A recent review published by the journal Critical Reviews in Food Science and Nutrition evaluated the current research on vitamin D in dogs.
In recent years, vitamin D has become one of the most frequently discussed micronutrients. In fact, several thousand studies have been published evaluating the relationship between vitamin D status and various health outcomes among humans.
This abundance of research has begun fueling the interest among the veterinary community, with a growing attention towards canine health. However, vitamin D intake and metabolism in dogs differs greatly from that of humans; therefore, findings from human studies cannot necessarily be translated to canine health.
To fully understand vitamin D’s role in disease pathophysiology among canines, there are several points of vitamin D research that must be addressed:
- Relationship between vitamin D intake and vitamin D metabolism
- Association between vitamin D intake and disease states
- Relationship between serum 25(OH)D levels and disease outcome
Therefore, in part one of this 2-part blog series, we will discuss the current research regarding vitamin D metabolism, requirements and maintaining healthy vitamin D levels in canines.
Vitamin D metabolism
The human body is designed to produce an abundance of vitamin D when bare skin is exposed to the sun (UVB). This is the most natural way for people to receive their vitamin D. However, according to the research in canines, minimal precursor vitamin D (7-dehydrocholesterol) is found in the skin. Furthermore, of the miniscule vitamin D precursor present, the UVB mediated vitamin D production is negligible. These findings were further validated by in-vitro studies, leading researchers to hypothesize that dogs have lost the ability to make vitamin D in the skin when exposed to the sun, and thus must rely on dietary sources to receive the vitamin D their bodies need.
Vitamin D requirements
So, what is the correct dosage of vitamin D for dogs? Until this year, no studies to date have evaluated this topic in adult canines. Despite this lack of research, the National Research Council (NRC) claims that adult dogs are unlikely to develop a vitamin D deficiency. The NRC is a non-profit organization that provides reports that form policies, increases public knowledge and carries out research in the fields of science, engineering and medicine. However, in more recent years, several studies have discovered a relationship between vitamin D status and several health outcomes in adult dogs, indicating a need for identifying vitamin D requirements in adult dogs.
Past research has assessed vitamin D requirements in puppies, determining that a diet with no vitamin D may result in rickets. Based on this finding, the NRC, Association of American Feed Control Officials (AAFCO) and The European Pet Food Industry (FEDIAF) have identified vitamin D as an essential nutrient in the canine diet. Though, the findings from the puppy studies may not be generalizable to the adult, pregnant and lactating canine population.
Recommended dietary intake of vitamin D based off primary regulatory agencies:
One study conducted by Tryfonidou et al (2002), found that feeding Great Dane puppies a diet containing around 535 IU vitamin D/kg DM supported normal growth. The NRC, AAFCO and FEDIAF’s recommendations for vitamin D align with this study. However, the energy density of the diets was not provided, preventing the direct comparison from the study to that of the AAFCO/NRC recommended diets.Kilocalories (kcal) are used to measure the energy content of different foods. The average 50 lb dog requires around 1,200 kcal/day to maintain a healthy weight.
Vitamin D doses between 4 – 6 times the recommended daily intake is considered a safe upper limit for dogs, according to the NRC. As a result, dogs receiving AAFCO/NRC compliant dog food for adult maintenance are given anywhere from around 125-800 IU/kcal vitamin D. However, the fact remains that there is no clear scientifically proven recommended intake and upper limit for dogs through the life stages.
Reference range for 25(OH)D status in canines
Like humans, the reference range for vitamin D status in canines varies depending on the laboratory used. Michigan State University’s Diagnostic Center for Population and Animal Health is widely used as the standard reference range for 25(OH)D status in dogs. This range considers levels from 24 – 86 ng/ml (60-215 nmol/l) as within normal limits.
However, according to a recent study, the healthy range for vitamin D status among canines may be higher than the Michigan State University’s Diagnostic Center for Population and Animal Health’s recommendations. In this study, researchers evaluated the relationship between vitamin D status, mean parathyroid concentrations and C-reactive protein (CRP), an indicator of inflammation, in 282 healthy dogs and 63 dogs with hemoabdomen. This condition occurs when blood has accumulated within the abdominal cavity, most commonly diagnosed as a result of a ruptured spleen or liver mass.
Here is what the researchers found:
- Vitamin D status was inversely associated with parathyroid concentrations (p < 0.001)
- Variation in parathyroid and CRP concentrations plateaued when 25(OH)D levels reached between 100-120 ng/ml.
- The relative risk of cancer increased 3.9 times in dogs with vitamin D levels below 40 ng/ml (p = 0.0002).
These findings indicate the reference range for 25(OH)D status in dogs warrants further investigation.
Relationship between vitamin D intake and status
The first study to aim to determine the effectiveness of vitamin D supplementation to improve vitamin D status in dogs was recently published. Young and Backus hypothesized that a daily dose of vitamin D at five times the NRC recommended daily allowance, without exceeding the safe upper limit, would safely and efficiently raise 25(OH)D levels in dogs.
The researchers evaluated the effects of vitamin D supplementation on 25(OH)D status in 13 privately owned canines with low vitamin D status (< 100 ng/ml). The dogs were randomly assigned a vitamin D supplement (100 IU/kg body weight) or a placebo pill. Their 25(OH)D levels were measured at baseline, weeks 1, 3, 6 and at completion of the trial (9-10 weeks).
Young and Backus concluded,
“We conclude that vitamin D3 supplementation at a dosage near the National Research Council recommended safe-upper limit was not effective for rapidly raising serum 25(OH)D concentrations in healthy, adult dogs.”
Further research is needed to determine the correct dosage of vitamin D supplementation needed to maintain optimal vitamin D levels (100-120 ng/ml) in adult canines to promote optimal health.
There remains a lack of consensus regarding the optimal dosage of vitamin D to supplement dogs. Additionally, recent research suggests that the current reference range for healthy vitamin D levels may be underestimated. Furthermore, when adult dogs were supplemented with vitamin D dosages at 5 times the recommended daily intake from the NRC, their levels did not significantly improve. These findings indicate further research is needed to determine the dosage needed for adult dogs to meet their metabolic needs and promote optimal health.
Stay tuned for part two of this blog series, which will focus on the current evidence regarding the role of vitamin D in various disease states and outcomes among canines.
Sturges, M. & Cannell, JJ. What role does vitamin D play in dog health? The Vitamin D Council Blog & Newsletter, 2016.
Young, L. & Backus, R. Oral vitamin D supplementation at five times the recommended allowance marginally affects serum 25-hydroxyvitamin D concentrations in dogs. Journal of Nutritional Science, 2016.