20/07/2026
Reduced Calcium Absorption in horses and ponies with Cushings Disease
By Dr. John Kohnke BVSc RDA
In aged horses and ponies with PPID Cushings Disease, there is concerning problem related to reduced calcium absorption and higher excretion losses caused by the side-effects of the PPID hormone changes. This secondary effect on calcium absorption in horses grazing 24/7 on high oxalate pastures, including highly hazardous kikuyu, buffel grass and setaria spp grasses, have an increased risk of developing Nutritional Secondary Hyperparathyroidism or NSH with symptoms of shifting lameness, loss of bone weight bearing strength and ‘Big Head’ due to severe calcium binding by oxalates.
The reduced calcium absorption is also linked to Cushing’s Disease, which results in the small intestine of a PPID affected and an aged horse or pony over 16 years of age, having a lowered ability to absorb calcium from the feed or supplements.
The hormone changes associated with PPID have an effect of reducing the absorption of many nutrients, including the vital bone-maintaining calcium from the small intestine. Studies have shown that the increased ACTH hormone blood levels released in PPID affected horses and ponies, acts to reduce to overall surface area for calcium absorption resulting from a reduction in the length and cell numbers of lining projections or ‘villi’ and the reduced depth of the folds or ‘crypts’ in the lining.
This is as result of the action of the PPID changes and high levels of ACTH hormone secreted from the pituitary gland due to the benign tumour or cancer development in the pituitary gland of PPID affected horses and ponies.
In a horse, or pony, 80% of the calcium taken in with food or supplemented in an available form, is absorbed primarily from the small intestine. The calcium binding effects of oxalate chemicals in tropical grasses to attach and bond to calcium in the small intestine, limits the amount of free calcium available for absorption.
Although, the calcium-oxalate bond is released during hindgut microbial fermentation of the feed, only 20% of the calcium is absorbed through the lining of the large intestine or hindgut, so it is too late to be absorbed effectively to maintain blood and ‘bone bank’ levels.
Unfortunately, this combined with the higher blood levels of cortisone released by the elevated circulating ACTH hormone, also acts to also reduce the normal small intestine surface area for optimum calcium absorption from the small intestine.
It also increases its rate of excretion from the blood and from bone stores as well, due to higher kidney excretion of calcium.
There is also a loss of 300 mg per litre of sweat as PPID affected horses and ponies often have an elevated body temperature and sweat more, especially with altered thermoregulation function in the body and thicker haircoat which often needs to be clipped so that they can maintain a more comfortable body temperature.
In summary, although a calcium supplement, such as Cal-Extra, provides sufficient calcium which is unable to be bound by oxalates due to its innovative formulation, the lower intestinal absorption and higher excretion rates of calcium in PPID affected horses and ponies may not be adequate at the normal recommended dosage amounts.
An increase of one third more of Cal-Xtra should help to counteract the reduced absorption of the calcium from the small intestine. The absorption of Vitamin D3, vital for the uptake and normal regulation of calcium, in Cal-Extra, is unlikely to be affected by the PPID effects.
It is also recommended that owners include a biscuit of lucerne hay in her diet each kilogram provides 12.5 grams of bioavailable calcium, a good content of natural sunlight activated Vitamin D and 170g per kilogram of good quality protein. Horses over 16 years of age have a reduced ability to absorb calcium and protein, so sun-cured lucerne hay is a good source of both to meet their needs.
