Nutrients and their functions
The principles of a healthy balanced diet:
Foods provide us with energy in the form of calories (Kcal).
Calories effectively act as the fuel that powers our bodies and enables us to function, in the same way that petrol fuels a car.
Some foods provide us with more energy than others, but by eating a wide variety of foods in the correct balance we can meet our requirements.
Foods that provide many nutrients relative to the amount of energy they contain are known as “nutrient rich” foods e.g. milk and dairy foods.
Energy requirements vary depending on age, sex, size, metabolic rate and activity level.
If we consume more energy/calories than we need, we deposit the excess energy in the form of fat or adipose tissue.
Conversely if we use more energy than we consume we use up fat to provide us with more energy.
Carbohydrates can be divided into two broad categories: available carbohydrate and unavailable carbohydrate.
Sugars and starch are categorised as available carbohydrate.
Sugars are present naturally in fruit, vegetables and milk and are also added to many processed foods such as confectionery, cakes and biscuits.
Starch is found in foods such as bread, cereals and potatoes.
Both starch and sugars are digested in the body and converted to simple sugars (mainly glucose), which are then used by the body to provide energy.
Unavailable carbohydrate includes dietary fibre or NSP (non starch polysaccharide). The term “unavailable” is used because fibre can’t be digested and therefore doesn’t provide us with energy. However it is helpful in many other ways described below.
Dietary fibre can be divided into two categories: insoluble fibre and soluble fibre.
Insoluble fibre (found in wholegrain cereals and grains, and some fruits and vegetables) adds bulk to the contents of the gut, speeding their transit and it is thought to help protect against constipation and other bowel disorders
Soluble fibre (found in pulses such as beans and lentils, fruit, vegetables and also oats, barley and rye) helps to reduce blood cholesterol levels and to regulate blood sugar levels.
At present the average intake of fibre in the UK is 14g/day in adults. Experts recommend that fibre intakes should be as high as 18g/day.
Consumption of brown, wholegrain, wholemeal and high fibre varieties of carbohydrate will help to increase fibre intake.
Proteins are essential for growth and maintenance of body tissues and for the production of substances such as hormones and enzymes which help to control many functions within the body. If insufficient carbohydrate and fat are available in the diet, then protein may also be used to provide the body with energy.
Proteins are made from building blocks known as amino acids.
There are 20 different amino acids.
Some amino acids can be made in the body and others can only be supplied by the diet -these are known as the essential amino acids.
Some foods are better providers of these amino acids than others. Those which contain all the essential amino acids are known as “high biological value” foods e.g. milk and dairy foods, meat, eggs etc.
Those which contain fewer of the essential amino acids are known as “low biological value” foods e.g. cereals, beans, lentils and nuts.
However if a wide variety of foods are consumed in the correct proportions the different protein sources can work together to provide the ideal levels of the different amino acids.
Fats are essential for many reasons:
- They are a provider of energy
- They are involved in forming cell membranes
- They are a vehicle for the provision of fat soluble vitamins such as Vitamins A, E, D and K
- They are involved in making hormones
- They provide insulation; keeping us warm.
- They provide us with a shock absorbing, protective layer
Fats are made from building blocks called fatty acids.
There are three types of fatty acids - saturated, monounsaturated and polyunsaturated fatty acids.
The fat in food contains a mixture of all three fatty acids, in different proportions in different foods.
Foods that contain the higher proportion of saturated fatty acids include fats and oils (e.g. butter, hard margarine, some blended cooking oils), meat and its products (e.g. pies, lard, suet), whole milk and its products, coconut and palm oil.
Olives, olive oil and rapeseed oil are the best providers of monounsaturated fatty acids.
Fats and oils containing large amounts of polyunsaturated fatty acids are derived mainly from seeds and nuts and include pure sunflower, safflower, sesame, soya, corn oils, and sunflower and soya margarine.
Two polyunsaturated fatty acids, linoleic and alpha-linolenic acids cannot be made in the body and must be provided in the diet. These are called essential fatty acids.
