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18 September 2012

Egg whites and other anti-nutritional factors in foods

The practice of eating egg whites instead of whole eggs or egg yolks appears to be quite popular. But is it safe?

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Following the article on the potential negative health effects of egg yolk because of their high cholesterol content, DietDoc received a number of queries from the public asking if it is safe to eat egg whites. She provides some answers:

Use of egg whites

The practice of eating egg whites instead of whole eggs or egg yolks appears to be quite popular. Some people eat egg whites because they still want to enjoy omelettes, but are scared of the cholesterol content of whole eggs. In other cases, sportsmen and -women and body builders who are trying to boost their protein intake to build muscle tissue, "protein-load" with egg whites.

Avidin in raw egg whites

It did become evident from the enquiries I received, that some individuals eat these egg whites raw, which is not a good idea. The reason is that raw egg whites, and raw whole eggs for that matter, contain a compound called avidin which is a so-called "anti-nutritional factor". Avidin binds biotin, one of the B vitamins, and makes it unavailable for absorption (Mahan et al, 2011). Excessive intake of raw egg white could, therefore, lead to a biotin deficiency and should be avoided.

Cooking or heat treatment destroys avidin and prevents biotin depletion by this anti-nutritional factor.

Other anti-nutritional factors in foods

a) Thiaminases in raw fish and the cabbage family

Thiaminases are other anti-nutritional factors found in raw fish or shellfish, Brussels sprouts and red cabbage. Now while few people will eat these cabbage species raw in large quantities, the fashion of eating raw fish in the form of sushi is growing exponentially.

Thiaminases are enzymes which destroy vitamin B1 or thiamine. It is believed that our ancestors, such as the Khoi who lived along the sea shore and ate raw shellfish as their main protein source, may have suffered from thiamine deficiencies. So while eating sushi once in a while is not going to deplete your vitamin B1 stores, having this raw delicacy too often may well start to affect a person’s thiamine intake.

b) Hemagglutinins in legumes

Lectins or hemagglutinins are glycoproteins found in legumes (World Food Programme, 2012). Hemagglutinins are defined as antibodies that cause the red blood cells to form clumps. If we as humans eat soaked raw beans or undercooked beans, we may develop severe foods poisoning. Most beans contain such phytohemagglutinins, but red kidney beans and yellow wax beans have the highest phytohemagglutinin content. Eating just 4 or 5 undercooked beans can cause symptoms such as nausea and vomiting, diarrhoea and abdominal pain.  Cooking well-soaked, raw beans (and all legumes) thoroughly for 10 or more minutes so that the water reaches boiling point (100oC) will destroys these coagulation factors or phytohemagglutinins (Helmenstine, 2012).

c) Goitrogens in plant foods

Goitrogens are chemical compounds (also called glucosinolates) found in foods such as sweet potatoes, beans, including soybeans, the cabbage family, turnips, rapeseed, peanuts, cassava, and kelp. Goitrogens hamper the absorption of iodine from the blood by the cells of the thyroid gland, thus causing the development of goiters. Populations that eat large quantities of raw cabbage and undercooked soy beans may be prone to goiters which are associated with cretinism in children born to mothers with goitre. Such infants may suffer from mental retardation, spastic diplegia (paralysis of corresponding parts on both sides of the body), deafness, mutism, shortened stature and hypothyroidism (Mahan et al, 2011). 

As is the case with most of these anti-nutritional factors, goitrogens can be inactivated by heat treatment or cooking.

d) Alpha-amylase inhibitors

Alpha-amylases are enzymes that break down the structure of carbohydrates during digestion. They hydrolyse sugar and starches so that the digestion of carbohydrates is improved. The alpha-amylase inhibitors are chemical compounds that are found in cereal grains, peas and beans. By inhibiting the breakdown of carbohydrates these inhibitors can impair growth and prevent the full utilisation of foods (World Food Programme, 2012).

e) Trypsin inhibitors

Trypsin is another digestive enzyme that is responsible for the digestion of proteins in the human body. The anti-nutritional factors called trypsin inhibitors, prevent this enzyme from digesting proteins effectively and can have a negative effect on growth and development.

Foods that contain trypsin inhibitors include legumes, egg whites and potatoes (Morelife, 2012). The Bowman-Birk trypsin inhibitor is, for example, found in soybeans.  Heat treatment also inactivates these inhibitors.

f) Phytates/phytic acid

Phytic acid and phytates are chemical compounds that are found in many different plant foods ranging from sorghum grain to wheat bran. On the one hand, phytic acid and phytates have been identified as valuable phytochemicals that may have powerful protective functions such as acting against harmful free radicals and slowing down the growth of tumours (Mahan et al, 2011). On the other hand, phytic acid is know for binding with essential minerals such as calcium, magnesium, iron and zinc making them unavailable to the human body  (World Food Programme, 2012). 

Anyone who uses phytates to fight free radicals should be aware of the fact that they may also affect mineral availability. Cooking does not influence the mineral-binding effect of phytates.

How to avoid anti-nutritional factors

The easiest way to avoid the majority of the anti-nutritional factors listed above, is to cook the foods that contain these factors thoroughly. Foods that contain phytates or phytic acid should if possible, be eaten separately from foods that contain important minerals.

The widespread use of soybean meal may increase our exposure to anti-nutritional factors. According to the FAO, soybean meal which is used extensively in many different forms contains practically all the anti-nutritional factors as well as others such as saponins and glucinins (FAO, 2012). To maximise the nutritional value of soybean meal these anti-nutritional factors must be inactivated by heat treatment. It is necessary to test for the presence of anti-nutritional factors after soybean flour has been processed to ensure that all these anti-nutritional factors have been inactivated.

According to the FAO information service, poor quality soybean flour that has been under-processed, has a "beany" raw taste. Now that our food supply is increasingly being supplemented with soybean flours to extend the protein content or provide a variety of food processing attributes (e.g. water-holding quality, etc), there is a potential that we may be exposed to such under-processed soybean flour and the anti-nutritional factors it contains. It is always prudent to buy foods from reputable manufacturers who use high quality soybean flour or textured vegetable protein in their products.

The movement that advocates that humans should eat all foods raw, may also expose its adherents to the risk of ingesting anti-nutritional factors. It has been suggested that one of the reasons why we as human beings developed so rapidly, was the discovery of fire which enabled us to cook our foods thus making such foods much more digestible and less potentially harmful. This line of reasoning probably also applies to avoidance of anti-nutritional factors as most of them can be made harmless by cooking the foods that contain them.

- (Dr IV van Heerden, DietDoc, September 2012)

(References: (FAO (2012). Soybean meal. http://www.fao.org.ag/; Helmenstine A M (2012. Hemagglutinin and food poisoning from beans. http://chemisty.about.com/; Mahan LK et al, 2011. Krause’s Food and the Nutrition Care Process. 13th Edition. Elsevier Saunders, USA; Morelife (2012). Heat-labile anti-nutritional factors inactivated by thermal processing. http://morelife.org/food/antinutrtionalfactors.html/ ; World Food Programme (2012(Toxins and anti-nutrients. http://foodquality.wfp.org/)

Any questions? Ask DietDoc

Read more:

Eggs: healthy or not?
Foods that increase longevity: soya
Eating fish: a slippery choice

 
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