The Antioxidant Status of Vegetarians

Even a single meal containing meat can result in higher oxidative damage markers in the bloodstream compared to a meat-free meal; so, one would expect those eating plant-based diets would suffer less oxidative stress. They certainly tend to consume more antioxidant vitamins, such as vitamins A, C, and E—in some cases, twice as many, closer to the intake of ancestral humans. This results in higher levels of these vitamins in their bloodstreams, though comparable or in some cases lower levels of quote-unquote “antioxidant” minerals, such as zinc or selenium. However, these aren’t antioxidants themselves, but rather co-factors in antioxidant enzymes; so, there may be no advantage to exceeding adequate intake.

In cross-sectional snapshot-in-time studies, the blood of vegetarians had either significantly higher or similar total antioxidant capacity, but you don’t really know, until you put it to the test in an interventional trial. Randomizing people to a lower meat diet or fully plant-based diet significantly increased total antioxidant power of their bloodstreams within three months, or three weeks, respectively. Vegetarians also have fewer free radicals in their poop. A study entitled “Total antioxidant capacity of feces of mammalian herbivores and carnivores” found that plant-eaters had an edge, but it wasn’t tested in people until a team of German researchers picked up the mantle. Those switched to a plant-based diet experienced a 13-fold drop in fecal free radicals in less than two weeks. (This wasn’t idle curiosity; fecal free radicals are implicated the development and progression of colorectal cancer).

Some studies show vegetarians have up to three times the expression or activity of antioxidant enzymes, but others have found no difference or even lower activity. It’s hard to interpret these findings, though. Is antioxidant enzyme activation a sign of robust defense or an indication of how much oxidant stress is being generated by the diet? Like, if you switch people from a junk food diet to a healthier diet, you see antioxidant enzyme levels plummet, described as an improvement in enzyme levels, since the healthier diet spared the body from needing to defend against so much damage. See how high or low antioxidant enzyme activity is hard to interpret? More important is the outcome: comparing rates of oxidative damage.

Both systematic and nonsystematic reviews have concluded that plant-based diets protect against free radical damage, which “may explain why vegetarians live longer.” Most studies do show that vegetarians suffer lower levels of oxidative stress, but some show no significant difference compared to meat-or fish-eaters, or even higher levels. The discrepant results may be due to vitamin B12 inadequacy among vegetarians and vegans who don’t supplement their diet with B12 or B12-fortified foods, as even subclinical (asymptomatic) B12 deficiency is associated with increased oxidative stress. This may also help explain why most studies show lower oxidative DNA damage among those eating more plant-based, but not all, and fewer chromosomal abnormalities, the same, or more. The bottom line is B12 deficiency is not something to be trifled with. A regular reliable source of vitamin B12 is critically important to take advantage of the full spectrum of benefits to healthier eating.