Bone grows and develops rapidly in adolescence, increasing by approximately 5%-10% annually in adulthood, reaching peak bone mass (PBM) in the early 30s [1]. The state of bone health after middle age is determined by the formation of PBM and the degree of bone loss experienced later in life [2]. Thus, PBM is a key determinant of skeletal health. Therefore, skeletal management that increases PBM completion by early adulthood is the best way to prevent fractures or osteoporosis due to age-related bone loss [3]. Significant variations in PBM or bone mineral density (BMD) have been reported in early adulthood, often attributed to individual eating habits given the expanded independent food choices associated with various types of residences and lifestyles [4, 5, 6].

Dietary factors that affect BMD have been reported to include calcium (Ca), magnesium, vitamin D, protein, sodium, and caffeine [7, 8]. It has been reported that excessive consumption of caffeinated beverages by young adults can negatively affect BMD by promoting urinary Ca excretion and deteriorating Ca balance when Ca intake is insufficient [9, 10]. In particular, college students foster a culture of coffee consumption, often consuming a large number of caffeinated beverages ranging from 1 to 10 cups a day for their stimulating effects and as a means of combating fatigue [11].

In contrast to the increase in caffeinated beverage consumption among young adults, milk intake is low. The 2021 Korea National Health and Nutrition Survey (KNHANES) reported that the daily milk intake of adults aged 19 years was 90.0 g, less than half a cup [12]. Ca is the most important nutrient for bone health. However, 2021 KNHANES reported that daily Ca intake was 486.3 mg [64.3% of the recommended intake (RI)] for adults, 533.0 mg (68.7% of the RI) for men, and 438.3 mg (60.0% of the RI) for women, which was insufficient to meet RI [12]. Considering the importance of Ca in bone functionality, a study on the bone status of young adults while they are nearing their PBM stage and its relationship with dietary intake status, dietary behavior, and coffee consumption, is paramount.

Accordingly, the purpose of this study was to evaluate the bone status by BMD of the calcaneus in young adults and to investigate the association of bone conditions with body size and composition, nutrient intake status, dietary behavior, including coffee consumption, and urinary bone resorption indicators by comparing an osteopenia or osteoporosis group with a normal group.

Among the study participants, 30% of men and 60% of women met the criteria for osteopenia or osteoporosis, showing a significant difference between men and women. In men, the mean height of the OSTEO group was significantly higher than that of the normal group. In women, the body weight and composition (muscle and body fat) of the OSTEO group were significantly lower than those of the control group. According to one study, increased adult height in men was associated with later puberty [13], which in turn is a recognized risk factor for fracture [14, 15, 16]. It has been speculated that late-maturing boys have lower sex steroid levels during puberty that may persist into adulthood [16]. Consequently, low levels of androgens and estrogen could predispose individuals to osteoporosis and increase the risk of fractures later in life [16, 17]. Conversely, in women, low body weight, lean body mass, and body fat, rather than height, were significant risk factors for low BMD. According to the results of previous studies targeting female college students, the average weight of the normal group was significantly higher than that of the osteoporosis risk group [3, 18], which was consistent with the results of this study. Edelstein & Barrett-Corener [19] also reported that bone density increases as weight increases. DeSimone et al. [20] reported that body weight exerts a load on the skeleton and positively affects bone density, and weight has a higher correlation with bone density than height. On the other hand, body fat produces estrogen, and estrogen plays a central role in preventing osteoporosis [21, 22]. Increased body fat also contributes to an increase in total mass, which puts a mechanical load on bones and contributes to BMD [23, 24]. There is also a possible role for leptin, which is mainly produced by the adipose tissue, in the positive association between circulating leptin and BMD [22, 25, 26]. Furthermore, lean mass contributes to total mass, increasing the bone load. It has also been suggested that a high percentage of skeletal muscle mass is likely associated with high physical activity, stimulating an increase in BMD [27]. In a study of premenopausal women from the Pacific Islands, lean mass was the strongest predictor of BMD, while many established contributors to bone health, such as Ca, physical activity, protein, and vitamin C, were not associated with BMD [28]. Our results suggest that low body weight, muscle mass, and body fat in young adult women may be associated with osteopenia and low BMD.

In our study, there was no significant difference in the urinary excretion of DPD and NTx between the OSTEO and normal groups in both men and women. Bone turnover markers were reported to be significantly different in elderly individuals with osteopenia and osteoporosis compared to healthy elderly individuals, whereas there was no difference in BMD [29]. Islamoglu et al. [30] reported that a low-protein diet had a negative effect on bone structure by significantly decreasing NTx levels in postmenopausal women with osteopenia. However, our study was conducted in healthy young adults with active bone turnover, and the OSTEO group was mainly composed of participants with osteopenia whose bone condition was not serious.

