Abstract
The fat but fit paradox has suggested that obese individuals with good fitness levels have lower cardiometabolic risk compared to individuals with normal weight but lower fitness levels. This paradigm has not been explored in the context of bone health. The aim of this study was to test whether categories of fat but fit paradigm assessed by body fat percentage and handgrip strength holds up in young adults and to analyze the relationship between fat but fit categories and bone outcomes. Cluster cross-sectional analyses of data from 499 young adults aged 18 to 30 from Toledo and Cuenca, Spain were conducted. Body fat percentage, handgrip strength, bone mineral content (BMC), bone mineral density (BMD), and dietary nutrients such as, proteins, magnesium, calcium, phosphorus, potassium, and vitamin D were assessed. Cluster analysis of body fat percentage and handgrip z scores resulted in a classification of four clusters that could be interpreted according to Fat Unfit (FU), Unfat Unfit (UU), Fat Fit (FF) and Unfat Fit (UF) categories. ANCOVA models showed that young adults in clusters with higher handgrip strength levels (FF, UF) and with higher key bone nutrients levels (UF) had significantly higher total BMC values than their peers in the UU and FU cluster categories, after controlling for sex, age and height. This study provides two novel conclusions in relation to the fat but fit paradigm: first, it confirms the construct of the four clusters of body fat percentage and handgrip strength, and second, it reinforces the predictive validity of the fat but fit paradigm categories, indicating the positive effect, although it may not just be a causal relationship, of muscular strength and key bone nutrients on counteracting the negative effect of obesity on bone health.
Highlights
First is the empirical confirmation of the construct validity of this theoretical paradigm using statistical clustering techniques to classify young adults according to their body fat percentage and muscular strength profile
In the case of bone health, the fat but fit paradigm is supported in terms of both bone mineral content (BMC) and areal bone mineral density (aBMD) since the influence of obesity on bone health is counteracted by acceptable-to-good levels of muscular strength
Our results showed the positive effect of cardiorespiratory fitness (CRF) on bone health, probably due to its relationship on the decrease in fat mass, as fat mass has been showed as a mediator in the association between CRF and bone [37]
Summary
Individuals who are overweight or obese but have moderate-good fitness levels, appear to be at lower metabolic risk when compare to those who have normal weight but lower fitness levels. To our knowledge, this paradigm has not been explored in the context of bone health. A large amount of evidence supports the benefits of physical activity for bone health among young people [6,7] Despite this evidence, the time that young people spend engaging in sedentary activities is increasing, which leads to higher adiposity, lower physical fitness levels [8] and presumable consequences for skeletal development [9]
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