I have a lot of vegan and vegetarian friends, and one argument they hate to deal with is the lack of B12 argument.

Well, here it is again, and this time researchers at the University of Bristol in the U.K. found that male vegetarians are twice as likely to develop depression because of the lack of B12 vitamins and iron, which often affects mental health, according to the July 28 study titled "Vegetarian diets and depressive symptoms among men."

The UK study was conducted on 9,668 adult men who identified as vegan or vegetarian. It showed that about 50 percent of vegan men, and seven percent of vegetarians had a vitamin B12 deficiency, and the longer they were vegan and vegetarian, the more likely they were to suffer from depression.

Not only are B12 vitamins not naturally produced by the body, but plants don't make them either, so we often have to look for foods that come from animals and supplements to get our fix.

The lack of B12 argument has been backed before by the Harvard Medical Health blog, saying that a "severe vitamin B12 deficiency can lead to deep depression, paranoia and delusions, memory loss, incontinence," among other problems.

Of course, meat isn't the only source of iron and vitamin B12, as it can easily be taken with over the counter supplements, so this study should serve more as awareness that the deficiency exists and is known to have negative consequences. It shouldn't stop anyone from being vegan or vegetarian, just make sure you get your vitamins in.