As a new biography Dutch Girl: Audrey Hepburn and World War II, excerpted exclusively in this week’s PEOPLE, reveals, it was her experiences during Germany’s five-year occupation of Holland during World War II that truly shaped her.

While more has come out in recent years about her war experience, Robert Matzen’s book reveals harrowing new details about how a young Hepburn battled severe malnutrition, particularly during what was known as the “Hunger Winter” of 1944-45.

Leading up to that brutally cold winter, as Germany tightened its grip on Holland, Hepburn and her family were often forced to live in the cellar for days and weeks at a time due to bombing overhead. And food became more and more scarce.