The main finding of the present study revealed that consuming inulin and lactulose fortified prebiotic food for 12 weeks balanced the gut microbiota, resulting in suppressed bone resorption marker without body weight gain among the Japanese female athletes. This finding provides novel insights into the existing literature indicating that the combination of prebiotics may play a pivotal role in modifying gut microbiota and bone turnover metabolism markers among the Japanese female athletes.
Inulin is an indigestible, water-soluble dietary fiber, that is, a polysaccharide with a glucose molecule attached to the reducing end of the fructose chain [18], whereas lactulose is a synthetic disaccharide comprising fructose and galactose that cannot be digested or absorbed by humans [19]. Several studies have reported that short-term administration of inulin or lactulose increased Bifidobacterium spp. in the gut microbiota [10, 20, 21], which reduced bone resorption markers in postmenopausal women and healthy young men [22–24]; however, only limited studies have demonstrated that supplemented prebiotics change the gut microbiota and bone turnover metabolism markers in humans [24]. According to a consensus statement, prebiotics were recognized to particularly stimulate Bifidobacterium spp. (bifidogenesis) [13]. Short-chain fatty acids (SCFAs) are metabolites generated by gut microbial fermentation from prebiotics [25]. Indeed, a previous study reported the direct suppression of osteoclast synthesis [26]. After ingesting prebiotic food for 2 weeks, the occupation of Bifidobacterium spp. was higher than that before intervention and was maintained at a higher level until post-intervention. This is in accordance with the results of previous studies [10, 20, 21]. Thus, the combination of inulin and lactulose in the present study might contribute to the “prebiotic effect.”
Energy intake deficiency may exhibit uncoupled bone metabolism turnover, which decreases bone formation, increases bone resorption and/or a combination of the two, thereby consequently inducing unfavourable bone disorders. Subsequent reduction in the estrogen levels and associated reproductive dysfunction may also be indirectly influenced by bone health among the female athletes. Participants in this study were in an energy-deficient state, and the bone resorption marker during their pre-intervention was above the reference value [27]. These results were in accordance to those observed in previous studies [4, 28]. In particular, energy deficiency elevates bone resorption markers in athletes. If the balance between bone resorption and bone formation is maintained (coupling), bone mass and bone mineral contents remain balanced; however, when uncoupling in bone metabolism turnover occurs and bone resorption becomes more dominant, bone mineral density decreases. TRACP-5b is recognized as a bone resorption marker with low intra- and inter-day variations [29]. The finding of reduced bone resorption markers in this study may prove beneficial for preventing issues related to bone health among female athletes.
Adequate consumption of calcium and enough vitamin D status contributes to bone health [30, 31]. Daily consumption of milk fortified with calcium 1,200 mg/d and vitamin D 15 µg/d resulted in improved serum 25 (OH) D3 and suppressed bone turnover marker in postmenopausal women [32]. Serum 25(OH)D3 plays a crucial role in maintaining bone health in humans [33]. In this study, any significant increase in serum 25(OH)D3 status did not observe. Taking these matters into account, the amount of calcium 100 mg/d and vitamin D 0.5 µg/d in the prebiotic food may have been insufficient to improve serum 25 (OH) D3; however, the combination of prebiotics in this study may have significantly suppressed a bone resorption marker.
Since serum E2 plays a crucial role in maintaining normal bone health in females, De Souza et al. reported that low E2 levels were involved with suppressed bone formation and increased bone resorption [34]. Since E2 did not show any significant changes during this intervention, it is inferred that E2 did not affect bone turnover metabolism.
Body composition is related to athletic performance [35–37]. During intervention, no changes were observed in BW, FFM, and FM. Cialdella-Kam et al. found that a 6-month intervention provided an extra energy (+ 360 kcal/day) that could successfully reverse the menstrual status, resulting in weight gain (+ 1.6 kg) [38]. In contrast, our study did not induce substantial weight gain (+ 0.1 kg). While habitual EI increment and body weight gain are useful strategies for solving women’s health problems [8, 39], such strategies may not be practically feasible for long-distance runners and rhythmic gymnasts. Increased body fat reduces athletic performance [40]. Among the competing female athletes that require daily weight control, the prebiotic food used in this study may provide “bone protection effect” without having a significant impact on body composition. Regarding energy, macronutrient, and micronutrient intake, participants did not change their dietary intake throughout the study. A previous study suggested that an increase in certain micronutrients (i.e., calcium and vitamin D) has a favorable effect on the bone health [41, 42]. Since daily meals include various nutrients, it is necessary to interpret the results from a comprehensive perspective. Hence, no differences were observed in the bone-related micronutrients in the participants, which indicates that TRACP-5b was significantly decreased during this study.
The advantage of this study is that it was conducted in multiple events where female athletes participated in intense training every day. Previous studies were conducted in postmenopausal women and adolescent girls. The rate of bone turnover varies with age; therefore, it was unclear whether prebiotic intake in female athletes would prove beneficial for bone health. Thus, the present findings can be used as a nutritional strategy in sports fields. In contrast, this study has certain limitations. First, our participants could not perform placebo trials. Because our participants in this study had a high-performance level, it was difficult to get female athletes to commit to long-term intervention. Second, we did not measure SCFAs in fecal samples or serum proinflammatory cytokines. Thus, our study could not explicitly explain the prebiotics-gut-bone axis. Future studies should clarify the impact of prebiotic food on gut microbiota and bone turnover metabolism in other athletes.