Obesity, Malnutrition and Lifestyle Factors

Lifestyle changes risk rather than guaranteeing disease

Non-communicable diseases (NCDs) cannot be transmitted between people. Many develop through an interaction between inherited characteristics, lifestyle and the environment. Diet and exercise are therefore risk factors: they can alter the probability of disease, but they do not determine with certainty what will happen to one individual.

This distinction matters when interpreting data. If a population with lower physical activity has more type 2 diabetes, the two variables are correlated. A biological explanation can support a causal link, but other variables—such as age, income or diet—may also affect the pattern.

Diet, exercise and energy balance

Food supplies energy for respiration as well as nutrients needed for growth, repair and healthy body processes. Physical activity increases energy expenditure. Over time:

• when energy intake is greater than energy expenditure, the surplus can be stored, mainly as fat, so body mass may increase;
• when energy intake is lower than energy expenditure, stored energy is used and body mass may decrease;
• when intake and expenditure are balanced over time, body mass tends to remain more stable.

This is not the whole explanation for obesity. Genetics, eating behaviour, illness, medicines and the availability and price of food can all affect body mass. Environments that provide cheap energy-dense food but few safe or convenient opportunities for activity can make healthy choices harder.

Regular exercise raises energy expenditure and can help reduce the risk of obesity. It can also improve the body's sensitivity to insulin and benefit cardiovascular health. By contrast, a long-term diet supplying more energy than is used increases the likelihood of excess fat storage. Obesity is associated with a higher risk of NCDs including type 2 diabetes and cardiovascular disease.

Malnutrition can mean too little, too much or the wrong balance

Malnutrition is broader than starvation. It occurs when energy or nutrients are deficient, excessive or badly balanced.

Undernutrition may result from too little total food or too little of a particular nutrient. Insufficient energy and protein can cause low body mass, tiredness, impaired growth and reduced ability to maintain and repair tissues. A specific deficiency has effects linked to the nutrient involved: for example, too little iron can limit haemoglobin production and contribute to tiredness and shortness of breath.

Overnutrition includes a sustained excess of energy that contributes to overweight or obesity. A person can also consume enough—or too much—energy while lacking essential micronutrients if much of the diet is energy-dense but nutrient-poor. Obesity and nutrient deficiency can therefore occur in the same community, household or person.

A balanced diet supplies suitable amounts of energy, protein, vitamins, minerals, fibre and other nutrients. The amount required varies with factors such as age, growth, pregnancy and physical activity, so the same intake is not appropriate for everyone.

Calculating body mass index

Body mass index relates mass to height:

$$\text{BMI}=\frac{\text{mass (kg)}}{(\text{height (m)})^2}$$

The height must be in metres and is squared before the mass is divided by it. BMI is commonly expressed in $\text{kg m}^{-2}$.

Consider an adult with a mass of $72\text{ kg}$ and a height of $1.80\text{ m}$. First square the height:

$$1.80^2=3.24\text{ m}^2$$

Then substitute the measurements:

$$\text{BMI}=\frac{72}{3.24}=22.2\text{ kg m}^{-2}\text{ (to 1 d.p.)}$$

The result relates the person's mass to their squared height; it is not a measurement of body fat itself. For adults, the World Health Organization uses a BMI of at least 25 as an indicator of overweight and at least 30 as an indicator of obesity. This result is below those adult thresholds, but BMI alone does not diagnose health. These thresholds should not be applied unchanged to children because their BMI must be interpreted in relation to age and sex.

BMI is useful as a screening measure, but it is only a surrogate measure of body fat. It does not distinguish fat from muscle, so a muscular person may have a high BMI without having excessive body fat.

Calculating waist-to-hip ratio

Fat distribution also matters. The waist-to-hip ratio compares the circumference around the waist with that around the hips:

$$\text{waist-to-hip ratio}=\frac{\text{waist circumference}}{\text{hip circumference}}$$

Both measurements must use the same unit. For example, suppose a person has a waist circumference of $84\text{ cm}$ and a hip circumference of $100\text{ cm}$:

$$\text{waist-to-hip ratio}=\frac{84\text{ cm}}{100\text{ cm}}=0.84$$

The centimetres cancel, so the answer has no unit. The calculation means that the waist circumference is 0.84 times the hip circumference. A larger ratio indicates that a greater proportion of body size is concentrated around the waist. This abdominal fat distribution is associated with increased risk of obesity-related conditions such as type 2 diabetes, but the ratio alone does not diagnose disease.

BMI and waist-to-hip ratio answer different questions. BMI compares overall mass with height, whereas waist-to-hip ratio gives information about fat distribution.

Effects at local, national and global levels

At a local level, communities differ in the affordability and availability of nutritious food and in access to places for physical activity. These conditions can influence patterns of obesity, undernutrition and related NCDs within neighbourhoods.

At a national level, the same patterns affect large numbers of people and increase demand on health services. Governments and organisations can alter the conditions in which choices are made—for example through food standards, pricing, marketing rules, education and urban planning—rather than treating obesity only as an individual's responsibility.

At a global level, both undernutrition and obesity are major problems. Many low- and middle-income countries experience a double burden of malnutrition: undernutrition remains present while obesity and associated NCD risks increase. Changes in food systems, urbanisation and physical activity help explain why obesity is no longer confined to high-income countries. In 2022, about one in eight people worldwide were living with obesity, showing how individual risk factors can combine with wider social conditions to produce a global public-health problem.

Core relationship

Diet and exercise affect energy balance:

• intake $>$ expenditure over time → more energy stored as fat → increased risk of obesity;
• intake $<$ expenditure over time → body stores are used;
• regular exercise increases energy expenditure and can reduce NCD risk.