Introduction

Cereal grains are the seeds of grasses (gramineae). A typical cereal grain consists of the testa, the aleurone layer, the embryo and the endosperm. The embryo is the living part of the grain that is very rich in nutrients. Most nutrients (for example, protein, vitamins and minerals) are concentrated in the aleuronic layer whilst the endosperm consists mainly of starch. Cereals constitute a major source of energy in most households today and it is no surprise they occupy the base of food pyramids. Examples of cereals are wheat, maize, rice, barley oats, millet and sorghum. With the advent of agriculture in the early history of human civilisation, cereal grain consumption became prominent, especially in developing countries. This article gives an overview of this food group in terms of its nutritional values.


Nutritive Value of Cereals

Cereals and cereal products are important sources of energy, carbohydrate, protein and fibre. Though cereals generally lack vitamins A, C and B12, they contain a range of micronutrients such as vitamin E, some of the B vitamins (for example B6), magnesium and zinc. The bioavailability of the B-vitamins, however, appears to be low. For example, the bioavailability of B6 from cereal grains tends to be low, whereas bioavailability of B6 from animal products is generally quite high. With the exception of calcium and sodium, cereal grains provide good amounts of minerals needed for adequate nutrition. Since the typical Western diet is high in sodium, the low sodium content of cereal grains is desirable. The low calcium content of cereal grains does not appear to be a problem because most people in developed countries consume a mixed diet rich in calcium. It is however advisable for people living in developed countries to obtain calcium from a wider variety of sources, such as in dairy and some vegetables. Ideally, calcium and phosphorus occur in the ratio of 1:1. However, cereal grains have a low calcium/phosphorus ratio which can negatively impact bone growth and metabolism. This is because consumption of an excess of dietary phosphorus when calcium intake is inadequate or low is reported to result in secondary hyperparathyroidism and progressive bone loss.

The high phytate content of whole grain cereals may reduce the availability of calcium for absorption because the phytate forms insoluble complexes with calcium. The combined effect of low calcium content, a low Ca/P ratio, and low bioavailability of calcium via high phytate content may pose problems for healthy bone development in populations that use cereal grains as a staple food. It has in fact been reported that in populations where cereal grains provide the major source of calories, osteomalacia, rickets and osteoporosis are common, even when there is sufficient sunshine to prevent vitamin D deficiency. The high phytate content of cereals also affects the bioavailability of non-haem iron. A number of factors including phytate and fibre, tannins, lectins and phosphate may contribute to the inhibition of non-haeme iron absorption. The high levels of phytate contribute most to the inhibition of non-haeme iron. To eliminate its inhibitory effect on non-haeme iron absorption, phytate must be almost totally removed.

Some studies of zinc absorption in rats and humans have clearly demonstrated that consumption of phytate contained in whole grain cereals (for example, wheat, rye, barley, oats) inhibits zinc absorption. The bioavailability of zinc from meat is four times greater than that from cereals, and it implies that total dependence on cereal-grain and plant-based diets (for example, vegetarian diets) may lead to impaired zinc metabolism in developed countries including Australia.

Cereal protein is usually incomplete as it lacks some amino acids. In particular, lysine is limited in cereals which form the basis for many diets. Furthermore, the essential amino acid, threonine, tends to be lower in cereal-based proteins compared to animal protein sources.
Whole grain foods such as wheat, rice, oats, and barley are relatively low in energy density and are reported to help maintain energy balance.
Effect of Processing on Cereals

Cereal grains are usually processed to:

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make them digestible
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inactivate natural toxins and prevent bacterial growth and food spoilage
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optimise the appearance, taste and texture of foods
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improve convenience to meet consumer demand for quick and easy meal solutions
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maximise their nutritional value.

Processing can make it easier for nutrients from grains to be digested or for nutrients to be added (fortification). There are a number of factors that determine the quality of grains and pulses for human consumption. For example, milling affects the nutritional value of grains in two ways:

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The physical separation of the different grain components drastically reduces the nutrient content of the grain.
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Grinding reduces the particle size which impacts on the glycaemic
index and resistant starch content of grains.

Before cereals are consumed, they must undergo some form of processing. The methods of processing cereals (e.g. milling) reduces the nutritional value of cereals. Generally, the final nutrient content of a cereal will depend on the extent to which the outer layers are removed during processing (extraction rate). Removal of these outer layers implies loss of fibre, vitamins and minerals. Extraction rate is the number of parts by weight of flour that is produced from 100 parts of the cereal (e.g. wheat). The higher the extraction rate, the more bran is included in the wheat flour and hence the higher the amount of dietary fibre, vitamins and minerals in the flour. Highly milled cereals such as white maize flour, polished rice and white wheat flour are therefore of less nutritive value because they have lost most of the germ and outer layers and with them most of the B vitamins and some of the protein and minerals. Flours can be produced to a range of different extraction rates, depending upon the amount of bran, germ and pericarp that is removed. Flours of high extraction rates retain many more of the micro-nutrients than those of lower extraction rates. The disadvantages of low-extraction flours to the consumer are that they contain less B vitamins, minerals, protein and fibre than high-extraction flours.

