Wheat is generally not considered toxic. However, a number of anti-nutritional factors and allergens occur in wheat and in extreme cases may have a toxic effect. These are described in Sections 5.2 and 5.3. Wheat grain contains haemagglutinin, amylase and protease inhibitors, but these are not present in large enough amounts to have adverse effects on humans (Simmonds 1989).
Wheat is one of the most commonly grown, processed and consumed human foods and is associated with intolerances and allergies (Tatham & Shewry 2008).
5.2.1 Dust and flour allergies
Wheat grain, dust and the milled products can cause physical irritation which may lead to a range of allergic reactions (Simmonds 1989). Allergy symptoms range from mild rhinitis to asthma and severe bronchial irritation in responses to the inhalation of flour or dust. Anaphylaxis has been reported to occur rarely in children (OECD 2003). Baker’s asthma and rhinitis are well characterised allergic reactions to the inhalation of wheat and cereal flours (Tatham & Shewry 2008). There are a number of candidates for the allergenic proteins in wheat (OECD 2003). Baker’s Asthma may be a response to a number of compounds in wheat flour including a number of α-amylase inhibitors, as well as cross-reactive carbohydrate determinants (Sander et al. 2011). In addition, wheat has been implicated in food-dependent exercise-induced anaphylaxis, where an allergic reaction is induced by intake of a causative food and subsequent exercise – the combination of both food and exercise are required (Tatham & Shewry 2008).
5.2.2 Coeliac disease
Coeliac disease is a condition in which the small intestine is damaged when exposed to gluten, which is found in wheat, barley, rye and triticale (Digestive Health Foundation 2012). This results in poor absorption of nutrients and a variety of related issues (Digestive Health Foundation 2012)
Inheritance of coeliac disease is multigenic and has been strongly associated with European populations (Kasarda 2004). It is more prevalent in females than in males (Hischenhuber et al. 2006). Estimates of the prevalence of coeliac disease vary widely across locations and times (Simmonds 1989; Fraser & Ciclitira 2001; Catassi et al. 1996). In Australia the prevalence of coeliac disease is estimated at approximately one in 100 (Digestive Health Foundation 2012), which is similar to recent rates estimated for Europe, North and South America, north Africa and the Indian subcontinent (Hischenhuber et al. 2006).
Symptoms of coeliac disease vary and sufferers may have many symptoms or none. They commonly include diarrhoea, weight loss, nausea, flatulence and abdominal discomfort, as well as tiredness and weakness often due to a degree of iron and/or folic acid deficiency and resultant anaemia (Catassi et al. 1996; Digestive Health Foundation 2012).
Onset of symptoms may occur very early in life or may be delayed even until very late in life, resulting in speculation about environmental triggers for the disease, potentially including viral infection, parasitic infection (Giardia) and surgery (Kasarda 2004).
5.3 Other undesirable phytochemicals
5.3.1 Enzyme inhibitors
There are two main types of enzyme inhibitors present in wheat, inhibitors of proteases and amylases. Protease inhibitors, especially trypsin inhibitors, may decrease the digestibility of dietary proteins while amylase inhibitors may affect the digestibility of dietary starch. However, these inhibitors do not appear to pose a serious risk to human health as they tend to be heat labile (OECD 2003) and references cited therein). Wheat germ is reported to contain a haemagglutinin that together with a protease inhibitor can affect the ability of poultry to utilise wheat germ effectively as a food source (Simmonds 1989).
Lectins are glycoproteins that bind to specific carbohydrate groups on cell surfaces, causing lesions to form (OECD 2003) and references cited therein). In the intestinal tract, these lesions can seriously impair the absorption of nutrients.
Lectins are usually inactivated by heat and are therefore of greater importance where wheat is consumed raw. For example, wheat germ muesli contains an unprocessed form of lectins, whereas wheat germ baked in bread contains an inactivated form which is not as easily recognised by the immune system (Gabor et al. 2003). Lectins may also be present in animal feeds containing wheat.
Singh et al. (1999) reported that physiological stresses to the wheat plant produced increased levels of a lectin, WGA (wheat germ agglutinin), in the germinating wheat embryo. The highest accumulation of WGA occurred when the germinating wheat embryos were exposed to salt stress (other stresses were temperature and osmotic stress). The authors concluded that WGA enhancement in germinating embryos appears to be a general stress response.
The insecticidal properties of lectins and their role in crop protection has been reviewed, including their potential roles in wheat (Macedo et al. 2015). A transgenic wheat line which expresses a plant lectin was shown to affect the fecundity, but not survival of insects fed on wheat leaves. The authors of this study suggest that this indicates potential for use of such plants in integrated pest management systems (Stoger et al. 1999).
5.3.3 Phytic acid
Phytic acid may reduce the bioavailability of trace elements in animal diets through chelation of minerals such as iron, zinc, phosphate, calcium, potassium and magnesium (OECD 2003). This anti-nutrient is of particular importance to monogastric animals, while ruminants possess digestive enzymes which degrade phytate, releasing the chelated minerals. The level of phytic acid is highest in wheat germ and lowest in wheat flour (OECD 2003).
5.3.4 Nitrate poisoning
Nitrogenous products can accumulate in plants, and ruminants have the ability to convert nitrates to toxic nitrites. Wheat, rye and rape have been identified as crop plants which can accumulate nitrate (Stoltenow & Lardy 2008; Yaremcio 1991), as can sorghum (Yaremcio 1991). In monogastric animals the risk of nitrate poisoning is much less because conversion to nitrites occurs closer to the end of the digestive tract (Yaremcio 1991). Cattle and sheep can generally tolerate up to 0.5% nitrate on a dry matter basis.
There are two forms of nitrate toxicity in stock. Chronic nitrate toxicity is commonly associated with reduced rate of weight gain, depressed milk production, reduced appetite and greater susceptibility to infection. This form of poisoning can occur when nitrate levels are 0.5 to 1.0 % of feed consumed (dry matter basis) (Yaremcio 1991).
The second type of nitrate toxicity, acute poisoning, occurs when nitrate is rapidly converted to nitrite in the rumen and is immediately absorbed in large amounts into the bloodstream. Signs of acute poisoning in cattle which can be fatal, include increased heart rate, muscle tremors, vomiting, weakness, blue/brown mucus membranes, excess saliva production and staggering (Robson 2007).
5.4 Beneficial phytochemicals
Wheat is considered a good source of protein, minerals, B-group vitamins and dietary fibre (Simmonds 1989) although environmental conditions can affect the nutritional composition of wheat grains. The nutritional content of a few important wheat products is shown in Table 5. More information is available (Simmonds 1989; Food Standards Agency 2002; OECD 2003) and references therein).
Table 5. The composition of wheat products per 100g edible portion (Food Standards Agency 2002).
Wheat bran can be a good source of dietary fibre, helping in the prevention and treatment of some intestinal disorders, although care must be taken for older populations (Simmonds 1989). In a study comparing the phytochemical profiles, total phenolic and carotenoid content and antioxidant activity in milled grain of eleven wheat varieties including red and white wheat and durum wheat, significant differences were found between varieties for the carotenoid and total ferulic acid content (Adom et al. 2003). Lutein is the predominant carotenoid present in wheat (Adom et al. 2003; Abdel-Aal et al. 2007) and the bran/germ fractions of wheat contained greater amounts of carotenoids and antioxidant activity than the endosperm fractions (Adom et al. 2005). These authors also suggest the combination of both fractions exert greater overall physiological effects than each separately (Adom et al. 2005).