I was vegetarian for almost 20 years ('91 - '10) and even now don't eat meat often.
I've also been donating blood for almost 40 years.
It was fine during the early years - I ran a number of distance races during that time, and did my best marathons in '92 & '93.
By '06 I found myself working very hard to maintain a pace much slower than what I had been doing a few years earlier. Attributed it to age.
Did some hiking in the Rockies a few weeks later and found the elevation really hard to cope with.
Still suspected nothing because I passed the sink or float blood sample test every time I donated. Actually around that time I failed one of these tests, but they redid it and I passed.
Had a routine physical that Fall, and my hemoglobin was down at 119. Thought it was a fluke, but a retest showed it was 114. (Adult male normal is 140 - 180.)
It turned out to be a simple iron deficiency. I started on iron pills, and a few years later reintroduced small amounts of meat into my diet.
Sometime after that, Canadian Blood Services moved over to a more sophisticated hemoglobin test, which presents an exact digital readout. The old sink or float test had a threshold of around 100, so my hemoglobin would have had to have been even worse for me to have failed.
Did some volunteer work out of town a couple of weeks ago, and ate about as much red meat during the week as I'd normally have in a year.
Donated yesterday and had a hemoglobin of 169!
I don't mean to put anyone off vegetarianism - there are many excellent ethical & health arguments to be made for it, but watch the hemoglobin! Not everyone is able to take iron supplements.