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.
Vitamin D helps the intestines absorb calcium. However, the vitamin D must first be activated or turned on by parathyroid hormone (PTH). Once activated, vitamin D acts to greatly increase the amount of calcium that the intestines can absorb from food, sometimes by as much as two to four times. The body can either make its own vitamin D using a process that requires sunlight or obtain vitamin D directly from the diet (e.g. in fortified milk or vitamins).