In the present study, our results indicated stronger correlation between FRAX and TBS than FRAX and BMD, similarly to the previous studies (16), that raises the question whether TBS is a more accurate tool for predicting bone involvement than BMD. It has been shown that FRAX adjusted with TBS has a better predictive role than FRAX alone (16).
Correlation between the spine BMD and TBS was significant in the present study, similar to the previous studies that suggested enhanced gradients of risks when the results of TBS and BMD are combined together compared to the TBS or FRAX alone (17). According to the Hans et al, when BMD levels are close to the intervention threshold, TBS can be more useful in predicting the risk of future fracture risk (4). Similarly, our results showed that according to the BMD results, six patients considered to have osteoporosis, while TBS results showed that two of them does not need any intervention and classified as the normal group.
Our results suggested that the correlations between TBS and WC, AC, and BMI are stronger than BMD which raises the question whether obesity affects bone microarchitecture more than bone densit. Rates of obesity are rising in patients with IBD, as in the general population; 15–40% of adults with IBD are obese, and 20–40% are overweight (18, 19). Previous studies showed that the higher adiposity in early life in pediatrics with IBD and also in newly diagnosed IBD patients is significantly associated with reduced bone mass, as shown by BMD z-score (20, 21).
Obesity or fat accumulation affects bone in different ways. The fact that mesenchymal stem cells generates both osteoblasts and adipocytes is the cornerstone of the connections between their metabolisms. Obesity has positive effects on bone formation by decreasing apoptosis and increasing proliferation and differentiation of osteoblasts and osteocytes (22-24).
BMI or bodyweight is also showed a positive correlation with bone mineral density or bone mass. But there are evidences that shows negative consequences of fat accumulation on total bone mineral density and content; for instance, increased adiposity is a risk factor for bone fracture (25-26).
As the source of these two types of the cells is the same, increase adipocytes differentiation may lead to decrease osteoblast differentiation (27). Also, elevation of proinflammatory cytokines such as including TNF-a, IL-1, and IL-6 due to obesity increase bone resorption and bone loss in IBD patients (28).
On the other hand, high-fat diet decreases intestinal absorption of calcium (29). These evidences show that the effects of obesity and its related condition on bone are two sided and yet these interactions are not clearly defined. Considering these facts, obesity effects could have different faces in BMD and TBS.
Introduction of TBS to the routine methods of bone densitometry, showed better prediction of the fraction risk. In a cohort study in Korea (2001-2014) 4000 Chinese men and women aged 65 years old or above were recruited; It showed that the combination of TBS to the BMD could better predict MOF compared to BMD alone, especially in older men (30). On the other hand, Nassar et al (2013, France) reported that TBS and BMD could detect patients with and without vertebral fractures, but when BMD is in the non-osteoporotic range, TBS would add more information to BMD lumbar spine alone and could better show spine deterioration (31).
According to our results, BMD was reduced in both UC and CD patients while some studies demonstrated that the reduction of BMD only appears in CD patients, not in UC patients (2). This conflict can lead us to further investigations.
The major limitation in this cross-sectional study was that we evaluated the density and trabecular bone mass in IBD patients at a time point with no follow up.