The powerful shrinkage technique, LASSO regression has been applied to analyze large databases with large amounts of dietary data like NHANES[6, 13]. But to our knowledge, this is the first study to use LASSO regression to assess the relationship between numerous dietary variables and obesity. Initial univariate results indicated that 29 out of 59 dietary factors were associated with obesity; then robust LASSO regression shrinkage to minimize multicollinearity between variables, 59 dietary factors were investigated as a whole, and 7 dietary factors were selected as the most associated with obesity. Afterward, by a series of logistic regression and ROC tests, folate, vitamin C, copper and PFUA 20:4 retained, where folate, vitamin C and copper were protective factors for obesity, and PFUA 20:4 was the opposite. The pathogenesis of obesity is more complex than overeating and a sedentary lifestyle, including a complex interplay between behavioral, environmental, genetic, physiological, social and economic factors. Finally, we retained the 4 adiposity dietary factors that were significant after adjustment for age, gender, ethnicity, family income, education, smoking, alcohol use and sedentary time.
Compared with normal-weight individuals, overweight and obese individuals are more vulnerable to vitamin deficiency due to differences in dietary intake of fruits, vegetables and energy[14, 15].Preliminary population studies have demonstrated that people with obesity have insufficient amounts of folate, which may be related to the volume dilution of blood in obese subjects and/or low folate intake in obese people[16]. Our research once again found that folate intake was lower in obese adults than normal adults. To make matters worse, vitamin deficiency, especially folate, may aggravate the pathological state of obesity[17]. Lower serum folate levels were linked to higher body fat percentages. In a population study, BMI higher than 50 kg/m2 was mainly associated with folate deficiency[18]. The development of obesity is often accompanied by insulin resistance, chronic inflammation, endothelial dysfunction and oxidative stress[19, 20]. Folate deficiency may exacerbate obesity in different ways. In vitro tests have shown that the main circulating metabolite of folate (5-methyltetrahydrofolate) can increase the production of nitric oxide, the main endogenous relaxation factor of the endothelium, which can scavenge superoxide free radicals and help in the management of progressive endothelial damage caused by obesity[21]. Studies have shown that folate supplementation can improve insulin resistance associated with obesity, the possible mechanism may be inhibition of β-subunit insulin receptor and its substrates by the active homocysteine, and reduce insulin stimulates glycogen synthesis[22].
Like previous studies, we found that obese people had lower vitamin C intake than normal people[15]. Furthermore, we found that vitamin C is a protective factor for obesity. And there are many studies reported the role of vitamin C in anti-obesity. Vitamin C is necessary for the post-translational modification of lipoprotein peptides to create collagen molecules that are cross-linked and stretchy. Defective collagen synthesis decreases exercise motivation resulting Obesity[23]. Additionally, vitamin C is necessary for the manufacture of carnitine, which is in charge of moving long-chain fatty acids across the mitochondrial membrane for -oxidation and subsequent fat oxidation[24]. Furthermore, carnitine transport in muscle depends on γ-butyrobetaine, which is synthesized by a vitamin C-dependent process. Thus, β-oxidation is reduced when vitamin C is insufficient. Johnston demonstrated this by showing that individuals with moderate vitamin C levels had elevated (30%) oxidized lipids compared with overweight or obese adults[25]. Leptin expression in both humans and animals has been demonstrated to be reduced or inhibited by vitamin C[20]. In addition, vitamin C modifies the expression of several important obesity-related proteins. At the same time, Vitamin C supplementation reduced C-reactive protein levels in 75% of obese subjects and could ameliorate endothelial dysfunction caused by oxidative stress[26].
Copper is a component of copper metalloenzymes, which are necessary for the healthy operation of the brain system, blood vessels, bones, and hematological[27]. This finding was consistent with previous findings of inadequate intake of minerals such as copper, in obese people, which were lower than in normal people. In addition, previous studies have shown that dietary copper intake is associated with a higher risk of obesity, and higher serum copper levels are associated with obesity risk in children and adults[28]. Our results, on the contrary, suggested that low levels of copper intake may be associated with obesity, and the reason may be that copper plays a crucial role in the regulation of enzymes, thereby catalyzing antioxidant metabolism, glucose metabolism, pancreatic β-cell function and insulin signaling[29]. Copper may be functionally related to fat accumulation [30].
Our study found that the intake of octadecatetraenoic (PFUA 20:4) in the obese population was higher than that in the normal population, which may be a risk factor for obesity. Animal studies have found that PFUA 20:4 can exacerbate diet-induced obesity. PFUA 20:4 metabolites are important factors in the initiation and resolution of inflammation and have been implicated in the pathophysiology of obesity[31]. Previous studies have found that BMI is positively correlated with pro-inflammatory 5-and 11-hydroxyeicosaenoic acid (HETE), PFUA 20:4 derivatives. 5-HETE can increase TNF-α-induced hepatocyte apoptosis, and inhibition of 5-HETE can reduce hepatic macrophage infiltration in HFD obese mice. Increased proinflammatory 5-HETE May contribute to pathological changes in obese tissues, thereby promoting oxidative stress and increasing ROS, thereby affecting PUFA peroxidation. 11-HETE is a non-enzymatic PFUA 20:4-derived oxygen lipid, which is a marker of lipid peroxidation, and 11-HETE may also be a pathological biomarker associated with obesity. However, anti-inflammatory PFUA 20:4 metabolites, such as lipoxinA4 (LXA4), attenuate proinflammatory M1 macrophages and boost anti-inflammatory M2 macrophages, which diminish obesity-induced adipose inflammation[32, 33]. PFUA 20:4 metabolism is one of the most complex regulatory systems in humans due to the enormous variety of PFUA 20:4 derivatives and their diverse ramifications, as well as the varied expression and activity of PFUA 20:4 metabolites in different cells and organs and their cross-cellular biogenesis[31]. Therefore, more research is needed to better understand the complexity of eicosanoid metabolism, how PFUA 20:4 influences obesity and may help improve current prevention and treatment strategies for obesity.
Limitations: We found that the dietary intake of folate, vitamin C and copper in obese people are less than that in the normal population, and they are protective factors for obesity, therefore necessary to assess the effective levels them supplementation in obese subjects, inversely, PFUA 20:4 is a risk factor for obesity. But further prospective studies and intervention experiments are needed to identify the underlying mechanisms of the association of dietary factors with obesity and to identify effective supplemental doses, as NHANES is a cross-sectional survey. In addition, the ROC test found that the AUC (0.579, 0.656) of the model was less than 0.75, so the use of dietary factors of intake for predicting obesity was limited.