Hm, I'm not quite sure that's an accurate description of what happens. If alcohol destroyed them, the acidic environment of your stomach would likely obliterate them, and then how would you get them at all?
Instead, it would appear that alcohol inhibit your body's ability to absorb the nutrients. See the Wikipedia page for links to references.
Wikipedia wrote:Although the yeast used to make beer results in beers being a source of B vitamins, their bioavailability ranges from poor to negative as drinking ethanol inhibits absorption of thiamine (B1), riboflavin (B2), niacin (B3), biotin (B7), and folic acid (B9). In addition, each of the preceding studies further emphasizes that elevated consumption of beer and other ethanol-based drinks results in a net deficit of those B vitamins and the health risks associated with such deficiencies.