In my continuing review of the literature of the glutathione and antioxidant status of vegetarians, today’s episode is a 2007 cross-sectional study from Ireland (1).
The study included 31 vegetarians (including 6 vegans) and 58 omnivores. The results were adjusted for age, gender, and body mass index (which did not significantly differ between the two groups). Antioxidant supplements were allowed and results were examined both for the entire group and for non-supplement users.
There were no significant differences between the groups for glutathione or any antioxidant enzymes. The only differences between the two groups of note were:
• Vegetarians had higher levels of carotenoids, associated with a higher vegetable intake.
• Among those not taking antioxidant supplements, the omnivores had a higher total antioxidant status (known as FRAP). This was due to higher uric acid levels (which is an antioxidant) in the omnivores, which was probably due to meat intake.
The authors say, “The results of this study indicate that there were no differences between vegetarians and omnivores in the level of cellular endogenous antioxidants…and in the plasma levels [of] antioxidant nutrients (vitamin C, retinol and a-tocopherol) despite the increased dietary intakes of these antioxidants by the vegetarian group. The reason for the lack of difference in the antioxidant vitamin status between the two groups might partly be due to homeostasis…” In other words, once you have reached a certain threshold of antioxidants in your system, adding more might not do much.
Regarding the uric acid and total antioxidant capacity, this study was interesting, and could possibly indicate that lacto-ovo vegetarians are at a disadvantage. As we saw from my recent post Higher Uric Acid Levels in Vegans, vegans would not be at a disadvantage.
In terms of glutathione levels, we now have a cross-sectional study (1) and a clinical trial (2) indicating that vegetarians have similar glutathione levels as omnivores.
As an aside, one other measure of interest was that plasma zinc levels did not differ between the two groups.
I’m going to take a brief break from the antioxidant discussion to report on a cross-sectional study just released from EPIC that shows vegans to have higher levels of uric acid than lacto-ovo and pesco-vegetarians (people who eat no meat other than fish).
To quote the authors, “Uric acid is the end product of purine metabolism, generated from the breakdown of DNA, RNA and ATP… High circulating concentrations of uric acid can lead to gout, a common form of arthritis, and have also been linked to chronic kidney disease, cardiovascular disease and cancer.”
In the model adjusted for age and alcohol intake (but not body mass index), the results were (µmol/l):
Meat-eaters – 323
Pesco – 307
Lacto-ovo – 301
Vegan – 336
Meat-eaters – 239
Pesco – 224
Lacto-ovo – 228
Vegan – 243
In our correspondence, author Paul Appleby told me that the differences between the vegans and the pesco/lacto-ovo groups were statistically significant for both genders, and the differences between the vegans and meat-eaters were statistically significant for men only.
How could this be?! Apparently, dairy products have been shown to lower uric acid levels. And in this study, there was a significant positive correlation between uric acid and soy protein intake in men.
The good news is that these levels are still within the normal range which is 202-416 µmol/l for men and 143-357 µmol/l for women (2). And even though the differences were statistically significant, they were not huge (about 11% in men and 8.5% in women).
These levels do not appear to be associated with an increased risk of kidney disease (3) or cardiovascular disease (4). One study found an increase in risk of stroke for uric acid levels above 297 µmol/l, but another did not find a statistically significant difference for levels above 410 µmol/l (4).
Uric acid is an antioxidant and has been studied for its potential to prevent cancer, but the findings have not been in that direction. One study found an increased risk for cancer above the normal levels (5), while another found an association of cancer in men (women were not in the study) in the range of the vegans in the EPIC study (6). The authors (6) noted that vitamin B12 deficiency can increase uric acid levels and also stated, “we hypothesize that elevated [serum uric acid] acts as a valuable, long-term, surrogate parameter, indicative for a life-style which is at increased risk for the development of cancer, but does not function as an independent risk factor or even carcinogenic substance by itself.”
Given all the above, I do not think there is much to worry about, though it would be interesting to see if cancer deaths are related to uric acid levels among vegans in EPIC (if there are enough participants to determine this).