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.
Calcium is probably the nutrient you think of first. But vitamin D is just as important for keeping bones strong and preventing the bone disease osteoporosis. Vitamin D helps your intestines absorb calcium from the food you eat. Getting enough of both nutrients is an important part of making sure your bones are dense and strong. Unlike calcium, which you only get through food, your body makes vitamin D when sunlight hits your skin.