Vitamin and Mineral Supplements in the Amphibian Diet

Vitamin and mineral supplementation of captive amphibian diets is done on an empirical basis. There are a tremendous number of supplements marketed for the herpetocultural hobbyist, but the bioavailability of the contents of these supplements as well as the need for the contents in any given species’s diet is undocumented.

Most supplementation programs are designed to ensure adequate levels of calcium (i.e., from 0.5% to 1.5% total diet dry matter) and vitamin D₃, as well as a balanced ratio of calcium to phosphorus (i.e., 1.5:1.0). This can be difficult given the inverse calcium-to-phosphorus ratio of many invertebrate species. The earthworm is a notable exception with a positive calcium-to-phosphorus ratio and adequate total levels of calcium (Dierenfeld & Barker, 1995).

There are “gut-loading” diets that can be fed to crickets and other insects to increase their total calcium content, a concept that can be traced back to a study that documented the increased calcium content of the mealworm, Tenebrio molitor, fed solely vitamin-mineral preparations for 24 hours prior to use as a food for insectivorous animals (Zwart & Rulkens, 1979). When fed solely a diet that contained 11.65% calcium and 0.55% phosphorus (Carnicon®, Trouw Company, Putten, Netherlands), the calcium-to-phosphorus values of mealworms increased from 1:3.7 to 1.38:1 and yielded a final calcium content of 0.84% calcium. Subsequent studies document the differing tolerances of insect species to high-calcium diets, and along the way the term “gut-loading” came into common usage in herpetoculture.

The ideal gut-loading diet for an invertebrate species should result in an active live prey item that has a nutritional composition no lower than 0.5% and no higher than 2% total calcium content (dry matter basis) and a calcium-to-phosphorus ratio of 1.5:1. The wax worm, Galleria mellonella, when maintained on a diet of 5.7% calcium, survived over 72 hours, and achieved a calcium content of 0.31% with a calcium-to-phosphorus ratio of 1.29:1 at 72 hours (Strzelewicz et ah, 1985) whereas one species of cricket, Acheta domestica, could not tolerate a diet in excess of 0.14% calcium for prolonged periods of time (McFarlane, 1991). Commercial gut-loading diets for crickets have calcium levels of up to 8% and cause significant mortality if fed for more than 48 hours.

Gut-loading diets are not balanced diets due to the high levels of calcium present, and are not intended for optimal growth and development of the insects. The gut-loading diets should be placed within the cricket cage no sooner than 48 hours prior to use as an amphibian food source. Either water or slices of fruit (e.g., apple or orange) can be used as a water source concurrent with the gut-loading diet, as the calcium content of the crickets were the same for either source in one study (Trusk & Crissey, 1987). Crickets given no access to moisture die within hours if fed the gut-loading diet. Given a diet containing 8% calcium, crickets achieved a dry matter calcium content of 1.3% (Allen, 1983; Allen & Oftedal, 1989; Allen et al., 1993). The crickets will begin to excrete their high-calcium ingesta almost immediately, so only as many insects as will be consumed by the amphibian within 2-4 hours should be offered.

It is noted that adults of the Cuban treefrog, Osteopilus septentrionalis, maintained adequate total body calcium levels while on either a high-calcium diet (i.e., crickets containing 1.26% Ca and 0.89% P) or a low-calcium diet (i.e., 0.23% Ca and 0.82% P) (Allen, 1983; Allen et al., 1993). No significant differences were noted in total body calcium, phosphorus, or in radiographic appearance between frogs raised on either diet. The frogs’ overall body calcium declined by 25% during the 7-month study, although it is possible that this decline was due to the frogs’ maintenance in deionized water and other factors, such as lack of ultraviolet-B irradiation, inappropriate levels of vitamin D₃, or a normal seasonal fluctuation in calcium. Whether the calcium levels would have been adequate in the face of growth, reproduction, or unusual activity is unknown.

