Not only is food which merely fills the belly not eternal food, it is not true food. For once you eat it and digest it, you will be hungry again, just like how the water you drink to quench bodily thirst will leave you thirst again. It will not, and will never truly satisfy you. This is because it is not true water. Here is what Jesus said to the Samaritan woman from whom he asked for water:
Jesus answered and said to her, “Whoever drinks of this water will thirst again, but whoever drinks of the water that I shall give him will never thirst. But the water that I shall give him will become in him a fountain of water springing up into everlasting life.” (John 4:13-14).
Food is not the things you eat. Food is to do the will of God. That is why Deuteronomy 8:3 says:
So He humbled you, allowed you to hunger, and fed you with manna which you did not know nor did your fathers know, that He might make you know that man shall not live by bread alone; but man lives by every word that proceeds from the mouth of the Lord.
This is affirmed by Jesus Christ Himself who quoted this glorious, beautiful and wonderful Scriptural truth to satan when he tempted Him to turn stone into bread:
Then Jesus was led up by the Spirit into the wilderness to be tempted by the devil. 2 And when He had fasted forty days and forty nights, afterward He was hungry. 3 Now when the tempter came to Him, he said, “If You are the Son of God, command that these stones become bread. But He answered and said, “It is written, ‘Man shall not live by bread alone, but by every word that proceeds from the mouth of God.’”(Matthew 4:1-4).
If your food is temporal earthly food which you eat and feel hungry again, you are not serving Christ. You are serving yourself, your belly. In serving your belly, you are serving mammon. Since no one can serve both God and mammon (Matthew 6:24), you cannot serve both God and your belly, seeking your temporal earthly needs. You cannot serve both God and your needs.To seek your own earthly needs, even legitimate basic needs is itself to serve mammon.