Not all mainstream doctors are meat and dairy pushers. I brought my omnivore husband to the Mayo Clinic in Rochester Minnesota (this is a world renown health system) due to issues he was having with severe anemia, rheumatoid arthritis, and immune deficiency. The doctors there were almost all pushing a plant based diet and were very interested in my veganism. I showed them research on rheumatoid arthritis and studies showing marked improvement in subjects with RA who followed a plant based diet.
Although my husband is very thin, he has very high cholesterol from years of guzzling tons of dairy. His parents, both dairy farmers, also have very high cholesterol and are on meds for it. They are also big meat eaters and hunters. His dad has hypertension and COPD. I am also a medical coder and used to work in Health Information Services at a large medical complex working with radiation oncology charts. I have coded and scanned literally thousands of cases by now over the years. Most of the cases all have sections that mention special diets and so on as part of their treatment. I have yet to see any vegans or even vegetarians mentioned. Even the numerous cases of iron deficiency anemia I have coded were all asked if they were vegetarian by their doctors and all of them said no, they were meat eaters. All of this information is in medical records.
I have been vegan (no slips) for three years and nine months. I have had my B12 checked, hemoglobin, vitamin D, blood calcium, HDL, LDL, total cholesterol, blood glucose, triglycerides, blood pressure, and more checked. Every one of these levels have been excellent and some even majorly improved since going vegan (for example my total cholesterol in 2010 as an omnivore was 155, and as a vegan in 2012 when I had a screening it was 125; vitamin D was 44 in 2010, in 2012 it was 77). I used to be on 112 to 125 mcgs of thyroid meds as an omnivore. As a vegan, even consuming soy here and there, I have only needed 88 to 100 mcgs of thyroid meds. I am currently on 100 mcgs. The ONLY thing that has gotten worse is that I have osteoporosis that was diagnosed when I was an omnivore in 2006 (I lost my ovaries and uterus when I was only 33 years old and also suffered with anorexia for years and was a very low weight for a long time). Over the years my bone density has declined, even when I was consuming Greek yogurt and sardines/salmon several times a week as an omnivore. I don't blame being vegan for it. There are many other factors at play. I did however intentionally put on over 22 lbs to get to a healthy weight to battle my osteoporosis.
It is such a stereotype that vegans are all too thin and weak and will have problems down the road. If you get involved in activist groups and get to know long term vegans, you will see that they are as vibrant as ever years later. There are more and more people being born and raised vegan now who are adults and doing just fine having never consumed meat. There really isn't anything to worry about. There are many vegan nutrition books and studies out there to check out for peace of mind to make sure you are meeting your needs if that is a concern.
I also like what another poster said about whether it is natural to eat meat or not. Murder and rape are also "natural" human acts that have occurred since the beginning of time. It doesn't justify them. Besides, there is NOTHING natural about keeping animals in cages and on farms and breeding and raising them for food, or keeping fish crowded in tanks as a food source, or scouring the ocean floors with huge nets to catch more and more fish to feed more and more people that this planet can not sustain. If we need to eat sea food, the least we can do is cultivate sea vegetables instead of sentient beings to meet our DHA/EPA needs. And indeed you can find vegan DHA/EPA supplements derived from sea vegetables, the same food that fish get their DHA/EPA from. You can also get other omega 3s from flaxseeds, walnuts, sunflower seeds, leafy greens, avocado etc. Animal ethics and health are not mutually exclusive.