There appears to be several definitions of gutloading circulating around the community:
Gutloading is the process of feeding your feeder insects high vitamin/mineral feed 12-24 hours before feeding them to your chameleon so as to change the nutrient composition of the insect to benefit your chameleon.
Gutloading is the process of continuously providing high quality, nutritious food to your feeder insects so that these nutrients will be passed up the food chain to your chameleon.
While 1 is more literal, I accept definition 2, and will discuss that below. So, why is gutloading thought to be important?
Dietary diversity: In the wild, chameleons eat a wider variety of insects than we typically offer in captivity. Each of these various insect species will have their own nutritional profile based not only on their particular constitutions, but also on the different items on which they themselves have been feeding--ie. a variety of plants, pollen, nectar, rotting fruit, carrion, etc. By the very fact that wild chameleons exist, we know that the diversity in their wild diets satisfies all their nutritional requirements including vitamins, minerals, proteins, fats, etc. In captivity we come no where near the kind of dietary diversity wild chameleons enjoy.
Dietary Specificity: Chameleons are diurnal animals. The do their chameleon business during the daylight hours and sleep at night. One main daytime activity is cruise foraging (hunting). It stands to reason that the insects they feed upon are likewise active during the day. This includes bees, wasps, many beetles, butterflies/moths, grasshoppers, spiders, and many more. Each of these diurnal insects will have a distinct nutrient profile--one that not only reflects the diversity in the species, but also the fact that these diurnal insect species are feeding on things during the day. How could that possibly be important; do plants have a different nutritional value at night? There are countless species of flower and plant that close their petals, or fold their leaves at night. That means that only diurnal insects will have access to the nutrients contained therein. The problem, however, is that most of the insects we feed in captivity are either nocturnal, or spend their time under the soil surface where chameleons would not typically encounter them. This includes crickets, various grubs (e.g. superwroms, black soldier fly larva) and cockroaches. In captivity, the specific insects that we typically use are different from the ones typically targeted by wild chameleons.
Dietary Logistics: Several well know scientists and researchers have noted that chameleons have a penchant for flying insects, especially bees, wasps, butterflies and some beetles. Unfortunately, keeping a beehive, wasp nest, or butterflies indoors presents some serious challenges. That is not to say that providing flying feeders is impossible. We now have access to items such as bluebottle/house flies, black soldier flies, and some moths. However, the fact is these feeders are rarely used as staple items, and even if flies became more mainstream, I doubt very much that indoor beehives and wasp nests will follow suit. So, the logistics of keeping certain insects also affects the diet of our captive chameleons.
Dietary calcium: While we don't fully understand how wild chameleons get calcium in the quantities we think they need, we know they do get it: MBD is not a problem for chameleons in their native ranges. With the exception of black soldier fly larva, all of our feeder insects have a poor calcium to phosphorus ratio--ie. they have more phosphorus than calcium. This is problematic because phosphorus negates calcium (this is a huge fudge of a complicated interaction, but it'll do for now), so any positive benefit the calcium in our feeders might have is diminished by their high levels of phosphorus. Unaltered, the captive diet is calcium poor.
While there is much more to discuss here, we can at least give a preliminary answer to the question, "why is gut loading important?" Gutloading is a tool for addressing the shortcomings of the captive diet with respect to dietary diversity, dietary specificity, dietary logistics, and dietary calcium. We use a variety of fruits and vegetables in the hopes of mitigating the lack of diversity in our feeders. We add things such as bee pollen and certain algal compounds in the hopes of tempering the fact that the specific insects we feed are not in the wild diet. Such ingredients are also meant to address the logistical problems of keeping bees indoors. Finally, we use high calcium greens in the hopes of altering the poor calcium to phosphorus ratio of our feeders.
Sick of theory? I will go into more exhausting detail in some blogs and list some sources for further reading; but for now, here is a list of ingredients in ratios I find useful:
~70% high calcium greens:
~15-20% highly nutritious vegetables, rich in calcium and other minerals, vitamins, carotenoids and other nutrients:
sweet bell peppers
~5-10% of these fruits, rich in calcium, antioxidants, vitamins, carotenoids, and more:
~5% can be made up of commercial gut load recipes and/or specialty items such as:
Repashy Bug Burger/Superload
Arcadia Insect Fuel
mulberry leaf powder
hibiscus flower powder
Some of you will notice this list is almost identical to the list that can be found on the chameleon forums. Well, you're right! I fully endorse that list!
Finke, M. (2003). Gut loading to enhance the nutrient content of insects as food for reptiles: A mathematical approach. ZOO BIOLOGY, 22(2), 147-162.
Ogilvy, V., Fidget, A., & Preziosi, R. (2012). Differences in carotenoids accumulation among three feeder-cricket species: implications for carotenoid delivery to captive insectivores. ZOO BIOLOGY, 31, 470-478.
Partali, V., Liaaen-Jensen, S., Slagsvold, T., & Lifjeld, J. (1987). Carotenoids in food chain studies--II. The food chain of Parus SPP. Monitored by carotenoid analysis. COMPARATIVE BIOCHEMISTRY AND PHYSIOLOGY PART B;COMPARATIVE BIOCHEMISTRY, 87(4), 885-888.
Check out my blog on this site: The philosophy of gut loading.