From holistic websites to nutritional seminars, messages bombarding consumers and the profession argue that “natural” vitamins are better than “synthetics” and often espouse the “more is better” line for orthomolecular or megavitamin therapy.
Skeptics counter this by insisting that “with minor exception, molecules made in the ‘factories’ of nature are identical to those made in the factories of chemical companies.”1 They maintain that a good diet should supply all necessary nutrients.
The true story builds a much more complex picture. With respect to the argument in favor of food-based vitamins, even natural sources of vitamin A in unnatural quantities produce problems like metabolic osteopathy in cats, a species with particularly high susceptibility to vitamin A toxicity.2 For precursors to vitamin A, the carotenoids, as many as nine factors influence their bioavailability when ingested in food. These variables include:
The carotenoid lycopene, which lowers prostate cancer risk, illustrates this last point. Cooking improves lycopene bioavailability because heating and homogenizing lycopene-rich tomatoes into paste disrupts the fibrous plant cell walls, releasing the lycopene for absorption.5 Partnering carotenoids with certain fats maximizes their intestinal and lymphatic uptake. Humans eating salads with either fat-free or full-fat dressing demonstrated significantly different plasma levels, with those ingesting vegetables dressed in the full-fat version enjoying higher amounts of circulating carotenoids.6
Members of the water-soluble vitamin B class have a reputation of being safe in any amount. Nutritionists have long-advocated vitamin B9, or folate, for cancer prevention. However, a concern erupted in the 1940s that is now receiving more attention: Too much supplementation with folic acid (the synthetic form of folate) appears to promote tumorigenesis. This springs from its role as a key cofactor in nucleotide synthesis. Large amounts of folic acid facilitate cellular proliferation, causing growth in neoplastic foci. This finding has provoked calls in the human food-processing industry for curtailing mandatory folic acid fortification.7,8 The lower amounts of folate ordinarily found in food are still viewed as cancer-preventive.
Many nonhuman species manufacture vitamin C (ascorbic acid). While they rely less on dietary sources, research testing their true capacity shows it falls far short of previous assumptions.9
Aging hepatocytes lose some ability to synthesize and recycle vitamin C, producing a conditionally essential status for vitamin C in geriatric dogs and cats.10 Health status may further influence vitamin C requirements. A 2009 study in the Journal of Animal Physiology and Animal Nutrition showed that healthy dogs supplemented with vitamin C produced no improvement in antioxidative capacity or serum IgA and IgG concentrations.11 In contrast, dogs with untreated lymphoma exhibited significant alterations in their antioxidant levels compared to healthy control dogs.12 After remission, dogs with lymphoma had significantly lower levels of vitamin C, raising questions about the potential need for post-chemotherapy supplementation with dietary antioxidants. Finally, Vitamin C supplementation, along with vitamins B and E, aminocaproic acid and N-acetylcysteine, disappointingly failed to forestall deterioration in the neurological status of dogs with suspected degenerative myelopathy.13
In the wild, carnivores obtain vitamin D by consuming the body fat, blood and liver of their prey; depending on the contents of homemade diets, dogs and cats may not be receiving enough vitamin D.14 They cannot meet their needs simply through sun exposure, as dogs and cats lack the ability to cutaneously synthesize D3 in adequate amounts.15 Problems such as “rubber jaw syndrome,” or secondary hyperparathyroidism, have arisen in young dogs.16,17 On the other hand, overnutrition with supplemented Vitamin D, calcium, and calories in puppy diets has been linked to predisposition for canine elbow dysplasia.18
Vitamin E refers to eight chemically different compounds obtained from plants: four tocopherols and four trienols. Synthetic vitamin E consists of eight stereoisomeric forms of a-tocopherol, with only one equal to the naturally occurring stereoisomeric form.19 Generally, the health-supporting and cancer-preventive superiority of either natural or synthetic vitamin E remains unclear;20 studies exploring species-specific differences have further muddied the waters, with some indicating better results with the synthetic and others with the natural form.21,22 Attention is turning to the formerly neglected half of the natural vitamin E family, the tocotrienols.23 Tocotrienols confer neuroprotective benefits and possibly more antioxidant properties than a-tocopherol. Tocotrienols distribute more widely throughout the fatty layers of cell membranes and tocotrienol, not tocopherol, suppressed growth of human breast cancer cells.24
Clearly, questions remain about when and how much vitamin supplementations small animals require, and whether or not pet food diets can meet their needs. However, veterinarians can convey to clients facts based on the information that is available, fulfilling their role of protecting animal health and welfare, rather than abdicating this effort to self-proclaimed animal nutritionists and lecturers paid to promote products.
Narda Robinson, DVM, DO, Dipl. ABMA, FAAMA, oversees complementary veterinary education at Colorado State University.
This article first appeared in the October 2009 issue of Veterinary Practice News.
1. Barrett S and Herbert V. Twenty-five ways to spot quacks and vitamin pushers. Obtained here on 081809.
6. Brown MJ, Ferruzzi MG, Nguyen ML, et al. Carotenoid bioavailability is higher from salads ingested with full-fat than with fat-reduced salad dressings as measured with electrochemical detection. Am J Clin Nutr. 2004;80:396-403.
11. Hesta M, Ottermans C, Krammer-Lukas S, et al. The effect of vitamin C supplementation in healthy dogs on antioxidative capacity and immune parameters. Journal of Animal Physiology and Animal Nutrition. 2009;93(1):26-34.
16. De Fornel-Thibaud P, Blanchard G, Escoffier-Chateau L, et al. Unusual case of osteopenia associated with nutritional calcium and vitamin D deficiency in an adult dog. J Am Anim Hosp Assoc. 2007;43(1):52-60.
20. Blatt DH, Pryor WA, Mata JE, et al. Re-evaluation of the relative potency of synthetic and natural α-tocopherol: experimental and clinical observations. Journal of Nutritional Biochemistry. 2004;15:380-395.
21. Weiss WP, Hogan JS, and Wyatt DJ. Relative bioavailability of all-rac and RRR vitamin E based on neutrophil function and total α-tocopherol and isomer concentrations in periparturient dairy cows and their calves. J Dairy Sci. 2009;92:720-731.