Srimad Bhagavatam Canto 07, Chapter 12, Text 23

SB 7.12.23

yadakalpah sva-kriyayam
 vyadhibhir jarayathava
anviksikyam va vidyayam
 kuryad anasanadikam
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
When because of disease or old age one is unable to perform his prescribed duties for advancement in spiritual consciousness or study of the Vedas, he should practice fasting, not taking any food.
Srimad Bhagavatam Canto 07, Chapter 12, Text 22
Srimad Bhagavatam Canto 07, Chapter 12, Text 24