Research Note

Transition Vitamin D?

Timing Key for Supplementing 25(OH)D₃

U.S. dairy animals have benefited from supplemental vitamin D continuously since the 1930s, with the most recent updated dietary inclusion recommendations in NASEM 2021. Yet year-on-year advances in dairy genetics and performance have helped rekindle concern about hypocalcemia, especially during the transition period.

Hypocalcemia occurs when cows cannot adapt to the rapid, irreversible loss of calcium at the onset of lactation. Adequate supplies of circulating vitamin D metabolites are critical to the intricate endocrine system that regulates Ca, phosphorus, and skeletal homeostasis. Hypocalcemia also negatively impacts immune function, digestion, and overall health status.

“There have been many attempts to prevent or treat hypocalcemia using various vitamin D metabolites,” says Dr. Corwin Nelson. “Conventional dietary supplements are in the form of synthetic vitamin D₃ or cholecalciferol (C₂₇H₄₄O), which also happens to be the metabolite produced naturally by the skin exposed to sunlight or artificial ultraviolet-B radiation and which is part of the complex feedback control system that regulates circulating vitamin D metabolites.”

A great advantage of synthetic vitamin D₃, Nelson points out, is its low cost. Also, supplementation takes up very little space in a formula.

However, to cope with hypocalcemia, the vitamin D metabolic pathway requires increased synthesis (or production) of the active metabolite 1,25-dihyroxyvitamin D (1,25[OH]₂D₃), “which is typically 20-50 pg/mL in lactating and dry cows, and upwards of 100-300 pg/mL at two to three DIM (days in milk).”

Importantly, plasma 1,25(OH)₂D₃ does not correspond to vitamin D intake, as shown in recent studies. Rather, the major metabolite circulating in blood is 25(OH)D₃. With a half-life of about two weeks, its plasma concentration — typically 40-100 ng/mL with standard rations — serves as the best indicator of vitamin D status.

Holstein cows (n = 173): (A) serum 25(OH)D₃ vs. serum vitamin D₃ in cows fed 1 or 3 mg/d vitamin D₃; (B) serum 25(OH)D₃concentrations in cows fed 1 or 3 mg/d of vitamin D₃ or 25(OH)D₃.

Recent studies also show that while increasing the rate of dietary vitamin D₃ from 1 mg/d (40,000 IU/d) to 3 mg/d (120,000 IU/d) can increase concentrations of plasma vitamin D₃, it does not increase 25(OH)D₃. Researchers are finding little benefit — and “potentially detrimental consequences” — to overfeeding vitamin D₃.

By contrast, Nelson says, if the dairy has good feed management and cow grouping, and is feeding low phosphorus and acidogenic diets prepartum, then supplemental 25(OH)D₃ can provide an effective alternative to vitamin D₃.

“Adding 25(OH)D₃ to closeup diets does not reduce risk of immediate postpartum hypocalcemia, but it does help restore calcium faster, thereby reducing the occurrence of delayed and chronic subclinical hypocalcemia.”

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