5.23 - 5.24 - Lifestyle and disease risk

5.23–5.24 · Interacting risks at three scales

A non-communicable disease cannot be transmitted from one person to another. Examples in this lesson include cardiovascular diseases, many forms of cancer, some lung and liver diseases, and diseases influenced by nutrition.

Many non-communicable diseases are multifactorial: several factors interact. For example, smoking can raise cardiovascular risk, but the final risk also depends on influences such as inherited susceptibility, age, blood pressure, diet, physical activity and the environment. Two people with the same lifestyle exposure can therefore have different outcomes, and a person without one named risk factor can still develop disease.

The same principle applies across the required disease groups. Exposure to smoke can interact with air quality and inherited susceptibility in some lung diseases and cancers; alcohol exposure can interact with obesity, existing liver disease and inherited susceptibility in some liver diseases; and diet, activity, food access and physiology can interact in nutrition-influenced disease.

Interaction does not mean that the effects simply add to a guaranteed result. It means that one factor can alter the effect or importance of another. For example, an energy-dense diet, low physical activity and an inherited tendency to gain body fat may act together to increase obesity risk more than any single factor considered alone.

The biological processes occur in individuals, but their effects can be considered at three scales:

The scale changes the pattern and impact being studied, not the underlying biology inside one person's cells and organs.

5.24 · Energy balance and body indicators

Food supplies energy and nutrients; physical activity is one way the body expends energy. Over time, if energy intake repeatedly exceeds energy expenditure, some excess energy is stored in the body, including as fat. Body mass can rise and the risk of obesity increases. Regular physical activity raises energy expenditure and can help maintain energy balance, but its effect still interacts with diet and the other factors already described.

The reverse problem is not simply being thin. Malnutrition means a deficiency, excess or imbalance of energy or nutrients. Too little total energy can cause loss of body mass, while too little of a particular nutrient can cause deficiency even when total energy intake is high. Obesity is therefore one form of malnutrition, and a person can have excess body fat while still lacking particular nutrients.

Two indicators can be calculated from body measurements. Neither is a diagnosis.

BMI compares mass with height. Its unit is usually written as $\mathrm{kg\,m^{-2}}$. For most adults, a BMI of 25.0 to 29.9 is classified as overweight and 30.0 or above as obesity, but BMI cannot distinguish fat from muscle. Adult cut-offs must not be applied directly to growing children, whose interpretation depends on age and sex.

Worked BMI example

An adult has a mass of $86.4\ \mathrm{kg}$ and a height of $1.80\ \mathrm{m}$. Find the BMI to one decimal place.

Equation: $\mathrm{BMI} = \dfrac{\mathrm{mass}}{\mathrm{height}^2}$

Substitution:

Answer: $26.7\ \mathrm{kg\,m^{-2}}$ to one decimal place.

The value is plausible for an adult above the usual healthy BMI range, but it does not reveal how much of the mass is muscle and cannot show that disease is present.