Heb. 13:9“Do not be carried away by varied and strange teachings; for it is good for the heart to be strengthened by grace, not by foods, through which those who were thus occupied were not benefited.”
God’s grace is what He has given us to strengthen our hearts. There is some limited benefit of diet or exercise to strengthen the heart (“bodily discipline is only of little profit, but godliness is profitable for all things” 1 Tim. 4:8). Our modern culture is exercise crazed, just as first century Jewish culture was greatly consumed with whether food was clean or unclean (Acts 10:14) or if hands and dishes were appropriately washed (Mark 7:2,3).
The thing about such physical things are that they don’t much benefit the person who concentrates on them. They same thing is true about moral restrictions not given by God, not matter how harsh or restrictive.
Col. 2:21-23““Do not handle, do not taste, do not touch!” (which all refer to things destined to perish with the using) --in accordance with the commandments and teachings of men? These are matters which have, to be sure, the appearance of wisdom in self-made religion and self-abasement and severe treatment of the body, but are of no value against fleshly indulgence.”
In Hebrews 13, we also see that doctrines of men don’t strengthen the heart. The only thing that helps our heart, our faith, our lives is the grace and truth of God. Thankfully for us, in the new covenant, He writes that right onto our hearts and minds (Jer. 31:33; Heb. 8:10).