The reference given for this statement is: Posnikoff J. Advances against Cushing's disease, available on HorseChannel.com. This article from 2005 suggests: "Antioxidants, such as vitamins E and C, could play a role in helping to support Cushing’s horses." No references given.
Equine Applied and Clinical Nutrition, p 297 in a chapter entitled Nutritional considerations for aged horses written by Sarah Ralston and Pat Harris, says "low plasma vitamin C concentrations were observed in horses with documented (on post mortem) adenomas of the pars intermedia (Ralston et al 1988).
The abstract (we haven't yet found the full paper online) for Ralston et al 1988 (Differences in diagnostic test results and hematologic data between aged and young horses) found "In group 1, plasma ascorbic acid values were lower (P less than 0.05) in aged (8 mares > 19 years old) horses than in young horses (6 mares < 6 yrs old) maintained under the same conditions and feeding regimens." Does this suggest there was no difference between old and young horses in group 2?
In Management of Geriatric Horses, Sarah Ralston writes "vitamin C supplementation (10 gm twice a day) increased antibody response to vaccines in aged horses, especially those with pituitary dysfunction (Ralston and Quackenbush, unpublished data), and in my experience helps old horses with chronic infections".
Redwing's Equine Cushing's Disease: The Facts "suggests Your vet may advise a supplement containing
B vitamins and Vitamin C if grazing is especially restricted, as these are helpful in supporting the immune system".
More about vitamin C and the older horse in Nutrition of the Aged Horse by Kathleen Crandell - Kentucky Equine Research.
A 2011 abstract (Kentucky Academy of Science) by Jeffrey Chalfant and Dianne McFarlane: CSF and plasma ascorbic acid concentrations in horses with PPID compared to age-matched controls
states that exposure to free radicals leads to oxidative stress damage, suggesting that horses with PPID may be deficient in one or more antioxidants. Ascorbic acid (vitamin C) decreases with age in horses and is an antioxidant. Age is the major risk factor for PPID. Therefore Chalfant and McFarlane hypothesizes that horses with PPID might "have decreased plasma and CSF concentrations of ascorbic acid (vitamin C) compared to age-matched controls".
3 groups of horses had blood (plasma) and cerebrospinal fluid tested for ascorbic acid concentration: horses diagnosed with PPID, horses of similar ages that didn't have PPID, and young horses without PPID. Preliminary results showed that mean CSF ascorbic acid concentrations were higher than mean plasma concentrations, perhaps because of vitamin C degradation, or perhaps because plasma samples were subject to increased handling - no further results were reported, just that further experiments are required to produce accurate samples.
The abstract concludes: "If our hypothesis proves true, and horses with PPID have lower plasma and CSF ascorbic acid, this would suggest that an intervention with vitamin C therapy could potentially benefit horses with PPID by reducing oxidative stress." However, we have been unable to find any follow up to this research.
OK, so a horse with PPID might benefit from supplementation with vitamin C. But how much vit C is there in the natural diet of a horse?
The OECD SIDS for L-Ascorbic Acid suggests that there is approx. 500 mg vitamin C per kilogram of grass - this would appear to be the amount as fed (AF), not dry matter (DM). Grass is around 20% dry matter (source NRC Nutrient Requirements of Horses 2007 p 306 "grass pasture, cool season, veg DM % as Fed 20.1"). If a 500 kg horse ate 2% of its bodyweight in grass per day, that would be 10 kg DM, which would be 50 kg AF. 50 kg x 500 mg = 25 g vit C.
This article from KER states that "plants are a natural source of vitamin C and green growing grass has plenty; however, hay is virtually devoid because of the oxidative instability of vitamin C".
Vitamin C in Horse Diets - Kentucky Equine Research Staff - December 2013
Hmm,interesting. So when a horse gets laminitis, the first thing we do is remove it from its natural source of vitamin C - grass - and feed it hay, often hay that is a year or more old (although it is a myth that old hay is "safer" for laminitics - generally it is simply lower in vitamin content). Interestingly a few years ago Andy Durham speculated that removing laminitic horses from grass could be contributing to the development of PPID - now researchers are reasonably certain that there may be a causal link between EMS and PPID, although this may well be due to the high levels of insulin, systemic inflammation... food for thought though - literally!
So are there any disadvantages with supplementing vitamin C?
Well, possibly - vitamin C increases iron absorption. And iron may be linked to the development of insulin resistance and PPID.....