There was another famine in the land in addition to the one that had occurred in Abraham’s time. And Isaac went to Abimelech, king of the Philistines, at Gerar. The Lord appeared to him and said, “Do not go down to Egypt. Live in the land that I tell you about; stay in this land as a foreigner, and I will be with you and bless you. For I will give all these lands to you and your offspring, and I will confirm the oath that I swore to your father Abraham. I will make your offspring as numerous as the stars of the sky, I will give your offspring all these lands, and all the nations of the earth will be blessed by your offspring, because Abraham listened to My voice and kept My mandate, My commands, My statutes, and My instructions.” So Isaac settled in Gerar. (Genesis 26:1-6)
Famine was common in the ancient world, and was in fact very common through most of human history. Obesity was certainly not an epidemic. Rather, people regularly worried about starving to death.
As famine came once again to the land where Isaac was living, it was only natural for him to think about traveling to a place where food was more abundant. Such a choice was common and the smart thing to do; people moved all the time for just that reason. But when God told Isaac to stay where he was, he believed God and acted on that belief. The time had not yet come for the Israelites to go down to Egypt. Not until the famine during the life of Isaac’s son Jacob (renamed Israel) would God send his people into Egypt, a place where they would, in the end, suffer as slaves for four hundred years. Isaac did not know what the future held; but he accepted that God knew what he was doing and that things would work out according to God’s own timing. The promise that God made with Isaac was the same one he had made with Abraham earlier (see Genesis 12:1-3).