Enter The Men's Journal (thank you, Gluten Freeway, for the link). In it's September 7th post of "The Men's Journal Guide to Going Vegan," MJ claims that just three weeks of a vegan lifestyle has you feeling better head to toe. I can definitely corroborate the quick time line.
Aside from listing vegan supplements, proteins, bars, and grocery tips, MJ also talks about "what to expect." I've inserted my comments in orange.
II. What to Expect
Your entire body will feel lighter, as the meat built up in your gut is literally forced out by the deluge of fiber from all the vegetables. You will also feel less sluggish. “You start to come out of this fog that many people have from eating heavy, fatty foods,” says Susan Levin, the director of nutrition education at the non-profit Physicians Committee for Responsible Medicine. “When you give up dairy, you immediately breathe easier.” You’ll also have to deal with cravings for things like cheese. Find an appropriate substitute, like soy cheese. Your taste buds will adjust within a week. Uhhh...I don't know about any "cheese craving." But if you don't get enough healthy fats, you will crave animal products. When I first became a vegan, I was so busy eating beans, veggies, cookies, and rice, that I found myself craving meat. My friend told me my fat levels were likely down, and so I immediately incorporated nuts, seeds, and olive oil into my cooking and snacking. Voila! Also, soy is not the solution to all. It is generally over-processed, and there is controversy as to whether over-consumption has adverse health implications. Try making your own cashew cheese or picking up rice milk or almond cheese (available at Whole Foods).
You will have noticeably increased energy, and you’re likely to see some slight weight loss, because your overall calorie intake has likely gone down. “Not much weight loss,” says Levin. “We don’t want people dropping weight like crazy.” With increased energy, she says, you will find your workouts getting better and, as pro athletes have noted, your recovery time will become shorter. By the end of your second week, says vegan ultramarathoner Scott Jurek, you won’t feel as achy after your workouts.
With more energy, says Levin, comes a brighter mood and outlook. According to a 2009 Arizona State University study, people who cut all meat from the diets, including fish, showed less tension and stress.
“Enjoy everything you had in week two, but even more energy and probably a final layer of weight loss,” says Levin. But really, this week is where it gets molecular. “If you were someone who was meticulous and into blood labs, you’d actually see your blood sugars and cholesterol levels go down,” says Levin. “Your blood pressure will also fall as you’re breathing better and your arteries are clearing out.” If you are already in your target range of caloric-intake, a vegan transition likely won't result in weight loss. If you over-eat dishes with animal fat regularly, then perhaps. And don't forget--veganism still means cupcakes, cookies, brownies, fried seitan, and other deliciousness. Also, a note on the increased energy: it is true that you feel a crazy boost of energy. But then that becomes your everyday life, and the feeling normalizes over time.