The prevalence of obesity is increasing worldwide. In particular abdominal obesity is a well-established risk factor for cardiometabolic diseases. The excess risk of abdominal obesity is due to fat... Show moreThe prevalence of obesity is increasing worldwide. In particular abdominal obesity is a well-established risk factor for cardiometabolic diseases. The excess risk of abdominal obesity is due to fat in the visceral area and in and around the organs (ectopic fat), such as in the liver. The main aim of this thesis was therefore to study whether lifestyle can reduce the amount of visceral fat and liver fat.Firstly, in a systematic review and meta-analysis of randomized controlled trials, we concluded that a diet high in proteins decreases liver fat compared with a diet high in carbohydrates. A diet high in fat did not lead to changes in liver fat compared with a diet high in carbohydrates. Within fat types, saturated fat leads to more liver fat accumulation than unsaturated fat. Secondly, we studied diet at multiple levels in the Netherlands Epidemiology of Obesity study, which is a population based cohort study of middle-aged men and women with directly assessed measured of adiposity. When studying food groups rather than nutrients, we observed that consumption of sweet snacks are positively associated with liver fat content, also after taking into account total body fat. Likewise, consumption of fruit and vegetables and plant-based fats and oils was associated with less visceral fat. A better adherence to the current Dutch dietary guidelines, as indicated by a higher score on the 15-component Dutch Healthy Diet Index, was associated with less total body fat, less visceral fat and liver fat. The associations with visceral fat and liver fat remained present after taking into account total body fat, indicating that the associations are indeed specific for visceral and liver fat rather than merely representing associations with overall adiposity. Associations were not driven by one component in particular, indicating the importance of an overall healthy diet. When we studied alcohol intake separately, each additional serving of alcoholic beverages per day was also associated with more liver fat. Moreover, replacing one alcoholic serving with one non-alcoholic serving was associated with less liver fat. Replacing the same amount of calories of alcoholic beverages with sugar sweetened beverages was equally associated with liver fat, whereas replacement with milk was associated with less liver fat. Lastly, we observed that objectively measured sedentary time was associated with more total body fat, visceral fat and liver fat. Replacing 30 minutes of sedentary time per day with moderate to vigorous physical activity, but not light physical activity was associated with less total body fat, visceral fat and liver fat. These associations with visceral fat and liver fat disappeared after additional adjustment for total body fat, indicating that there is no extra effect on visceral fat and liver fat beyond effects via total body fat.Results described in this thesis hint towards the importance of considering diet as a whole, instead of separate components, which is in line with the current dietary guidelines. Sedentary behaviour should be replaced with moderate to vigorous physical activity rather than light physical activity. Alcohol should not be replaced with sugar sweetened beverages, but rather with milk, coffee or tea. Show less
Weighing the strengths and limitations of our studies, we believe our results contribute to the unraveling of causal pathways between obesity and lung function impairment. The main conclusion of... Show moreWeighing the strengths and limitations of our studies, we believe our results contribute to the unraveling of causal pathways between obesity and lung function impairment. The main conclusion of the first part of this thesis is that visceral fat is associated with lung function impairment in men with the metabolic syndrome. Furthermore, we conclude that in the general population there is no causal association between insulin resistance and lung function, nor an association between visceral fat and exhaled nitric oxide. In the second part of this thesis we showed that vitamin D is associated with lung function and nitric oxide in obese participants.In the study presented in this thesis antimicrobial peptides levels were lower in allergic asthmatics than in healthy controls. Unfortunately, we were not able to conclude if vitamin D supplementation influences these antimicrobial peptide levels. Larger studies are needed to investigate whether vitamin D also increases antimicrobial peptides in the lung.In summary, obesity is a global burden that influences lung function. Future research should reveal if obesity, and in particular visceral fat, causes lung inflammation and thereby impairs lung function.Large randomized trials are necessary to establish the effect of vitamin D therapy on infections in vitamin deficient patients. Show less
