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 Breeding & Genetics

Health & Nutrition

Miscellaneous diet and health topics

Antinutrients

Every food in the world comes with natural negatives attached to it, in the form of chemical compounds that are bad for us in some way. These natural negatives come in a variety of types including antinutrients, toxins, and carcinogens.  As the name suggests, antinutrients are compounds that interfere with the absorption of nutrients. Different types of antinutrients act in different ways, but the common forms of antinutrient action include inhibiting the action of digestive enzymes, interfering with the absorption of minerals, and even making plants taste bad so we don't want to eat them.  This article will deal with the antinutrients that get the most attention in the pet bird community.

Oxalate

Oxalic acid (aka oxalate) is a compound found in many plants that has a strong affinity for binding with calcium. It can bind with other minerals as well, but its calcium-binding properties get the most attention because of the potential health effects. Oxalates "tie up" the minerals that they bind with; the body can't absorb these oxalate-mineral compounds so they are passed out of the body without being used. 

In the pet bird community there's a great deal of concern about oxalates causing calcium deficiency in birds, but this concern seems to be misplaced.  In the human health community there doesn't seem to be any concern at all about calcium deficiency; instead the focus is on the role that oxalate plays in the formation of kidney stones and other health problems including gout.

Bird owners tend to be concerned about the amount of oxalate in vegetables, and some vegetables do have a high level of it.  Oxalicacidinfo.com has a chart showing oxalate levels in vegetables as measured by three different sources, and the different sources came up with radically different amounts for some vegetables.  The site notes that the these numbers may not be particularly meaningful anyway:  the type of oxalic acid (soluble or insoluble) in the food has an effect on absorption, growing conditions and cooking method affect the amount of oxalate in the food, and the National Institute for Health believes that oxalate interactions have little or no nutritional consequence for people who eat a variety of foods.

Do you know what else has a lot of oxalate?  Seeds, whole grains, nuts, and beans (Chai & Liebman, Denver Nephrology, Kaiser Permanente). The bird community is generally unaware of this, even though these are the foods that make up the bulk of our birds' diet in one way or another. Oxalate is pretty pervasive in plants and there's no getting away from it. It's highly probable that the plants that make up our birds' wild diet have a similar oxalate profile to what we're feeding them. So birds must have some kind of adaptive mechanism to help them deal with it, and I think I know what it is. It's common for wild birds to eat dirt and grit, which can be excellent sources of minerals.  The most obvious solution to the problem is to make sure birds have access to free-choice calcium sources like cuttlebone, mineral block, or mineral grit to make sure they can get enough calcium/minerals to offset the oxalate. It doesn't do any harm to try to limit high-oxalate foods, but oxalate is too widespread and there are too many variables that affect oxalate level for this tactic to be very practical or effective.

Where does oxalate bind with calcium and other minerals - does it only bind with minerals in the food before you eat it, or can excess oxalate in the food bind with minerals in the body that came from other sources?   Sources including the href="http://my.clevelandclinic.org/services/urology-kidney/treatments-procedures/hic-kidney-stones-oxalate-controlled-diet> Cleveland Clinic and Johns Hopkins say it can bind with calcium in the intestines and that's a good thing, because unbound oxalate can be absorbed into the body and cause kidney stones. They urge you to eat plenty of calcium, not to prevent calcium deficiency, but because they WANT it to bind with oxalate and be passed out of the body.

Consuming very large quantities of oxalates can cause oxalate poisoning, resulting in kidney damage or death. Rhubarb leaf is considered to be toxic because of the high levels of oxalic acid it contains.  There are other plants that contain dangerous levels of oxalate, and even excessive consumption of black tea can cause oxalate poisoning.  But these are extreme cases, and oxalate poisoning isn't a concern with ordinary consumption of plants that are generally recognized as food.

Phytate

Phytic acid (aka phytate) is another compound found in plants with a strong affinity for binding with calcium and other minerals, making them unavailable for use by the body. However it is considered to be a less potent inhibitor of mineral absorption than oxalate (Oregon State).  Phytate is the primary storage form of phosphorus in plants. Phosphorus is an essential nutrient, but in phytate form it is not bioavailable to most animals (including birds and humans).  The digestive enzyme phytase is required to digest phytate, and ruminant animals (like cows) are pretty much the only animal that have this enzyme. Phytic acid has many functions within the plant. It has functions in animal cells too, but the animals have to synthesize it for themselves since they can't absorb it from the diet (Wikipedia).

There are numerous factors that affect the binding of phytic acid to other minerals in the body, including pH, gut flora, and the presence of phenols and tannins.  The way it binds with minerals varies depending on which mineral it is, and the process is more complex in some cases than in others.  From the human health standpoint, its affinity for binding with zinc and iron may cause more problems than binding with calcium, depending on the relative abundance of these minerals in the diet (Wikipedia). But most people consume enough minerals that phytate isn't a problem. The phytate consumed during a meal may affect the absorption of minerals from that meal, but it doesn't affect mineral absorption from subsequent meals. Phytic acid is beneficial in some ways, to the point that it can be bought in supplement form from vitamin sellers (Precision Nutrition, Authority Nutrition).

Because of the mineral-binding issue, it is often recommended to maintain a calcium to phosphorus (Ca:P) ratio in the diet of about 2.5:1.  But it appears that this ratio is most relevant during bone growth in infants and juveniles, and its relevance to adults is questionable (Dietary Reference Intakes, Australian Ministry of Health). However, it is generally accepted in the reptile-keeping community that calcium supplementation is required to prevent calcium deficiency when feeding a diet of high-phosphorus insects (LLLReptile, DrsFoster&Smith). Therefore it appears that a severe imbalance between calcium and phosphorus can be problematic, and it's wise to pay some attention to the balance of these nutrients.

Grains, seeds, nuts, beans and legumes contain far more phytic acid than vegetables do, and their Ca:P ratio skews heavily toward the phosphorus (see tables at Precision Nutrition). The chart at left shows the Ca:P ratio and other nutritional information for some seeds, grains, and nuts that are commonly fed to pet birds, with a couple of vegetables thrown in for contrast. 

Soaking, sprouting and cooking all help to reduce phytic acid in foods. As with oxalate, providing free-choice minerals will let your bird regulate its mineral intake to offset the mineral-binding effects of phytic acid.

There have been some studies on the effect that phytic acid has in poultry (Onyango et al, Likuski & Forbes, Cowieson et al).

 

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