Ok-so I've been riding for 40 years, and teaching and training for 20. I have always based a horses' jump on his athletic abilities, scopiness, etc. I've seen the solid, 2'6 packers that can stretch to 3' but are better suited to the 2'6 job.
Bioactive vitamin D or calcitriol is a steroid hormone that has long been known for its important role in regulating body levels of calcium and phosphorus, and in mineralization of bone.
Your body needs several other nutrients in order for calcium to be absorbed and used properly, including magnesium, phosphorous, and especially vitamins D and K. Many factors, including age, disease states, and medications, can affect calcium absorption.