Oxidative stress is caused by an imbalance between free radical generation and antioxidative protection systems in the human body (Persson et al., 2014). Free radicals are highly reactive and destructive molecules generated by an electron transport chain (ETC), cytochrome P450, and other cellular and sub-cellular functions. They can also be produced by non-enzymatic and enzymatic reactions, such as Fenton's and Haber's reactions, and monoamine oxidase (Noori, 2012; Orient et al., 2007; Bedard and Krause, 2007). Over production of free radicals leads to oxidative stress, which may create disturbances in body homeostasis and cause disease (Zampetaki et al., 2013; Lastra et al., 2014). Moreover, free radicals can also be generated from external sources (e.g., environmental pollution, cigarette smoke, alcohol, sunlight, and toxic metals) (Aseervatham et al., 2013), which can also damage body systems. To maintain body homeostasis, both endogenous and exogenous antioxidant systems must be involved. Endogenous antioxidants are composed of enzymatic and non-enzymatic approaches. Superoxide dismutase (SOD), catalase, and glutathione peroxidase are an example of enzymatic antioxidants, while glutathione, ferritin, uric acid, lipoic acids, and coenzyme Q are non-enzymatic antioxidants (Poljsak et al., 2013). Both antioxidant systems help protect our bodies from the dangerous effects of free radicals. For instance, SOD and catalase are used to catalyze superoxide anions and hydrogen peroxide, which are toxic substances, to non-toxic products (water). In addition, glutathione scavenges hydroxyl anions, hydrogen peroxide, and chlorinated oxidants (Sung et al., 2013), while uric acid scavenges reactive free radicals from hemoglobin auto-oxidation and peroxide radical production from macrophages (Sautin and Johnson, 2008; Ames et al., 1981). Moreover, exogenous antioxidants from fruits and vegetables, such as vitamin C, vitamin E, phenolic compounds (e.g., phenolic acid, cinnamic acid), carotenoids (e.g., beta-carotene), and flavonoids (quercetin and rutin), demonstrate a synergistic effect on endogenous antioxidants as a total defense mechanism (Willett, 2006). It has been suggested that consuming various exogenous antioxidants from food and dietary sources leads to a healthy life (Lopez and Denicola, 2013). Among the exogenous antioxidants, vitamin C is an important vitamin that is involved in many anti-oxidative processes.

Vitamin C (L-ascorbic acid) is an important water-soluble vitamin. It is primarily found in many fruits and vegetables (Du et al., 2012). Due to a lack of gulonolactone oxidase (GLO) in the final step of vitamin C synthesis, humans, other primates and guinea pigs cannot synthesize this vitamin (Chatterjee et al., 1961); hence, they depend on exogenous sources to maintain homeostasis, particularly from fruit and vegetables. Aside from its antiscorbutic property, vitamin C enhances immune functions, iron absorption, and collagen synthesis (Mandl et al., 2009; Schlueter and Johnston, 2011). Vitamin C also plays a role in the brain as a cofactor of dopamine betahydroxylase, which is involved in catecholamine synthesis (May, 2012). As a free radical scavenger, vitamin C is the most effective external antioxidant in plasma due to its water solubility and ability to scavenge a wide range of reactive oxygen species (ROS) (Frei et al., 1990), resulting in the protection of biomolecules from oxidative stress (Sung et al., 2013; Du et al., 2012; Descamps et al., 2001). Vitamin C works synergistically with vitamin E to remove lipophilic radicals in lipid peroxidation (Du et al., 2012; Kojo, 2004). In humans, the vitamin C dose required to saturate plasma and tissues in healthy adults is 500 mg (Levine et al., 2001), which is far from the recommended daily intake (60 mg) for disease prevention and general health promotion (Carr and Frei, 1999; Deruelle and Baron, 2008). However, higher doses (more than 500 mg) cannot change plasma levels due to the increased vitamin C excretion in urine (Levine et al., 1996: Friedman et al., 1940). Although a low amount (10 mg/day) of vitamin C is needed to prevent scurvy (Smith and Hodges, 1987), a large cross-sectional study has demonstrated that vitamin C deficiency is commonly observed in humans and affects 5– 10 % of adults in the industrialized world (Lindblad et al., 2013).

Cigarette smoking and alcohol consumption have been associated with increased free radical production, leading to oxidative stress and antioxidant reduction (Wu and Ceder-baum, 2013; Valavanidis et al., 2009). Schectman et al. (1989) have reported that an inverse association between cigarette smoking and serum vitamin C level were independent of dietary intake. Furthermore, low blood levels of vitamin C and other antioxidants (vitamin A, C, E, and coenzyme Q10) have also been reported in smokers (Song et al., 2009). Smoking, a lack of physical activity, and insufficient daily fruit consumption were all associated with vitamin C deficiency (Pincemail et al., 2011). Regarding alcohol consumption, Zloch and Ginter (Zloch and Ginter, 1995) have reported that moderate alcohol consumption in guinea pigs caused a decrease of vitamin C in tissue and body pool concentration when compared to a control group. In Thailand, a non-significant difference in the vitamin C dietary intake between smokers and non-smokers has been reported (Jitnarin et al., 2008, 2014), and a decrease of blood levels of vitamin C has also been observed in priests or monks (Viroonudomphol et al., 2005). In one survey, a 9.9 % blood vitamin C deficiency was observed in older Thai adults (Assantachai et al., 2005). In the Bangkok Metropolitan, a large urban area, the lifestyle behaviors of cigarette smoking and alcohol consumption are still quite high. Much of the population works as laborer in the city, such as working under the hot tropical sunlight in the construction industry. The lifestyle behaviors and dietary intakes vary. Therefore, the present study aimed to explore the association between total serum vitamin C levels and vitamin C dietary intake and lifestyle behaviors that are believed to induce body oxidative stress (e.g., smoking, alcohol consumption, and working outdoors, particularly under the tropical sunlight) among people in the Bangkok Metropolitan.