ayunar

To fast, abstain from food — a verb with deep religious significance across Christian, Muslim, and Jewish practice (Cuaresma, Ramadán, Yom Kippur) as well as everyday medical contexts, like ayunar antes de un análisis de sangre.

• Muchos musulmanes ayunan durante todo el mes de Ramadán. — Many Muslims fast throughout the month of Ramadan.
• Debes ayunar doce horas antes del análisis de sangre. — You need to fast for twelve hours before the blood test.
• Algunos cristianos ayunan durante la Cuaresma. — Some Christians fast during Lent.

Ayunar means to abstain from food entirely (or nearly so), not simply to eat less or diet — hacer dieta is the correct verb for eating less or more healthily without full abstention.

• quote El ayuno de Jesús en el desierto — Matthew 4:1-2 in Spanish Bible translations describes Jesus fasting for forty days and forty nights in the desert before being tempted ("Después de ayunar cuarenta días y cuarenta noches, tuvo hambre") — one of the best-known fasting episodes in Christian scripture.