These essential fatty acids are known as the "parent" fatty acids of 2 families of unsaturated fatty acids. The parent fatty acids undergo various different chemical reactions to produce the different fatty acids within each family, which have numerous different and important functions within the body.
Linoleic acid is the parent fatty acid of the n-6 family of fatty acids and alpha-linolenic acid is the parent fatty acid of the n-3 family of fatty acids.
Vegetable oils, eggs and poultry are good providers of n-6 fatty acids which are important for the formation of membranes in the body.
Unrefined fish oils and oily fish such as salmon, mackerel and sardines are good providers of n-3 or Omega 3 fatty acids which are important for the correct formation of nerves and have been linked to numerous health benefits such as reduced risk of cardiovascular disease and improved brain function.
Experts advise that too much fat, in particular saturated fatty acids, may lead to raised levels of blood cholesterol in some people which, in turn, is a risk factor for coronary heart disease.
As a result, government guidelines recommend that fat should provide no more than 35% of daily food energy, with saturates providing no more than 11%, polyunsaturates contributing no more than 6.5% and trans fatty acids no more than 2% of the daily food energy intake.
NB: These recommendations for fat intake do not apply to children under five years of age.
Trans fats are formed when the structure of monounsaturated and polyunsaturated fats are altered during a process called hydrogenation. They are often found in processed foods such as biscuits, cakes and margarines.
Trans fatty acids found in industrially produced products have been shown to have a negative effect on risk factors for CVD. In the UK efforts have been made to reduce or remove trans fatty acids from margarines and spreads through the use of new techniques and many now have minimal amounts of trans fatty acids.
Trans fatty acids also occur naturally in small amounts in milk and milk products and have been created in the stomach of ruminant animals (such as cows and sheep).
Vitamins, minerals and trace elements
Vitamins, minerals and trace elements are required for numerous functions within the body and deficiencies can lead to serious health problems.
They are required in much smaller amounts than fats, carbohydrates and proteins and are therefore known as micronutrients.
The department of health recommends specific amounts of each micronutrient for certain sub groups of the population known as dietary reference values (DRVs).
These recommendations only apply to healthy people and should only be used as a general guideline as individual requirements are likely to vary.
There are two types of vitamins, water-soluble and fat soluble.
Water soluble vitamins travel around the body in the bloodstream and are picked up by cells when they are needed. Water-soluble vitamins that are not required by the body are excreted in the urine.
Fat soluble vitamins
Fat soluble vitamins are stored in body fat (for a few days or as long as 6 months) until the body needs them.
- Water soluble vitamins:
- Vitamin B1 (thiamin)
- Vitamin B2 (riboflavin)
- Vitamin B6
- Vitamin B12
- Pantothenic acid
- Vitamin C
- Fat soluble vitamins:
- Vitamin A
- Vitamin D
- Vitamin E
- Vitamin K
Minerals and trace elements
Minerals and trace elements are similar to vitamins and are required in very small or trace amounts to maintain good health.
Minerals tend to be required in milligram (mg) quantities and trace elements tend to be required in much smaller amounts - microgram (μg) quantities.
Some are found only in a few foods, so it is important that these foods are included in the diet on a regular basis e.g. the main providers of calcium in the diet are milk, cheese and yogurt . Some foods are also fortified with minerals, for example, iron is added to some breakfast cereals.
- Trace elements:
Consumption of a balanced and varied diet should ensure adequate levels of all vitamins, minerals and trace elements are received.
It is always better to receive the recommended levels of vitamins, minerals and trace elements through consumption of food sources rather than artificial supplements. However supplements are sometimes useful, particularly if you have an increased requirement for one or several nutrients e.g. pregnant women, infants, older people who don't go out of doors or ethnic groups who wear coverall clothing etc.
It is always a good idea to seek advice from a state registered dietitian if you feel that supplements are necessary.
The principles of a healthy balanced diet: