Modern day giants (e.g. Robert Wadlow) often have serious health problems. Yet according to 1 Samuel 17:4 Goliath was six cubits and a span high (ca. 320cm). Any hypotheses as to how the giants survived without health problems? I know some will say Angelic DNA, but how spiritual beings can have DNA I don't know
Don't know if that is a good answer but couple of years ago i read interesting essay that connect the existence of "giants" to neurological disease on the pituitary gland that affect on growth hormone.
According to this when one suffer from that disease he become a giant' and he suffers from side effects that well describe on the story:
He moves slowly - as opposite to David that described running.
He suffer from lack on the field of vision - that's why he couldn't see the stone that David throw.
And he die younger - (as @DrPeterMcGowan point on his comment) we don't know how old he was, but assuming that he would win he would still (probably) wouldn't live long life.
"There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown."
(Gen. 6:4 KJV)
The passage referred to in Gen. 6 must be understood in context: Angelic(or demonic) hosts cannot cohabit with men. This point is made clear when Jesus said stated in Matt. 22:29-30, when He said,
"Ye do err, not knowing the scriptures, nor the power of God. 30 For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven."
The angelic hosts, or fallen angelic hosts, lack the ability to procreate with man. But we see from Gen. 6 that they were able to "affect" them as spiritual beings in such a way as to alter their DNA, "creating", or perhaps "morphing" a giant. Since from the Scriptures we see Satan inflicting disease, sickness, and causing superhuman strength to exhibit itself in those who Jesus healed or delivered(See Mark 5, Luke 13:16). Apparently, this "DNA alteration" took place over a period of time, since in Gen 6:3 it says,
And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
The descendants of Adam lived a thousand years, it allowed a race of people who's genetic structure was altered to become "giants". From the Book of Enoch, we read these giants tormented mankind and dominated them, possessing(and being "possessed") with superhuman qualities that weren't part of God's Original Creation.
Only 8 people(Noah, his wife, his 3 sons and their wives-Gen. 7:7) made it on the Ark, this generation of DNA altered individuals died off; although it appears that some survived in the Canaanite Nations that the Israelite's were intended to destroy,(Num. 13:33)
And there we saw the giants, the sons of Anak, which come of the giants: and we were in our own sight as grasshoppers, and so we were in their sight.
Since none of Noah's children were giants, we can only surmise that the remnant DNA exhibited itself in the Canaanites-especially in the area called, "Valley of the Giants", or the Valley of Hinnom(Joshua 15:8).
In response to the question: "Giants" were not originally designed by God, they were "morphed" because of Satanic interference, therefore because of Satan's interference with God's Original Plan, defective DNA has interfered with His Creation throughout man's history. The physical prowess of these giants, from the story of Goliath was "of renown", the term used in Gen. 6:4, yet their strength was attributed to evil sources, not God. Though they were destroyed as a race by the Flood, they continued on in DNA, concentrated in the Valley of the Giants which Moses wiped out(Jos. 13:2). Any remnant today is a weak copy of what previously existed.