Besides the toxicity resulting from excessive vitamin A, vitamin D is the vitamin next most likely to be consumed in concentrations toxic to livestock. Although vitamin D is toxic at high concentrations, short-term administration of as much as 100 times the requirement level may be tolerated. For most species, including swine, the presumed maximal safe level of vitamin D3 for long-term feeding conditions (more than 60 days) is four to ten times the dietary requirement. For swine the upper safe dietary level for short-time exposure is 33,000 IU per kg (15,000 IU per lb) and for over 60 days, 2,200 IU per kg (1000 IU per lb) of diet (NRC, 1998). In practical conditions, vitamin D toxicosis for swine would only be expected as a result of feed mixing errors and then over a relatively long period.
Excessive intake of vitamin D produces a variety of effects, all associated with abnormal elevation of blood calcium. Elevated blood calcium is caused by greatly stimulated bone resorption, as well as increased intestinal calcium absorption. The main pathologic effect of ingestion of massive doses of vitamin D is widespread calcification of soft tissues. In pigs, signs of toxicity are anorexia, stiffness, lameness, arching of the back, polyuria, and aphonia. Death was reported in four days when young pigs received 473,000 IU per kg (215,000 IU per lb) in the diet (Long, 1984). Whiting and Bezeau (1958) reported that a high vitamin D content (800 IU per lb of ration) may be involved in parakeratosis by reducing absorption and retention of zinc. Hancock et al. (1986) suggested reduced gains and feed efficiency when young pigs received 22,000 to 44,000 IU per kg (10,000 to 20,000 IU per lb) of diet. Studies in a number of species, including swine, indicate that vitamin D3 is 10 to 20 times more toxic than vitamin D2 when provided in excessive amounts.
A possible new role for vitamin D3 is being evaluated: extremely high levels of vitamin D3 are fed to cattle for short periods of time prior to slaughter to increase tenderness of the meat. Several research studies (Swanek et al., 1997, 1998) have been designed to evaluate the effects of feeding cattle supplemental vitamin D3 at 0, 2.5, 5, or 7.5 million IU per head per day for 5 or 10 days immediately prior to slaughter. The researchers determined that feeding the high vitamin D3 levels reduced the Warner-Bratzler shear force values of tough beefsteaks aged for short periods. The possible mechanism by which beef tenderization occurs is that high levels of vitamin D3 cause increases in calcium concentrations in blood and muscle; at higher concentrations of calcium, calcium-activated proteases associated with the calpain system are more active, increasing meat tenderness. Using a similar approach, Enright et al. (1998) fed swine diets containing three levels of vitamin D3 (331, 55,031, or 176,000 IU/kg) to finishing pigs for the last 10 days prior to slaughter. The researchers reported that feeding the higher levels of vitamin D3 reduced feed intake and average daily gains while increasing serum calcium concentration. Reduced drip loss and color values (Hunter L* measurement) and increased subjective color and firmness scores were exhibited in the carcass as a result of increasing dietary vitamin D3. Tenderness was not evaluated in this study. Sparks et al. (1998) determined that the optimal dosage of vitamin D3 was 500,000 IU daily for three days prior to slaughter as based on blood calcium concentration. The researchers stated that the results of feeding high levels of vitamin D3 in this initial study did not improve pork tenderness and other measures of pork quality.
