Complex sentence: Nicotine constricts blood vessels, including those of the gums, which has led to speculation that long-term use of nicotine gum may contribute to risk for gum disease.
Split into simpler sentences: Nicotine constricts blood vessels, including those of the gums. Prolonged use of nicotine gum may lead to inadequate blood flow through the blood vessels in the gums, which can make them susceptible to gum disease.