Kark contributed to medical research, the Jerusalem Lipid Research Clinic (LRC) longitudinal study of cardiovascular, cancer, diabetes and cognitive risk factors (from the 1970s-today) and studies on adolescent predictors of cancer and cause-specific mortality in adulthood of over two million people, with a follow-up period extending over four decades. His study in Israel during the 1991 Gulf War showed the impact of stress on premature death. His work on the relationship between religious observance and health compared religious and secular Israeli kibbutzim and discovered lower mortality rates in the religious kibbutzim for both men and women, suggesting a protective effect.