The main findings of this study were as follows:
1) Serum 25(OH)D concentration was inversely correlated with levels of LDL-C/HDL-C, TG, ApoB, and ApoB/ApoA-1 levels regardless of several confounding factors including visceral fat accumulation, and cardiorespiratory fitness (Chapter 2).
2) Higher serum 25(OH)D concentration and cardiorespiratory fitness levels were associated with low risk of insulin resistance, independent of visceral fat levels. Furthermore, among participants with high CRF levels, higher serum 25(OH)D concentration may largely strengthen the effect of CRF on reducing insulin resistance (Chapter 3).
3) Daily supplementation of 420 IU of vitamin D for 1 year significantly increased serum 25(OH)D and 1,25(OH)2D concentrations, and improved glucose metabolism in generally healthy Japanese adults. These effects were independent of changes in moderate to vigorous physical activities and visceral fat accumulation (Chapter 4).
Thus, the present studies revealed that increasing circulating 25(OH)D concentration was related with low risk of cardiometabolic risk, especially improved glucose metabolism in generally healthy Japanese; the results were independent of visceral fat and physical fitness levels.
Obesity is a well-known risk factor for the development of cardiometabolic diseases.
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Importantly, obesity is associated not only with lipid accumulation in adipose tissue but also ectopic lipid accumulation, for example, liver and skeletal muscle tissues. Evidence from animal and human studies has showed that ectopic lipid accumulation is associated with the development of cardiometabolic diseases. In the present studies, the reduction of insulin resistance after the 1 year vitamin D supplementation compared with placebo group was independent of either the changes in total body fat (BMI and percent body fat) or abdominal body fat ( waist circumference and visceral fat). Thus, the significant improvement of vitamin D supplementation in insulin resistance maybe associated with reduced ectopic lipid content in general Japanese adults.
Although the vitamin D supplements effect on lowing risk of insulin resistance was independent of cardiorespiratory fitness, the results from chapter 3 showed that a high serum 25(OH)D concentration may largely strengthen the effect of CRF on reduction of insulin resistance.
Furthermore, evidence also showed that high cardiorespiratory fitness was associated with substantial reductions in ectopic lipid levels without weight loss, resulting in reduced cardiometabolic risk Thus, the combined effect of serum 25(OH)D concentration and CRF on ectopic lipid content and other cardiometabolic risk factors are needed to be evaluated through randomized, controlled intervention studies.
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Acknowledgements
I appreciate all members of Higuchi laboratory for their encouragement and support in my doctoral time at Higuchi laboratory in Waseda University. Especially, I would like to thank Dr.
Mitsuru Higuchi, who has provided great support and encouragement for me. He is one who led me into the world of sport nutrition and exercise biochemistry. Without him, I could not successfully finish this dissertation and could not have the opportunity to be successful in the future. In addition, I would like to thank Drs. Isao Muraoka and Shizuo Sakamoto for their many valuable comments on this dissertation.
I would like to express my deepest thanks to Dr. Zhen-Bo Cao for providing continued encouragement and teaching throughout my studies. Dr. Cao not only teaches me how to conduct scientific research, how to write scientific papers, but also teach me how to face with hard time during doctoral time.
Lastly, I would like to say “Thank you” to my parents for their great admiration on my own freedom, and my sweet friends for their thousands of love, understanding, encouragement and support throughout this dissertation work.
This study was supported in part by a Grant-in-Aid for the Global COE Program (Sport Sciences for the Promotion of Active Life to Waseda University) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (to MH); a grant from the Ministry of Education,
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Culture, Sports, Science, and Technology-Supported Program for the Strategic Research Foundation at Private University (to MH); a Grant-in-Aid for Scientific Research (C) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (to ZBC); a grant from the National Natural Science Foundation of China (No. 31571226); the Program for Professors of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No.
TP2014057) and Shanghai Key Laboratory of Human Performance (Shanghai University of sport) (No. 11DZ2261100). XS (No. 15-111) was a recipient of a scholarship from the Otsuka Toshimi Scholarship Foundation.
60
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