Bone Remodeling in Horses

Bone remodeling is an ongoing process in horses, and in all mammals larger than rats.  This lifelong process repairs micro-damage to bones caused by everyday stress, and promotes the deposition of newer, better material.  It includes bone resorption, where mature bone tissue is removed from the skeleton, and ossification, the process by which new bone is formed. Bone remodeling also responds to the demands of mechanical loading and impact, which can affect normal bone homeostasis—that is, the coordinated balance of the cells responsible for bone formation, known as osteoclasts and osteoblasts.

When there is bone homeostasis, the osteoclasts break down bone and the osteoblasts make new bone.  But when bone homeostasis becomes imbalanced, the osteoclasts are breaking down bone faster than the osteoblasts can rebuild.  In humans this is known as osteoporosis.

The role of minerals in bone remodeling
Calcium, phosphorus, and magnesium are important co-factors in the bone remodeling process.  The osteoclasts break down bone, thus freeing the calcium, while the osteoblasts deposit calcium into new bone.  The bone matrix itself is made of a mineral known as hydroxyapatite that contains calcium, phosphorus and collagen.

The role of Tildren and Osphos
When the coordinated activity of osteoclasts and osteoblasts becomes unbalanced, the osteoclasts break down more bone material than the osteoblasts can replace.  Tildren and Osphos belong to a group of drugs called bisphosphonates.  This class of drugs has been used in human medicine for osteoporosis, bone cancer, and periodontal disease.  Bisphosphonates inhibit bone resorption by reducing the activity of the osteoclasts, thus increasing bone density.

Osteocalcin and vitamin K
Research has highlighted osteocalcin, which is a calcium-binding protein found in the extracellular matrix of bone and produced by osteoblasts during bone formation.  Osteocalcin is the most abundant non-collagenous protein in bone.  It regulates bone mineralization and is routinely measured in human medicine as a serum marker of bone formation.

Osteocalcin is vitamin K-dependent, and low dietary intake of vitamin K can lower bone density and bone mass.  There are three forms of vitamin K: vitamin K1 from green leafy plants; vitamin K2, synthesized by the horse in the liver; and vitamin K3, a synthetic version added to many commercial feeds.

Vitamin K was once thought to be important mostly for its role in blood clotting.  But research on humans and horses show that vitamin K plays a major role in bone metabolism and subsequent cartilage and joint integrity, and is essential for the production of osteocalcin.

Horses out on fresh pasture 8-10 hours per day generally receive their daily requirement of vitamin K1 from the grasses.   But horses on limited turnout, or confined to dry paddocks, may not be getting enough vitamin K1.

Vitamin K1 is a product of photosynthesis, which means that when a pasture is cut for hay, the vitamin K1 content quickly diminishes.  A study in Australia showed that freshly cut rye grass yielded 8.9 mg of vitamin K1 per kilogram. However, when the pasture was cut for hay and stored for two weeks in a shed, the vitamin K content dropped to 1.9 mg per kilogram.

Winter pastures that have turned brown have virtually no vitamin K1.

Supporting bone homeostasis
Daily training and exercise (including short periods of intense exercise) can produce a beneficial response in bone, stimulating bone modeling and improving skeletal strength.  However, sustained high-loading of limbs can cause the remodeling rates to be reduced.

Long breaks from training, either voluntary or because of injury, can lead to a decrease in bone mass and strength.

Several small studies on thoroughbred yearlings and adult horses showed that there is an enhancement of bone mineral content associated with “free exercise”: allowing horses access to pasture as much as possible.  Confinement of yearlings to box stalls was shown to decrease the mineral content of the cannon bone compared to yearlings kept out on pasture.

Horses on spring, summer, fall pasture 8-10 hours per day are getting a good supply of vitamin K.  Horses that are primarily kept in stalls and horses on winter pasture benefit from vitamin K support.  The blue-green algae Spirulina is a wonderful food source of vitamin K1 and can be found in all of  BioStar’s Optimum formulas.

Adding shilajit, a bio-resin from the Himalayas, has shown to increase osteoblast formation and the up-regulation of six different types of collagen.  This is important both for horses in work and horses on layup.

Supplementation
Because bone remodeling is an active, ongoing process,  BioStar provides shilajit with Re-Natured® vitamin K1 in a matrix of supportive foods and minerals in a formula called Quantum EQ.

The vitamin K1 we use is grown and cultured in alfalfa, which is a natural food source (green leafy plant) of the vitamin. The cultured broth of alfalfa and vitamin K is then blended with live yeast which metabolizes the broth, providing receptor sites for Vitamin K.  This is a far more expensive source of vitamin K1 than the synthetic version, but the bioavailability is significantly higher.

Bone remodeling is an ongoing process throughout our horses’ lives.  Exercise, hours spent out on green pasture, and the minerals calcium, phosphorus and magnesium are the foundation of a healthy balance between the breakdown and building of new bone.