Complex sentence: Plasma zinc levels have been found to be dependent upon vitamins A and D. This suggests that a Vitamin A or D deficiency could cause a secondary zinc deficiency and that for treatment of zinc deficiency one should ensure adequate vitamin A and D intake.
Split into simpler sentences: Plasma zinc levels have been found to be dependent upon vitamins A and D. This suggests that a Vitamin A or D deficiency could cause a secondary zinc deficiency. And that for treatment of zinc deficiency one should ensure adequate vitamin A and D intake.