There is a lot of circumstancial evidence on the issue. If for example, we look at invertebrates, we can see that on analysis they are poor sources of vitamin A, yet vitamin A deficiencies are rare in the few cases where analysis have been done of the predators. Yet problems like spindly leg are virtually unreported in wild populations even in those that are being intensively studied or cases where wild populations have thier enviroment manipulated with the results followed but spindly leg is not an uncommon problem with captive bred anurans. On review, there is a lot of evidence (anecdotal and documented) of spindly leg or poor development in captive bred or maintained anurans due to deficiencies in vitamins like vitamin A... as a result we have tweaked the supplements to resolve this issue (even though we know that frogs don't get much preformed vitamin A unless they are consuming vertebrates).... In addition, it is well documented that the anurans in captivity tend to be obese as they are typically fed to excess with "high quality" prey species in a manner that results in little or no significant energy expenditures for foraging. (See this old thread for some discussion on how many flies are needed to meet the metabolic needs of a resting frog http://www.dendroboard.com/foru m/food-feeding/9031-how-many-flies-mealtime.html
These extra calories result in fat deposition which permits the females to deposit eggs more frequently. (We also know that in the natural enviroment that in obligate egg feeders that females do not lay clutches of eggs to be fertilized while they are egg feeding yet this exact thing is anecdotally reported fairly frequently by those who work with pumilio...) We know that the provisioning of the eggs is done through depleting maternal stores of the nutrients which can reduce some functions such as color (conversion to vitamin A or simply carotenoid provisioning of the egg), immune function, ability to handle stress and other potentially other systems.
So there is some decent circumstancial evidence of the problem that we are currently "treating" by tweaking the supplements. In the wild the frogs would see a reduction in breeding during the dry seasons which allows the females to sequester the nutrients for the breeding season. These then go into the formation of eggs when the enviromental conditions are correct. So we have a couple of things that lead to depletion of nutrients because of continual rapid formation of eggs by the frogs the first is overfeeding, the second is keeping them in conditions to stimulate breeding 24/7/365. Cycling the frogs down and reducing feeding in an artificial dry season can reduce the problem.
Most of the rant is over....