Energy-yielding nutrients

The nutrients that break down to yield energy to the body are called energy-yielding nutrients or macronutrients. These include:

·    Carbohydrates

·    Proteins

·    Fats

One other substance contributes to energy: alcohol. Alcohol is not a nutrient, because it interferes with growth, maintenance and repair of the body, but it does yield energy when metabolised in the body. In contrast, vitamins, minerals and water do not yield energy in the human body.

Carbohydrates

A large proportion of energy in our diets comes from carbohydrates, which can be found in their simple form (sugars) or more complex forms such as starch and fibres. Sugars and starches can be digested, but fibres cannot.

Sugars and starches

These digestible carbohydrates can be found in fruits, vegetables, barley malt, sweet corn, corn syrup, honey, table sugar, sugar cane, molasses, maple syrup, milk, legumes, roots, nuts, tubers and cereals.

Functions of sugars and starches:

·    Supply energy to the body

·    Stored as fat in the adipose tissue

The Glycaemic Index (GI) is a rating system that measures the effect that carbohydrate-containing foods have on blood sugar levels compared to a reference food. In other words, how quickly glucose is absorbed from food after a person eats it, how high blood glucose rises after eating the food, and how quickly it returns back to normal.

Slow absorption, a modest rise in blood glucose, and a smooth return to normal are desirable (a low glycaemic effect); fast absorption, a surge in blood glucose, and an overreaction that plunges glucose below normal levels is undesirable (a high glycaemic effect). The general recommendation is to select low glycaemic carbohydrate-rich foods for a more moderate and longer lasting rise in glucose.

For more information, refer to the Glycaemic Index article in The Basics.

Fibre

Fibres are indigestible carbohydrates. All plant foods and their products contain some fibre, such as cellulose, pectins, and gums.

There are two types of fibre:

·    Soluble Fibre. This type of fibre tends to hold water and forms a gel. In this way it can aid the movement of the contents through the digestive tract. Soluble fibre is also fermented by colonic bacteria, which produces fatty acids that are used as fuel by the cells in the colonic wall. Soluble fibre is found in fruits, vegetables, oats, oat bran, barley, pulses (lentils, chick peas, beans) and legumes.

·    Insoluble fibre. This type of fibre is found in brown rice, high fibre cereals, bran, wholewheat bread, nuts and seeds.

Functions of fibre:

·    Increases bulk of stool through the absorption of water. This puts pressure on the walls of the alimentary tract, which in turn stimulates peristalsis

·    Fibres help to regulate the passage of food through the gastro-intestinal system, but contributes little, if any, energy.

Benefits of fibre:

·    Protection against diseases of the colon (constipation, diarrhoea, diverticulitis and colorectal cancer)

·    Reduces blood cholesterol levels because it can bind cholesterol in the bile and prevent the cholesterol from being reabsorbed by the body

·    In the small intestine, soluble fibre slows down the absorption of glucose into the blood stream, preventing sudden rises in blood sugar levels, which is particularly beneficial for diabetics

·    Beneficial for weight loss as it bulks up the food and helps delay gastric emptying, thereby prolonging the onset of hunger.

Drawbacks of too much fibre:

·    Too much fibre can lead to flatulence and other gastric disorders

·    A very high-fibre diet can restrict kilojoule intake

·    Too much insoluble fibre can reduce the absorption of calcium and iron

Proteins

Responsible for the growth and repair of the body, protein is a vital nutrient. It helps to form muscles, bones, connective tissue, hair, nails, antibodies, enzymes and certain hormones. The amount of protein you need daily depends on your body size, rate of growth and the presence or absence of disease.

Amino acids

The basic building block of protein is an amino acid. There are twenty-two amino acids, of which nine are essential to humans. Because the body cannot manufacture these essential amino acids, it is necessary to eat foods that contain them. A deficiency in amino acids leads to weight loss, impaired growth in infants and other clinical symptoms. Non-essential amino acids can be synthesised by the body should they not be provided by the food we eat. Conditional essential amino acids become essential when the body is put under stress.

Proteins are classified into two categories:

·    High quality protein. These proteins contain significant amounts of the essential amino acids and can be found in red meat, chicken, fish, eggs, milk and dairy products, and soya products

·    Low quality protein. These proteins contain all the amino acids, but not in adequate amounts. They can be found in cereals, nuts, pulses and vegetable protein. Although vegetable proteins are considered lower quality protein than animal protein, a combination of cereals and pulses will provide all the amino acids in adequate amounts.

Functions of protein:

·    Tissue repair and growth

·    Provides energy if the diet is inadequate in carbohydrates and fat.

Protein deficiency:

·    Can lead to a low resistance to illness and infection

·    Can result in more serious symptoms such as growth retardation, oedema (swelling), muscle wasting and hair loss

·    In severe cases, protein deficiency results in Kwashiorkor, which is a severe nutritional deficiency illness found in children, the symptoms of which include anaemia, poor growth, weakness and oedema (characterised by a pronounced pot-belly).

Excessive protein intake:

·    Because protein cannot be stored in the body, excess protein is converted to glucose and to nitrogen, which is excreted by the kidneys, skin and intestines. This excretion can stress the kidneys and may intensify gout.

Fats

Fats contain twice as much energy as protein and carbohydrates.