A 5.7% calcium gut-loading diet for the wax worm, Galleria mellonella, has been reported (Strzelewicz et al., 1985). It consisted of 12.0 ml honey, 18.9 g high- protein baby cereal (Gerber High Protein Baby Cereal, Gerber Products Company, Remont, MI; this product may no longer be available), 5.7 g calcium carbonate, 10.0 ml glycerol (Glycerine USP 99.5%), and 4.0 ml distilled water. Wax worm larvae should be maintained on this diet for a minimum of 72 hours at 30°C (86°F) in order to achieve a calcium content of 0.31% and a calcium-to-phosphorus ratio of 1.29:1.

An alternative diet for gut-loading crickets and mealworms consists of 20% calcium carbonate powder and 80% nutritionally complete layer chicken mash (Dierenfeld & Barker, 1995). This diet has a final calcium content of 8%. Crickets may eat less in the final weeks of their life, so gut-loading may be less effective for older crickets. This may hold true for other insects as well.

A small amount of powdered supplement can be placed into a container, and prey items can be shaken in the container to become thoroughly coated with the supplement. This technique is known as dusting and is based on the assumption that finely ground vitamin- mineral powders adhere to prey items, which in turn bolsters the prey’s intrinsic nutritional value. Dusting is usually used with small prey items such as fruit flies and pinhead crickets for which gut-loading diets are lethal or impractical. Dusting does itself significantly decrease the life span of very small insects and should only be done immediately prior to feeding out the insects. It is generally assumed that these smaller insects have inverse calcium-to-phosphorus ratios and thus require dusting; however, one study suggests that this may not be a valid assumption as pinhead crickets had an average dry matter calcium of 2.1% (Dierenfeld & Barker, 1995). If this analysis of pinhead cricket calcium content holds true (i.e., pinhead crickets are truly a high-calcium food), then dusting pinhead crickets may have had little to do with the successes of rearing small amphibians.

One study failed to achieve a positive calcium-to- phosphorus ratio in adult crickets (unknown species) using a dust that contained 11% calcium and 3.2% phosphorus (Trusk & Crissey, 1987). The highest calcium-to-phosphorus ratio achieved was 0.18:1 within 5 minutes of dusting, and this tapered off to 0.15:1 within 3 hours. As with gut-loading, it is suggested that no more insects be offered than can be consumed within a few hours to maximize the calcium gain achieved by dusting. Pure calcium carbonate or other nonphosphorus forms of calcium should be the calcium source for any mineral supplement to offset the inverse calcium-to-phosphorus ratio of most prey species.

The benefits of dusting with regard to improving the vitamin content of prey species appears to be undocumented. There are a variety of vitamin-mineral supplements marketed for amphibians and reptiles; however, full-spectrum human vitamin-mineral supplements are cheap, readily available products that can be used for dusting if one is willing to grind the tablets into a fine powder. The bioavailability of the human products may actually be better than those marketed specifically for amphibians and reptiles (Donoghue & Langenberg, 1996). One is cautioned to review the label of any vitamin, pet or human, prior to purchase, as substitutions and reformulations frequently occur. As an example,, in many human formulations vitamin D₃ has been replaced recently with vitamin D₂.

It is noted that further studies are needed to validate the conclusions of one dusting study (Trusk & Crissey, 1987). If dusting achieves much lower levels of supplementation than does gut-loading, then the recommendations for supplementation of invertebrate prey may change. Currently it is recommended that small insects, such as fruit flies, pinhead crickets, and houseflies, should be dusted with a vitamin-mineral mix prior to being fed to young growing amphibians. Adult amphibians may do well on a schedule with prey insects dusted from 1 to 3 feedings a week.

Vertebrate prey rarely need additional supplementation in this manner (but see Section 6.5, Rodents as a Food Source), although frozen whole fish may need additional thiamine to offset thiaminase activity. (See Section 7.3, Thiamine Deficiency.) If only muscle portions of fish are used, supplementation is recommended to offset calcium, vitamin A, vitamin D, and iodine imbalance of the muscle meat.