In the 1930s, the English physician Lucy Wills was investigating a severe form of anaemia affecting pregnant textile workers in Bombay, India. She observed that improving their diet often cured the condition. In her search for a simple, effective supplement, she turned to Marmite.

Her experiments were a resounding success. Giving the women a daily dose of the yeast extract spread cured the anaemia. Dr. Wills had discovered what she called the "Wills Factor," an unknown nutrient in Marmite that was essential for preventing this type of anaemia. This factor was later identified as folic acid (Vitamin B9), which is now known to be crucial for preventing birth defects.

Marmite was also used to combat malnutrition during a major malaria epidemic in Sri Lanka (then Ceylon) in the mid-1930s. This solidified its reputation not just as a food, but as a life-saving nutritional supplement.

Source: https://en.wikipedia.org/wiki/Marmite, https://medium.com/@jessica.h.s.wallace/the-medical-history-of-marmite-33d379124396