Fatty acids

These are the basic component of fats and oils. There are two types of fatty acids:

·    Saturated fatty acids which can be found in animal products (beef, pork, chicken, dairy) and some vegetable products (palm, palm kernel, and coconut oil)

·    Unsaturated fatty acids of which there are two types:

·    Monounsaturated fatty acids (MUFA). These protect against diabetes and cardiovascular disease and can be found in olive oil, canola oil, peanut oil, pecans, almonds, and avocados

·    Polyunsaturated fatty acids (PUFA). These are dietary essential fatty acids and can be found in seed oils such as sunflower oil, corn oil, soya oil and soft margarines

Essential fatty acids. Essential fatty acids play an important role in eye functioning and brain development, as well as protection against inflammation. The body cannot manufacture linoleic acid (omega 6) and linolenic (omega 3). These fatty acids must therefore be supplied by the diet and are classified as "essential."

Trans fatty acids. These are formed during food processing when oils are turned into semi-solids. This is done through the process of hydrogenation, where PUFAs are changed to MUFAs. For example, brick margarine is made by hydrogenating vegetable oil. Trans fats are found in margarines, shortenings, salty snacks, high-fat baked goods and commercial frying fats. In the body, trans fatty acids behave more like saturated fats than unsaturated fats.

Triglycerides. These are divided into fats and oils. Because of their high energy density and low solubility, they are the chief form of fat in the diet and the major storage form of energy in the body. Triglycerides with more unsaturated fatty acids are soft or liquid oils at room temperature, while those with more saturated fatty acids tend to be solid fats at room temperature.

Phospholipids. These are used in the food industry to combine two ingredients that do not ordinarily mix, such as water and oil. They are found naturally in the following foods: eggs, liver, soybeans, wheat germ and peanuts. Phospholipids are important constituents of cell membranes. Because they can dissolve in both water and fat, they can help lipids move back and forth across cell membranes into the watery fluids on both sides. They therefore allow fat-soluble substances, including hormones and some vitamins, to pass easily in and out of cells. Phospholipids also act as emulsifiers in the body, helping to keep fats suspended in the body fluids and the blood.

Cholesterol. This is an essential component of cell membranes, and is the major component of brain and nerve cells. It is used to synthesise steroids, bile acids and hormones and plays a role in human fertility. The body synthesises up to 80% of the cholesterol circulating in the blood, while the remaining 20% is obtained from the diet. Cholesterol is found in animal products such as red meat, chicken, fish and dairy products. High amounts of cholesterol are found in egg yolk, liver, offal, shrimps and prawns.

Functions of fats:

·    Triglycerides, both from food and from the body's fat stores, provide the body with energy

·    Fat insulates the body. It is a poor conductor of heat, so the layer of fat beneath the skin helps to keep the body warm

·    Fat also acts as a 'shock absorber', supporting and cushioning the vital organs

·    Fat helps the body to use carbohydrate and protein efficiently. Fat molecules combine with glucose molecules during energy metabolism and help to spare protein by providing energy so that the protein can be used for other essential functions in the body.

Excessive fat intake:

·    A high fat diet can contribute to being overweight and has been linked to certain cancers

·    A high intake of saturated fat raises cholesterol levels, which leads to an increased risk of heart disease

Reduction of fat intake in the diet:

·    Overall fat intake should be decreased, but it is still very important to balance the types of fats (saturated, monounsaturated and polyunsaturated) in your diet

·    Limit take-away and convenience foods

·    Avoid fried foods. Rather grill, bake, steam, boil or microwave and use as little fat and oil in your cooking as possible

·    Use light margarine, olive oil or canola oil

·    Use low fat mayonnaise and salad dressing, if any

·    Reduce your intake of red meat and rather have chicken and fish more often

·    Cut off all visible fat from meat and remove the skin of the chicken before cooking

·    Choose low fat or fat free dairy such as skim milk, low fat yoghurt, low fat cheese and fat free cottage cheese

·    Beware of foods with hidden fats such as sauces, gravies, white sauce, cheese sauce, cream based pasta sauce etc.

Water

An adult's body is made up of 60 percent water, which needs constant replacement. Next to oxygen, water is the most important substance for sustaining life.

Functions of water:

·    Essential for metabolism to occur

·    Circulates substances around the body

·    Essential for the digestion and absorption of food and the elimination of waste products

·    Contains many valuable mineral salts

·    Regulates body temperature

Water regulation:

·    The kidneys regulate water balance. Water is lost through the skin (sweat), kidneys (urine), lungs and rectum (feaces) as a means of removing waste from the body (2-3 litres per day). When water loss is excessive (diarrhoea, burns, excessive sweating, bleeding) or water intake is insufficient, the kidney compensates by excreting more concentrated urine and reabsorbing more water. Excessive water loss can cause a loss of electrolytes. Electrolytes dissolve salts in water and therefore enable the body to move fluids into appropriate compartments to maintain the correct fluid balance throughout the body

·    The body has no way of storing water and so it is very important that water be constantly replaced to prevent dehydration and shock. Thirst is not a good indicator of water balance - by the time you start to feel thirsty, you are already slightly dehydrated

·    Dehydration causes fatigue, accumulation of waste products in the blood system and headaches.

Sources of water:

·    Plain water (tap or botlled)

·    Juices e.g. tea, coffee fruit juice, fizzy drinks, sports drinks, cordial and alcohol

·    Milk, yoghurt, milk shakes

·    Fruit and vegetables, especially juicy vegetables and salad such as oranges, watermelon, salad leaves

·    Bread and cereals

·    Meat

Prudent dietary guidelines

Sources

Understanding Nutrition (Rolfes & Whitney)

Food, Nutrition & Diet therapy (Mahan & Arlin)

Manual of Dietetic Practice (Thomas for the British Dietetic Association)

(Reworked from article by Discovery)