Large studies of human populations have suggested for some time that having adequate vitamin D concentrations in the body protects not just against rickets but also against all kinds of other diseases, notably a number of cancers, including breast cancer. Research has indicated, in fact, that women who live in southern climates in the US. are less prone to developing breast cancer than women who live up north. The association makes sense when you consider that 90 percent of people’s vitamin D is made in the skin upon exposure to sunlight. The closer one lives to the equator, the more of the sun’s rays there are to get the vitamin D-synthesizing process going.
For dogs, the opposite is true — the part about where they get their vitamin D, that is. The great majority comes from their diet, not from the sun, because they lack the enzyme that converts sunlight into vitamin D. And a new piece of research out of the Cummings School of Veterinary Medicine at Tufts suggests that the amount of D dogs ingest from commercial dog foods may not be enough. It’s a concern because dogs, like people, avoid rickets as puppies when they get enough vitamin D and, it seems, also are protected from cancer and other diseases in adulthood when their diets are high enough in that nutrient.
Tufts veterinary scientist Claire Sharp, BVMS, and colleagues at the University of Missouri’s Veterinary Teaching Hospital and the Veterinary Diagnostics Institute in California made the finding that canine consumption of vitamin D appears suboptimal when they studied 320 pet dogs, with a focus on German shepherds and Golden retrievers.
They asked the dogs’ owners to complete a survey indicating the food their dogs were fed, along with any nutritional supplements. Then they tested the concentration of vitamin D circulating in the dogs’ blood. Based on previous research, a blood serum concentration of more than 100 (nanograms per deciliter of serum) was considered adequate. Frank vitamin D deficiency was set at less than 25, while vitamin D “insufficiency” — greater than an out-and-out deficiency but sub-par for ideal balance — was anywhere from 25 to 100.
It turned out that more than 300 dogs were fed commercial dog foods from 40 different brands — a good sampling. Almost 300 of the dogs were fed store-bought food only, while about 20 were fed strictly homemade diets. Ten other dogs received a combination of commercial pet food and homemade.
But it didn’t matter. The researchers found that no matter where the dogs’ food originated, the majority of dogs had insufficient blood vitamin D levels, suggesting that their diet contained too little vitamin D. Average blood serum concentrations for store-bought, strictly homemade, and combo diets were 70, 55, and 88 respectively — well below the threshold of 100 for which the investigators were checking. Some dogs did have more than 100. But most fell far short. And some had blood serum concentrations as low as 9.
Changes in Our Dogs’ Diets May Explain Low Vitamin D Levels
Dogs probably did not always have difficulty getting enough vitamin D from their diets. In fact, from an evolutionary sense, it’s logical for that nutrient to have come from their food. Vitamin D is fat soluble, which means it hangs around in animals’ fat stores. Before dogs were domesticated, they presumably got all the vitamin D they needed from the fat stores of killed prey, the researchers posit.
Today, however, even though commercial dog foods are high in fat (by human standards), those foods still have to be supplemented with D. And the supplementation may just not be aggressive enough. It definitely works against rickets but may very well not be enough to help interrupt disease processes down the line. Investigators have found associations between low vitamin D stores in dogs and lymphoma, certain types of mast cell tumors, kidney disease, inflammatory bowel disease, cardiovascular disease, and infections, among other ills.
The researchers do note that because just about all of the dogs were one of two breeds, either from the shepherd or retriever family, it needs to at least be considered that there may be something about the biochemical makeup of those particular breeds that doesn’t allow them to absorb enough D from their diets rather than a problem with the diets themselves. Indeed, on average, the German shepherds had significantly higher blood serum concentrations of the vitamin than golden retrievers — 77 versus 61. Based on the low numbers, however, the scientists do not anticipate that they would see huge upward ticks in other breeds.
More Research on Vitamin D Requirements in Dogs is Needed
It would be reasonable to wonder if you should start calling manufacturers to see how much vitamin D they put into their food and buy the brand that contains the most (pet food manufacturers do not typically list the amount of vitamin D on labels). Or to ponder whether you should perhaps start giving your dog vitamin D supplements.
The vitamin D supplementation route is a particularly attractive notion, especially in light of the fact that the researchers looked specifically at the blood serum concentration levels of the nutrient in those dogs who were supplemented and found something intriguing. While those supplemented with fish oil (a natural source of D) or dog biscuits fortified with vitamin D did not have higher concentrations of the vitamin in their blood than dogs not supplemented, those dogs routinely given salmon oil did in fact exhibit significantly higher levels of vitamin D in their blood than all the other dogs: an average of 89 versus 69 for all the un-supplemented animals.
Still, the researchers say it’s too soon to start giving your dog salmon oil or calling manufacturers to compare D levels. The results of their study, while suggestive, are not definitive.
But stand by. Further studies are needed, and veterinary nutrition scientists may be working with pet food companies in the not-too-distant future to consider adding more vitamin D to the dog foods you find at the grocery store.