Cereals feature prominently in most breakfast cereals, bread and pasta. As a result of losses that occur during the processing of cereals, those used as breakfast cereals are often enriched.
Anti-nutritional Factors in Cereal Grains

Cereal grains constitute a biologically novel food for mankind, but questions remain unanswered as to whether they are particularly suitable for the genetic constitution of humans. It has long been realised since the early 1950s (with the discovery of wheat gluten as the causative agent in coeliac disease) that cereal grain peptides interact with and induce change in human physiology and therefore elicit disease and dysfunction. Cereal grains contain a variety of secondary metabolites which tend to be either toxic or anti-nutritional.

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Alkylresorcinols:
Alkylresorcinols are phenolic compounds which are reported to be in the highest amounts in rye (97 mg/100 g), in high amounts in wheat (67 mg/100 g) and in lower amounts in other cereals such as oats, barley, millet and corn.
The traditional role of these compounds is to provide resistance from pathogenic organisms during dormancy and germination. Though alkylresorcinols pose problems in animal feeding, their relevance in human nutrition is unclear. In particular, giving large amounts of rye to cattle, sheep, horses, pigs and poultry has been shown to cause slower growth than feeding of other cereal grains.
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Alpha-Amylase Inhibitors:
Alpha-amylase inhibitors are found in wheat, rye, barley, oats, rice and sorghum. By virtue of their resistance to heat, alpha-amylase inhibitors persist through bread baking and are therefore found in large amounts in bread, breakfast cereals, pasta and other wheat products. The acute effects of alpha-amylase inhibitors therefore appear to have therapeutic benefit in patients suffering from diabetes mellitus, obesity and other diseases of insulin resistance, though chronic administration in animal models has been shown to induce deleterious histological changes to the pancreas and pancreatic hypertrophy. In view of the potential adverse effects these dietary anti-nutrients can have on human health, it is advisable to be cautious with alpha-amylase inhibitors in human foodstuffs.
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Protease Inhibitors:
Protease inhibitors have been found in most cereal grains. Protease inhibitors are proteins which have the ability to inhibit the proteolytic activity of certain enzymes and are common throughout the plant kingdom, particularly among the legumes. As with alpha-amylase inhibitors, there are a multiplicity of plant proteins which have protease inhibitor activity. Two examples of protease inhibitors are the Kunitz inhibitor, which has a specificity directed mainly towards trypsin in human gastric juice, and the Bowman-Birk inhibitor which is capable of inhibiting chymotrypsin as well as trypsin. The Bowman-Birk inhibitor is relatively stable to both heat and digestion and can therefore pass through cooking and the stomach with little change. There are no known dietary effects of chronic low level exposure to plant protease inhibitors in humans. If anything, there is some evidence to suggest they may have beneficial, anti-neoplastic effects.
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Lectins:
Lectins are proteins that have a strong affinity for carbohydrate-containing molecules, particularly toward the sugar component. They have the ability to bind to specific glycoconjugate receptors on the surface of the erythrocyte cell membranes and are recognised as the major anti-nutrient of food. Lectin activity has been demonstrated in wheat, rye, barley, oats, corn, and rice but not in sorghum or millet. Lectins, especially wheat germ agglutinin, (WGA) bind surface glycans on gut brush border epithelial cells, which leads to impairment in digestive/absorptive activities, stimulates shifts in bacterial flora and modulates the immune state of the gut.

Health Benefits in Cereal Consumption

Despite the nutritional deficiencies associated with cereals as a food group and the potential health risks their over-consumption may pose, they offer health benefits when included in moderate amounts in the diets of most people. There is evidence to suggest that regular consumption of cereals, in particular whole grains, may have a role in the prevention of chronic diseases. People who consume diets rich in wholegrain cereals seem to have a lower incidence of many chronic diseases. Consumption of high-fibre foods is associated with a lower risk of several chronic diseases, including diabetes, cardiovascular disease, and diverticulitis. Consumption of high-fibre foods, such as legumes and whole grain breads, cereals, rice, and pasta, is therefore highly recommended because evidence for an association between fibre and cancer risk is not strong.
Conclusion

Cereal grains lack a number of nutrients which are essential for human health and well-being. Furthermore, cereal grains contain antinutritional factors (phytates, alkylresorcinols, protease inhibitors, lectins, etc.) which are not well tolerated and in most cases tend to be health risks in humans. Of more concern is the ability of cereal grain proteins (protease inhibitors, lectins, opioids and storage peptides) to interact with and alter human physiology.

When combined with a variety of both animal- and plant-based foods, cereals are a reliable source of cheap energy, capable of sustaining and promoting human life. Like all food groups, over-consumption of cereal grains may disrupt health and well being in virtually all people. The take home message is that cereal grains should be consumed moderately in a mixed diet.