Actually there is a school of thought that says lower weight leads to longer longevity. In fact a low calorie diet rich in whole grains and fiber along with essential nutrients is what one scientist (Ray Kurzweil) is taking and he claims he got rid of his diabetes and is now looking and feeling 10 years younger than he did 10 years ago.
So, may be Steve should quit eating ice cream and remain healthy. All that milk fat is hard on the liver too.
Not too low, though. Extremely thin people have higher risk of bone fractures when they get older (not enough cushion to soften the blow, and extreme thinness is often accompanied by calcium deficiency). Regular-sized to moderately overweight women also have fewer problems with menopause because the body can draw from the estrogen stored in fat.
(yeah yeah, [citation needed] and all that...)
There are plenty of ways to regain weight, and still be healthy and vegetarian. The aforementioned Burger King's BK Veggie can definitely help (hold the mayo, though-- extra cholesterol is not good for the liver). There are also soy-based "ice cream" confections that taste quite good, and there's always Silk, which has no cholesterol yet contains a good bit of polyunsaturated fat. Decaf soy latte, Mr. Jobs?