Vitamin B6 is one of the B vitamins that is least likely to be deficient in livestock and pet diets. Because of its wide distribution in feedstuffs, nutritionists generally expect adequate levels in typical pet diets. Nevertheless, vitamin B6 deficiencies have been reported for dogs and cats when diets were prepared and processed in such a way as to destroy the vitamin or use of vitamin B6 sources that are biologically unavailable.
Concern that a dog or cat is vitamin B6-deficient probably will not arise unless the animal is convulsive and/or has a microcytic hypochromic anemia. However, a nondetectable subclinical deficiency of vitamin B6 may affect performance (e.g., the immune response) but not exhibit more traditional signs such as microcytic hypochromic anemia. If the clinical diagnosis of a dog or cat is consistent with vitamin B6 deficiency, the diagnosis can be confirmed by an injection of 25 mg of pyridoxine, which will stop convulsive seizures within one to two hours and will initiate a hematologic response (Ralston Purina, 1987).
Perry (1978) listed reasons for needed supplementation of vitamin B6 for animal diets as follows: (a) great variations in amounts of B6 in individual ingredients, (b) variable bioavailability of this vitamin in ingredients, (c) losses reported during processing of ingredients, (d) discrepancies between activity for test organisms vs. those for animals, (e) a higher vitamin B6 requirement due to a marginal level of methionine in the diet and (f) high protein diets.
Variability of vitamin B6 in feeds depends on the sample origin, conditions of growth, climate, weather conditions and other local factors. Yen et al. (1976) determined available vitamin B6 in corn and soybean meal using a chick growth assay. Corn was found to be 38% to 45% available and B6 in soybean meal 58% to 62% available. It is probably equally available for the dog and cat. There was little difference in availability between corn samples not heated and heated to 120°C. However, corn heated to 160°C contained significantly less available B6. Level of vitamin B6 contained in feedstuffs is also affected by processing and subsequent storage. Bioavailability can be as low as 40% to 50% after heat processing of feedstuffs.
Predominant losses of vitamin B6 activity in feedstuffs occur in the pyridoxal and pyridoxamine forms, with pyridoxine the more stable form. Supplemental vitamin B6 is reported to have a higher bioavailability and stability than the naturally occurring vitamin. Naturally occurring vitamin B6 in retorted milk products exhibited only 50% of the bioavailability of synthesized B6 or B6 in formulas that were fortified with the vitamin prior to thermal processing (Tomarelli et al., 1955).
The recovery of vitamin B6, as pyridoxine hydrochloride, in a multivitamin premix not containing trace minerals, was 100% even after three months of storage at 37°C. However, stability in a premix containing trace minerals was poor with only 45% recovery after three months at 37°C (Adams, 1982). Verbeeck (1975) found vitamin B6 to be stable in premixes with minerals as sulfates. However, if minerals in the form of carbonates and oxides are used, 25% of the vitamin can be lost over a three-month period. Stress agents such as choline chloride help catalyze this destruction. Gadient (1986) considers pyridoxine to be very sensitive to heat, slightly sensitive to moisture and light, and insensitive to oxygen. Retention of B6 activity in pelleted feeds after three months at room temperature should be 80% to 100% as a general rule.
