For every one that useth milk is unskilful in the word of righteousness: for he is a babe. But strong meat belongeth to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil.
(Hebrews 5:13, 14 KJV)

Athletes and people who work in manual labor professions require the protein in meat for building muscle strength. Meat is much more difficult to digest then milk. There is work required chewing meat and the stomach has to work harder to enzymatically degrade meat into small absorbable particles for digestion. Milk and meat are symbols used in the Bible to differentiate levels of scriptural truths. The meat of the Word of God requires more work for a deeper level of understanding. Extra time is needed to study and reflect upon exactly what God is saying to us. Increased commitment to apply the Biblical truths to our lives gives us spiritual strength. Activity in difficult ministry work demands the strong meat of the Scriptures to help us discern good and evil. Mature spiritually and be meat eaters of God’s Word.