Citing research suggesting that diet soft drinks and other artificially sweetened products actually contribute to weight gain, a new advocacy group is asking federal regulators to investigate whether manufacturers including Coca-Cola and PepsiCo have engaged in false or misleading advertising. The California-based group, U.S. Right to Know, filed citizen petitions Thursday calling on the U.S. Food and Drug Administration and the Federal Trade Commission to stop those companies from branding artificially sweetened products with the word "diet."