Food is an integral part of Thanksgiving. It brings to mind the complexity of food and meal planning. Rightfully, many consider food (nutrition) as the foundation for health. Some feel that illness is often a reflection of nutritional imbalance. Understanding the imbalance can be challenging because the body has an incredible ability to adapt to imbalance and because food has an amazing intricacy.

This brings us to sweet potatoes and liver. Both are great sources of a wide range of vitamins, minerals, antioxidants and other valuable substances. Both are listed as sources of vitamin A without clearly identifying how different they are as sources of vitamin A. Like other yellow and orange vegetables, sweet potatoes contain high levels of carotenoids. The carotenoids contribute the yellow, orange and red colors. Carotenoids are also found in dark leafy greens like spinach and mustard greens. Over 750 carotenoids have been identified. Many play roles in health and nutrition but not all contribute to increasing vitamin A levels in the body. The carotenoid of particular interest for vitamin A is beta-carotene (b-carotene).

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While often discussed as vitamin A, b-carotene is considered a provitamin A. This means it has to be converted to vitamin A (retinol) before it can be used as vitamin A. Most of the conversion of b-carotene to vitamin A occurs in the cells lining the intestine. Since b-carotene is fat-soluble, getting access to the intestinal cells requires the presence of sufficient fat in the meal. In addition, b-carotene is found in a free form in leaves but in a bound form in vegetables. It has to be released into the free form to be used. Obviously a healthy digestive tract along with a balanced diet are important for the efficient conversion of b-carotene to vitamin A.

This fascinating process gets more complex when species differences in the ability to convert b-carotene to vitamin A are considered. The most efficient converters are the omnivores (like humans and rats) and herbivores (like cattle and sheep). This is thought to be related to their basic diets that include many b-carotene sources. Carnivores (like ferrets and cats) are a different story. Cats are considered obligate carnivores which means they require a meat based diet. They do not convert b-carotene into vitamin A. This means they have to have vitamin A (retinol) in their diet to prevent vitamin A deficiency. Even though dogs are considered carnivores, they feed more like omnivores and they have the ability to convert b-carotene to vitamin A enough to generally meet their metabolic needs.

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Species that cannot convert or weakly convert b-carotene require preformed vitamin A in the diet. That brings us back to liver. It is a source of preformed vitamin A meaning it is already in the vitamin A retinol form. This is the form that cats need in their diets. Preformed vitamin A is found in animal source foods like liver, meat, dairy, and fish oils.

Vitamin A is critical for health. It plays a role in growth and development, immune function, and maintenance of the heart, lungs, kidneys, eyes, and mucous membranes. However, excess vitamin A (10 times or more the recommended amount) can be toxic resulting in loss of appetite, nausea, weakness, tremors, and convulsions. Vitamin A is stored in the liver so excess consumption can become a problem over time or with acute very high doses. There is minimal concern with vitamin A toxicity when using b-carotene because it has to be converted to vitamin A and the rate of conversion is reduced when there are adequate vitamin A stores in the liver.

It is important to recognize that there is individual variation in ability to convert b-carotene to vitamin A. This is in addition to the species variation already mentioned. Not all dogs are efficient converters, especially as they age. Recognizing when a food is providing “vitamin A” as a provitamin or as a preformed vitamin A can be important when considering managing some health conditions.

If you have questions about your pet companion’s health, contact your veterinarian.

Ron Carsten, DVM, PhD, CVA, CCRT was one of the first veterinarians in Colorado to use the integrative approach, has lectured widely to veterinarians, and has been a pioneer in the therapeutic use of food concentrates to manage clinical problems. He is also the founder of Colorado Animal Rescue (CARE). In addition to his Doctor of Veterinary Medicine, he holds a PhD in Cell and Molecular Biology and is a Certified Veterinary Acupuncturist and Certified Canine Rehabilitation Therapist. He practices integrative veterinary medicine in Glenwood Springs. Dr. Carsten is the 2022 Colorado Veterinary Medical Association Distinguished Service Award recipient.