Introduction to nutrition
Nutrition is defined as the processes by which an animal or plant takes in and utilises food substances. Essential nutrients include protein, carbohydrate, fat, vitamins, minerals and electrolytes. Normally, 85% of daily energy use is from fat and carbohydrates and 15% from protein. In humans, nutrition is mainly achieved through the process of putting foods into our mouths, chewing and swallowing it. The required amounts of the essential nutrients differ by age and the state of the body, for example: physical activity, diseases present (e.g. prostate cancer, breast cancer or weakened bones – known as osteoporosis), medications, pregnancy and lactation.
Nutrition is essential for growth and development, health and wellbeing. Eating a healthy diet contributes to preventing future illness and improving quality and length of life. Your nutritional status is the state of your health as determined by what you eat. There are several ways of assessing nutritional status, including anthropometric (i.e. physical body measurement), food intake and biochemical measurement.
Your body mass index (BMI) is a good indicator of your nutritional status. It takes into account your weight and height, and correlates well with total body fat expressed as a percentage of body weight. The correlation depends on age, with the highest correlation seen in ages 26–55 years and the lowest in the young and the elderly. If you take your weight in kilograms and divide it by your height in metres squared, the figure you obtain is your BMI.
BMI calculations will overestimate the amount of body fat for:
- Body builders;
- Some high performance athletes;
- Pregnant women.
BMI calculations will underestimate the amount of body fat for:
- The elderly;
- People with a physical disability who are unable to walk and may have muscle wasting.
BMI is not the best measure of weight and health risk. A person’s waist circumference is a better predictor of health risk than BMI.
The healthy weight range for adults of a BMI of 20 to 25 is not a suitable measure for children.
For adults who have stopped growing, an increase in BMI is usually caused by an increase in body fat. But as children grow, their amount of body fat changes and so will their BMI. For example, BMI usually decreases during the preschool years and then increases into adulthood.
For this reason, a BMI calculation for a child or an adolescent must be compared against age and gender percentile charts. These charts should be used only by health professionals such as your general practitioner, child health nurse, or dietitian.
A person’s waist circumference is a better predictor of health risk than BMI. Having fat around the abdomen or a ‘pot belly’, regardless of your body size, means you are more likely to develop certain obesity-related health conditions. Fat predominantly deposited around the hips and buttocks doesn’t appear to have the same risk. Men, in particular, often deposit weight in the waist region.
Generally, the association between health risks and body fat distribution is as follows:
- Least risk – slim (no pot belly);
- Moderate risk – overweight with no pot belly;
- Moderate to high risk – slim with pot belly;
- High risk – overweight with pot belly.