations at the time of dissection did not show significant differences between any of the groups. In contrast, plasma total cholesterol concentration at the time of dissection showed no impact from the diet, though the Ex group with a normal diet had significantly lower values (p < 0.05). In addition, plasma triglyceride concentration at the time of dissection showed no impact from exercise. However, the impact of the diet was reflected in the significantly lower values in the Ex group with a normal diet compared to the Non-Ex group with SMP (p < 0.05). Leptin concentration showed significantly higher values in the Non-Ex and Ex groups with a normal diet compared to the Ex group with SMP (p < 0.05). The bone weight of the femur and tibia per body weight, the bone density (BMD), bone strength, and plasma osteocalcin (Gla type) are shown in Table 5. The bone strength of the femur and tibia as well as the plasma osteocalcin (Gla type) did not show significant differences between the groups. Though the bone weight of the femur and tibia per body weight and the bone density (BMD) did not show significant differences due to dietary content, the Ex group in both dietary groups had significantly higher values (p < 0.05). The measurement results for the tibia proximal cancellous bone structure are shown in Table 6. Compared to the Non-Ex group with a normal diet, BS/BV (mm²/mm³) was significantly lower in the Ex group with a normal diet and in the Ex group with SMP (p < 0.05). Tb.Sp (μm) showed significantly lower values only in the Ex group with SMP compared to the Non-Ex group with a normal diet (p < 0.05). BV/TV (%) and Tb.N (/mm) showed significantly higher values only in the Ex group with SMP compared to the Non-Ex group with a normal diet (p < 0.05). Tb.Th (μm) had significantly lower values in the Ex group with a normal diet and the Ex group with SMP compared to the Non-Ex group with a normal diet (p < 0.05). TBPf (/mm) had significantly higher values in the Ex and SMP groups compared to the Non-Ex group with a normal diet (p < 0.05). Exercise did not change the osteocalcin concentration, preventing bone absorption and improving bone density in the tibia. In this research, both SMP intake and exercise induced weight loss regardless of the bone structure of obese rats without impacting bone weight and density. Exercise improved the bone density of obese rats without affecting the tibia and cortical bones. Therefore, both SMP intake and exercise adjust the energy of skeletal muscle to the minimum but do not change energy metabolism. Physical activity leads to gradual weight loss [16]. Physical activity along with dietary therapy decreases the body fat, weight, and abdominal circumference in obese individuals as verified by the rats in this experiment. Obesity treatment also corrects hormone balance [17,18]. In this research, decrease in the concentration of adipokines, which induces remodeling of bones, likely impacted the bone cells. Both SMP intake and exercise likely reduced the leptin level as a result of reduced fat. Various research papers have reported that when obese rats exercise, the leptin level decreases [19-23]. This is because leptin is made by fat cells at a rate proportional to the amount of fat stored [24]. Leptin differentiates bone marrow stromal cells into osteoblasts, suppressing their differentiation into fat cells. Therefore, the results indicate that leptin promotes bone formation. These conflicting results have not yet been explained. In this research, SMP intake and exercise likely had mutual effects that lowered leptin concentration. However, the age of the rats in this experiment at the time of dissection was 39 weeks, which was middle aged to elderly. Therefore, to take into consideration the decline due to age, younger rats should be used to conduct a comparative study. There was also a difference in the intake between the group that ingested normal food and the group that ingested a high-fat high-carbohydrate diet. This may be a result of the intake amount rather than the dietary composition. Therefore, using the pair-feeding method, an investigation should be performed using groups that consume identical amounts of food.