In men, the preference for caffeinated beverages in the OSTEO group was significantly higher than that in the control group, but not in women. However, the intake frequency of caffeinated beverages in men and women did not differ significantly between the OSTEO and control groups. The association between caffeine or coffee intake and BMD has been inconsistent among studies [31, 32, 33, 34]. Caffeine is known to increase Ca excretion through the kidneys and intestines; however, there is no evidence that caffeine has any detrimental effects on bone status or Ca economy among individuals consuming the recommended daily allowance level of Ca [35]. Conlisk & Galuska [36] reported that the average caffeine intake from coffee, tea, and caffeinated drinks in young adult women was 99.9 mg/day, which was not significantly associated with BMD. It was also reported that participants with high coffee consumption (over 4 cups a day) had a 4% lower BMD at the proximal femur compared with low or non-coffee consumers in older men, but not in women [37]. Taken together, when Ca intake is insufficient, caffeine intake through coffee or beverages has a negative effect on bone density, which appears to be greater in men than in women. Although coffee preference was high in the OSTEO group, there was no significant difference in the frequency of direct coffee consumption.

Inadequate nutrient intake is an important risk factor for osteopenia and osteoporosis. In this study, the average Ca intake in the OSTEO group was 547.9 mg in men and 564.5 mg in women. Although these values were not significantly lower than those in the normal group, they remained below the recommended intake (800 mg for Korean men aged 19-39 years, 700 mg for Korean women aged 19-37 years) [38]. These results are similar to recent KNHANES data for men but higher for women [12]. Interestingly, in our study, animal Ca intake in the male OSTEO group was significantly higher than that in the normal group. In women, the daily intake of dietary fiber, vitamins A, Ca, plant Ca, and K, mostly derived from plant foods, in the OSTEO group were significantly higher than in the normal group (P < 0.05). Although the difference was not statistically significant, the energy intake of the female OSTEO group was lower than that of the control group. Thus, we conducted an additional comparison of nutrient intake between the two groups adjusted for energy intake. After adjustment, there was no significant difference in the male groups; however, in females, the intake of carbohydrates, vitamin E, vitamin B₂, folic acid, phosphorus, iron, and plant iron was also significantly higher in the OSTEO group. This indicated that the overall dietary quality and nutritional density tended to be higher in the OSTEO group than in the normal group. In addition, the eating attitudes and eating habits of both men and women in the OSTEO group were not significantly different from those in the normal group, except for the eating attitude of males in the OSTEO group. In fact, the OSTEO group tended to have more favorable dietary attitudes and behaviors than the normal group, which is consistent with the observed lower quality of nutrient intake within the normal group. The findings in the normal group raise concerns that people who are confident in their BMD or health status in early adulthood, when genetics and body size play a large role in the status, may be at risk of negative consequences for their future BMD and health if they maintain an unhealthy lifestyle.

Our study had several limitations. First, the cross-sectional nature of this study inherently limited our ability to establish causal relationships between various risk factors, osteoporosis, and osteopenia in young adults, thereby introducing the potential for reverse causality. Specifically, our study showed that the OSTEO group had more favorable dietary habits and better nutrient intake than did the normal group. This could be because individuals with health issues consciously or subconsciously modify their dietary habits because of health issues. Second, differential misreporting of dietary intake based on body weight could also be possible, with underweight females in the OSTEO group potentially overreporting, and overweight individuals in the normal group may underreport their intake. Consequently, these biases could influence the observed higher nutrient intake and better dietary behaviors in the OSTEO group. Third, BMD, the main dependent variable, was measured by quantitative ultrasound of the calcaneus instead of the major sites of the femoral neck and lumbar spine using DXA measurements; therefore, interpretation of the prevalence of osteopenia and osteoporosis may be less reliable. Finally, this study did not analyze bone formation-related biomarkers, which may reflect the current status of bone formation. As the participants were still in the bone-forming stage, analyzing bone formation-related biomarkers in conjunction with bone resorption markers may help explain their current bone status. Despite these limitations, this study has strengths in its novel examination of the correlations between comprehensive dietary habits, body components, and an array of objective bone biomarkers, specifically in young Korean adults with an elevated risk of osteoporosis. By integrating these multidimensional factors, our research provides a scientific contribution to understanding susceptibility to osteoporosis in this age group.

The observations from this study indicated that our study participants had comparatively lower levels of BMD. In young adult males with greater stature and in young adult females, a leaner physique was associated with lower BMD. The findings of this study suggest the potential for reverse causality, a common occurrence in cross-sectional research, especially in relation to the differential association between dietary factors, including dietary Ca sources, and BMD in men and women. Interestingly, the OSTEO group, despite having a lower BMD, consumed a diet rich in nutrients and exhibited healthier dietary attitudes than the normal group. Therefore, further research is warranted to elucidate the complex interactions between dietary factors and the risk of osteoporosis in young adults. Specifically, comprehensive prospective cohort studies with larger populations should be conducted to address these discrepancies and offer more conclusive insights.