Charles I likely had rickets, and there are a few sources that indicate this is the case. The first is a letter written by King James' physician, Henry Atkins. When James traveled south from Scotland to claim the throne in 1603, he left the three year old Charles behind. In 1604, Atkins was sent back to Scotland to check on Charles, and found the child "well out of order." (ODNB, "Henry Atkins", by Norman Moore, rev. Patrick Wallis). Atkins wrote James that Charles' "joynts of his knees, hips and ankles being great and loose are not yet closed and knit together." (Quoted in Keevil, "The Illness of Charles, Duke of Albany (Charles I)" in The Journal of the History of Medicine 1954). Within a few months, Charles was able to walk, which he had never done before Atkins visited (CSPD, 1603-1610, p. 128). All this points poor early bone development consistent with Rickets.

A latter reference to Charles' difficulties is an anecdote reported in an early biography of Charles' older brother Henry. There are a few versions of the story, but most include the relevant point. Sometime c. 1610, Henry was mercilessly teasing Charles over the deformity in his legs, and stated that Henry would have to make Charles Archbishop of Canterbury, because only a bishop's robe would hide the deformed legs (The oldest reference to this in Cornwallis' biography of Henry, printed in 1641, but originally written in 1626. Later authorities covering either part or all the story include Birch, Life of Henry, Prince of Wales 1760, pp. 399-400; ODNB "Henry Frederick, Prince of Wales"; Gregg, King Charles I; Carlton, Charles I: The Personal Monarch; and Purkiss, Literature, Gender and Politics During the English Civil War). Charles was also known to have had a pair of boots specially made for him to correct the deformity in his legs and allow him to walk.

As to how Charles developed rickets, the main authority is still Keevil's essay cited above. Infants were typically weaned from breast milk at one year, and then fed a mixture of milk or whey with flour. The diet of a child, even a prince, would have been mainly carbohydrates, with little in the way of fruits, vegetables, eggs or dairy products. He may have also had limited exposure to sunlight in his early years in Scotland, but there aren't really sources to tell us this. Overall, the conclusion most scholars have come to has been that his diet may have been enough calories for a growing boy, just simply not enough vitamins C, D, phosphorus or calcium (Gregg, King Charles I, pp. 11-12).