There are a few things in food that actually attack or block nutrients, or are designed to hurt animals. Many grains and seeds, for example, contain toxins- some of which can be removed in the milling or heating process.
Some sites and books make a big deal of these anti-nutrients. The good news is that they are not a big deal in a balanced diet. Certainly we should be aware of them and limit the worst offenders, but they are not generally worth a lot of worry. Some common anti-nutrients include:
Oxalic Acid combines with calcium to make insoluble, indigestible crystals. Spinach contains a lot of great calcium- but the oxalic acid in it binds up all but about 5% of it, working at a rate of 1 unit of oxalic acid binding up almost 100 units of calcium. Vitamin A can help reduce the effect a little. The upside of oxalic acid is that the crystals help clean the digestive system and actually offers some other benefits in small amounts. Foods high in oxalic acid include spinach, rhubarb, lambsquarters, alfalfa and parsley. Oxalic acid is, by itself, a toxin for many animals in sufficient quantities although it does not seem to affect tortoises much. In general, this is not a health issue if the tortoise is properly hydrated and has a varied diet.
Phytic Acid is found in peas, beans, nuts, grains and seeds and can ‘chelate’ away calcium, magnesium, iron and zinc. (Chelate comes from the word for ‘lobster claw’, and came to mean ‘shell’ in general. Here it is used to refer to the way chelating agents ‘grab’ things.) On the upside, phytic acid is also present in many greens where it helps make phosphorous bio-available to the animal eating it. Phytic acid found in most greens can be handled by the intestines. Avoid raw, unprocessed seeds and grains. Otherwise, this is not a major issue in a varied diet.
Tannins are found in many fruits and berries that make you pucker- sour grapes, pomegranates, and persimmons. They bind with proteins, but are not a problem with most tortoise foods. These are one of the reasons that oak leaves and acorns are considered toxic to reptiles.
Goitrogens are a group of elements that contribute to goiters and other health issues by blocking iodine absorption. They are found in many foods, such as bamboo, strawberries, peaches, radishes, brassicas (cabbages, broccoli, and all of the greens- turnip, radish, mustard, collards, etc.).. They are not considered much of a risk in a balanced diet.
Purines are found in organ meats, red meats, oily fish, brassicas, spinach, etc. and were thought to contribute to gout. Studies have disproven this.
There are several lists of toxic plants on-line (see Helpful Links), but many of them are on the list because they are toxic to humans, mammalian pets, and domestic animals. Reptiles, and especially chelonians, are resistant to many toxins, so can safely eat a lot of those plants. Some box turtles eat so many toxic mushrooms that eating their flesh can be fatal! The plants known to be a risk to reptiles includes
- Nicotine- including tobacco products and things like cigarette butts, as well as marijuana, etc.
- Oak- the sprouts, leaves, acorns, etc. have enough tannins in them that they are considered dangerous for reptiles. There do not seem to be any cases of tortoises eating these in captivity, however, so an oak tree in the yard may not be a major risk.
- Most ivys,
- foxglove, and
Reviewed 6-4-2013 (C) Mark Adkins