Rickets & VIT. D
Rickets or epiphyseal displasia is particularly likely to develop during rapid growth such as in low brith weight infants and in adolescents.
(NELSON textbook of Pediatrics 16th edition P. 184)

Davidson's 16th ed. P68-69
The infant with rickets has often received sufficient calories and may appear well-nourished, but is restless, fretful & pale with flabby muscles and is prone to respiratory & gastro-intestinal infections. Development is delayed. The teeth often erupt late & there is failure to sit, stand, crawl, and walk at the normal ages. The bony changes are the most characteristic signs of rickets.

Delayed miestones & Vit. D deficiency
There is an increasing likelihood that the mild cases will be missed. A flabby baby towards the end of the first year, unable to pull itself up, fredful and easily irritated, with too few teeth showing and liable to profuse sweats, should always be suspected of having rickets.

Diarrhea & Vit. D
ED-3 preserves the Intestinal Mucosal Barrier.
University of Chicago AM J Physiol Gastrointest & Liver Physiol. 2007 Oct 25.
The study concluded vitamin D deficiency may compromise the mucosal barrier, increasing susceptibility to mucosal damage and the risk of bowl disorders.