Obese individuals have lower circulating levels of certain nutrients compared to lean individuals. For example, it has been reported that the plasma hydroxyvitamin D levels of obese individuals are low compared to normal weight controls (here). In addition, it has been observed that the obese have low levels of antioxidants compared to normal weight controls. Further, the plasma vitamin C concentrations of the obese have been reported to be low (here). Other antioxidants have also been shown to be lower in subjects with more adiposity, compared to leaner controls. In one recent study1, researchers assessed the vitamin E and carotenoid serum (blood) concentrations of schoolchildren in the United States between the ages of 8 and 15 years. The results showed that the body mass index of the children was associated with the serum levels of α-carotene, α-tocopherol, trans-β-carotene and cis-β-carotene. Similar associations were also found between these antioxidants and total fat mass and truncal fat mass.
The authors calculated that higher α-tocopherol and cis-β-carotene serum concentrations in the children was associated with reduced probability of being overweight or obese. In fact high concentrations of these antioxidants decreased the odds of being overweight by 40 % and being obese by 60 %. Interestingly having higher serum retinol concentrations was associated with an increased probability of being overweight or obese buy 2 and 3-fold, respectively. It has been suggested that the lower levels of certain nutrients in the obese may relate to the increased oxidative stress seen in such individuals. Adipose tissue accumulation when excessive can lead to the influx of macrophages and an immune response that increases systemic inflammation. This increase in inflammation then in turn causes increases in systemic oxidative stress. It is hypothesised that the higher levels of oxidative stress in the obese, may over time, deplete the individual of antioxidants, reducing plasma levels.
Therefore being obese could deplete the body of antioxidants in a similar way to smoking. For example, the antioxidant capacity of the lungs of smokers becomes depleted through chronic exposure to the free radicals in cigarette smoke. However, there could be other alternatives that explains the low plasma concentrations of antioxidants in the obese. For example, it has been suggested that the additional fat mass of the overweight may be able to sequester fat soluble nutrients thus reducing plasma levels. Some evidence does support the sequestration of vitamin D in adipose tissue (here), and as both carotenoids and vitamin E are fat soluble they could similarly be sequestered in adipose tissue (here). However, retinol is also fat soluble and yet plasma levels were increased in the children. Low antioxidant plasma levels may also be markers for poor diet, which is a known cause of weight gain and is also low in plant foods. In other words eating poor quality foods makes you fat and also provides few plant derived antioxidants.
However, it is interesting to speculate as to whether antioxidants are able to cause weight loss. Evidence is starting to emerge that the polyphenol group of antioxidants have have beneficial weight loss effects. For example, green tea has particular weight loss effects, and it is often assumed this is due to the caffeine that it contains. However the catechin (flavan-3-ol) polyphenols in green tea may contribute significantly to these effects (here). It is unclear how polyphenols can cause weight loss, but one suggestion is that they inhibit starch digesting enzymes in the gut and therefore have the ability to slow the digestion rates of starch and thus decrease the rate that glucose is absorbed (here). This may then allow better control over blood sugar and provide insulin sensitising effects. Additionally, the antioxidants may have direct effects on the insulin receptors (here), protecting them from oxidative damage. Blueberries have been particularly well studied with regard their beneficial glycaemic effects (here).