This seems to be at odds with the information published in NRC's Nutrient
Requirement of Horses (1989). "Krook and Maylin (1988) proposed that
osteochondrosis may be associated with excess dietary calcium (e.g., from
alfalfa hay) and its production of HYPERcalcitonism." (my emphsis) Am I
misreading something or has further research invalidated this? The full
citation is Krook, L., and G. A. Maylin, 1988, Fractures in Thoroughbred
race horses, Cornell Vet., 77:172.
I agree with Dr. Newell that an occassional bran mash is not going to
change the overall Ca/P, but I hear of people substituting rice bran for
grain (Perhaps these are commercial preparations with supplemental Ca).
Again, quoting NRC: "Ratios less than 1:1 (i.e., when phosphorus intake
exceeds calcium intake) may be detrimental to calcium absorption. Even if
the calcium requirements are met, excessive phosphorous intake will cause
skeletal malformations (Schryver et al., 1971b)." Full citation: Schryver,
H. F., H. F. Hintz, and P. H. Craig, 1971b, Phosphorous metabolism in
ponies fed varying levels of phosphorus, J. Nutr., 101:1257. Has this
study been invalidated?
Duncan Fletcher
dfletche@gte.net
----------
> From: Gwen Dluehosh <dluehosh@vt.edu>
> To: endurance@moscow.com
> Subject: Re: Calcium metabloism (long, from class)
> Date: Tuesday, November 26, 1996 1:30 PM
>
> HI Joyce!
>
> she is correct about this. When the osteoclasts stop knocking on the
bone
> to get calcium because there is enough already in circulation, production
of
> PTH slows down and when the cow then calves, the sudden need for CA is
not
> met by the feed, the PTH can't get the Ca out of the bone fast enough,
so
> the cow experiences hypocalcemia. If this is corrected the cow will be ok
> once the bone gets stimulated enough (aobut 24 hours later). THis is
> probalby exactly what's happening in the endurance horse. We need the
excess
> calciun during the ride for an acute effort, but since the horse isn't
> getting it from alfalfa while he is being ridden, he ties up. I guess the
> theory behind it is that if the horse is being stimulated by lack of Ca
then
> there is always a "background level" of stimulation going on which won't
> occur when Ca rich feed is being given.
> Here is what it says from my CLin Pathology book:
>
> "Bone is made up of a matrix of collagen fibers that is mineralized by
> hydroxyapatite crystals. THese bone salts are largely composed of Calcium
> and Phosphorus. A portion of theses salts is readily mobilized, which
forms
> the so-called exchangeable calcium adn phos which are in equiloibrium
with
> the serum [blood]. Theses exchangeable ions are responsible for buffering
> and evening out sudden changes in serum calcium to keep it within normal
> limits.
>
> " Bone is constantly being remodeled as it is torn down by osteoclasts
> [cells that eat bone] and rebuilt by osteoblasts [bone making cells]. HTe
> presence of PTH (parathyroid hormone) enhances the absorption of Ca and O
> from the bone and increases the concentration of theses electrolytes in
the
> serum. THe presence of Calcitonin shifts teh balance to mineralizing and
> laying down bone. THerefore, CT causes a decrease in serum calcium.
> {skip some}
> PARATHYROID HORMONE
> "PTH is responsible for the minute to minute regulation of Ca
concentration
> in the blood. Hypocalcemia (not enough Ca) causes the parathyroid glands
to
> secrete PTH and conversely, hypercalcemia (too much Ca) causes the
glands
> to cease secretion of PTH. PTH also has an indirect effecton the
intestine
> as it promotes formation of active metabolite of Vitamin D in the kidney.
> HTe presence of Vitamin D increases absorption of both calcium and
> phospohorus in the intestine (now you know why they put Vitamin D in
milk!).
> The net effect of PTH is to increase the concentraion of Ca and decrease
P
> in the extracellular fluids becasue the renal effects are so profound.
> {skip some}
> HYPOCALCEMIA
> "MOst of the disturbances that result in hypocalcemia cna be attributed
to
> disturbances of PTH, CT or Vitamin D. Depending on the hormoine that is
> disturbed, the effect on circulating calcium and phosphorus can generally
be
> predicted by its physiologic actiona s described above.
> DISORDERS OF PTH
> {skip some}
> "MOre commonly, we see functional hypothyroidism when a sudden heavy
demand
> for calcium accompanies lactation. THis is most often seen in cattle, but
> similar conditions are found in bitches, mares, and ewes. Parturient
> hypocalcemia is a complex metabolic disease of high producing dairy cows.
It
> is generally found in COWS THAT ARE FED A HIGH CALCIUM DIET during
> pregnancy. THis causes calcitonin release and a chronic reduction of bone
> remodeling. When parturition causes a sudden, heavy demand for lactation
and
> a drain on blood calcium, the bone resorptive cells are not present in
large
> enough quantities to respond to PTH. HYPOCALCEMIA, HYPOPHOSPHATEMIA, and
> MUSCLE PARALYSIS result. Hyperglycemisa is often found as adequate blood
> calcium is needed for insulin secretion.
> Hmm, more interesting stuff...
> "Nutritional secondary hypERparathyroidism occurs most often in young
> growign dogs, cats, and HORSES that are fed a diet that is high in
> phosphorus and low in calcium. ALl meat diets are most often the culprit
in
> dogs and cats and EXCESSIVE bran supplementation is often implicated in
horses."
>
> OK that's enough for today, but that's what my book said.
> I also had to review a case of tying up for this class, and the
mechanisms
> are the same according to our professors. There may be other mechanisms
> involved, but this is the main one and will cause all the major signs of
> tying up in conjunction with muscle damage.!
> HOpe this clarifies and sorry about the big words but I am confident most
> everyone will understand, if not, write me...
> Gwen
> .Seems to me that an
> >> analogy with distance horses wouldn't be too far-fetched.
> >> (I mean, this is why horse fed diets high in alfalfa tie up.)
> >>
> >Someone please stop me before I hurt myself, or get hurt! I just can't
let
> >this one go, and no, I'm not educated in animal husbandry, and I may be
way
> >off base here, but when, oh when, is a dairy cow like an endurance
horse?
> >I can just see me riding 'ol Bossie to my top ten finish! OK, maybe
horse
> >and cow physiology is similar, but I just have to check for myself on
this
> >one. Sorry.
> >
> Gwen Dluehosh
> Desert Storm Arabians
> 1156 Hightop Rd, #89
> Blacksburg, VA 24060
> 540/953-1792