Key nutrients in grains

Table 1. Nutrient content of cereal grains

US Department of Agriculture, Agricultural Research Service. 2001. USDA Nutrient Database for Standard Reference, Release 14. Nutrient Data Laboratory home Page, http://www.nal.usda.gov/fnic/foodcomp

Energy

Grains provide approximately 1400-1600 kJ/100 g. They are an important source of energy in the diet, especially in developing countries in Asia and Africa where they provide up to 70% of total energy intake.

Most of the energy is provided by carbohydrate rather than fat, consequently grains have a relatively low energy density. Grains, such as oats, which have a higher fat content, have a higher kilojoule content since fat provides more kJ per gram than carbohydrate.

Carbohydrate
Grains are major sources of carbohydrate, providing more than 70 g of carbohydrate per 100 g. Most of the carbohydrate is in the form of starch (about 90%) which is found in the endosperm of the grain.

Grains generally have a low glycaemic index (i.e. the carbohydrate is slowly digested) especially when the outer layers of the grain remains intact after processing. Milling increases the glycaemic index by decreasing the particle size of the flour.

Dietary fibre

• Soluble fibre -oat, barley and rye fibres have a higher proportion (about one-third) of soluble fibre (mainly as (-glucan in oats and pentosans in rye).
• Insoluble fibre -wheat, rice, maize and other cereals have a higher proportion of insoluble fibre.
• Resistant starch -certain grain varieties, such as Hi-maize( and Doongarra rice, are high in amylose making them high in resistant starch.
• Oligosaccharides -4% fructan, which is similar in structure to inulin, is found in grains. It has similar properties to dietary fibre in that it affects gastric emptying, transit time, colonic pH and faecal mass. It also functions as a prebiotic agent by significantly increasing the concentration of beneficial bifido bacteria.

Protein

The protein content of grains varies depending on:

• Grain variety (e.g. the protein content of hard-grained wheat is typically higher than that of soft-grained wheat varieties).
• Climatic factors (e.g. temperature, rainfall).
• Environmental factors (e.g. soil fertility, fertiliser use).

The protein in grains is low in lysine, an essential amino acid. The amount of lysine present varies between different grains and between different components of the grain. The content of lysine in the aleurone and germ is greater than in the endosperm. Maize is also low in the amino acid tryptophan. Since grains are generally eaten with other foods, such as meat, fish, dairy products or pulses, the overall nutritional quality of protein in grain-based diets is usually good because these foods provide adequate quantities of the limiting amino acid. Pulses and grains are termed complementary foods because each provides the amino acid lacking in the other food.

Wheat, oats, rye, barley and triticale contain proteins which form gluten. People requiring a gluten-free diet can use gluten-free grains such as maize and rice.

Fat

Grains are generally low in fat, containing 2-4% fat, with the exception of oats which has a fat content of around 7%.

The fat in grains is mostly unsaturated, containing polyunsaturated fats (mainly as linoleic acid) and monounsaturated fats (mainly as oleic acids). Saturated fats (mainly palmitic acid) account for less than 25% of total fats.

Being plant foods, grains contain no cholesterol but do contain plant sterols. Plant sterols are known for their cholesterol-lowering effect.

Vitamins

• B vitamins -grains contain thiamin, riboflavin, niacin, vitamin B6 (pyridoxine), folate and pantothenic acid.
• Vitamin E - the germ of grains contain vitamin E, a known antioxidant. Grains contain more tocotrienols than other food products. Tocotrienols possess vitamin E activity and also inhibit cholesterol synthesis.
• Carotene - beta-carotene, found in yellow maize, may have antioxidant activity.
• Vitamin C - grains do not contain vitamin C, but when grains are germinated, they become a useful source of vitamin C. Vitamin C is not only an essential nutrient, it also has antioxidant activity.

Minerals

• Potassium -The higher concentration of potassium compared to sodium is considered beneficial, particularly in the management of hypertension.
• Magnesium, iron, zinc and phosphorus (mostly present as phytate).
• Selenium -the amount in the grain is proportional to the selenium content of the soil in which it is grown. Studies suggest that cereals are an important source of selenium in the diet.
• Small amounts of calcium, manganese, chromium and copper are also found in grains.

Bioavailability of minerals

Phytate, found in the outer layers of the grain, binds minerals, especially zinc, making them less available for digestion and absorption in the small intestine. Zinc absorption from high phytate meals can be less than 15% depending on the ratio of zinc to phytate. For example, almost 40% of the zinc is absorbed from a food with a low phytate content (e.g. white bread), while only 17 % is absorbed from a food high in phytate (e.g. wholegrain bread).

However, the total amount of zinc absorbed from wholegrain bread is almost 50% more than that absorbed from white bread because wholegrain bread contains more than three times the level of zinc found in the white bread. Hence, consumption of 20-35g/d of dietary fibre and three servings/day of wholegrain foods have not been found to have any adverse effects on mineral status.

Phytochemicals

Phytochemicals are naturally occurring chemicals found in plants which are biologically active and have been shown to have antioxidant and/or hormone-like activities. They are not recognised as nutrients but are believed to contribute to the overall health benefits of foods made from grains. Grains, especially wholegrains, contain a wide range of phytochemicals.

The main phytochemicals found in grains:

• Lignans are found in the cell walls of wheat, barley, rye, rice and oats. They have anticarcinogenic, weak antioestrogenic and antioxidant properties.
  
  
• Phenolic acids are located in the bran layer, especially ferulic and caffeic acid. The total phenolic acid content of wheat, rice and oat flour ranges from 71 to 87 (g/g whereas maize flour is 309 (g/g. They have been shown to have antioxidant activity. They may also bind to iron, preventing iron absorption.

The following antinutrients, so called because they reduce availability of nutrients, have recently been shown to have potential health benefits:

• Phytic Acid is concentrated in the bran and aleurone layers of wheat, maize, rice, barley and oats. Hence, bran and wholemeal flour have the highest phytate content whereas white flour has relatively little. In maize phytic acid is concentrated in the germ. Phytic acid has been shown to have antioxidant activity. It may also reduce the solubility of starch by binding it, reducing its absorption and hence contribute to lower glucose levels.
• Hemagglutinins (lectins).
• Phenolic compounds, including tannins.
• Inhibitors of digestive enzymes (protease and amylase) are concentrated in the endosperm and embryo of raw cereals. They are denatured and inactivated by heat. The adverse effect of protease inhibitors have mainly been found in animals and the effect in humans is unclear.
• Saponins.

Protease inhibitors, phytic acid, phenolic compounds, and saponins have been shown to reduce the risk of cancers of the colon and breast in animals.

Phytic acid, lectins, phenolic compounds, amylase inhibitors and saponins were also shown to lower one or more of the following: glucose, insulin and cholesterol.

For Further research

Additional Link:

• Antioxidants and Phytoestrogens