A 2017 study in the journal Nutrients pulled national health survey data (NHANES) on blood levels of vitamins A, B6, B12, C, D, E and folate in more than 15,000 children and adults.
31% were at risk of at least one vitamin deficiency or anemia. Among people who took no supplements at all, it was 40%.
Among people who took a full-spectrum multivitamin with minerals, it was 14%.