Childhood obesity

The proportion of obese children is on the rise - from 4.2 per cent in 1990 to 6.7 per cent in 2010 worldwide. Of 42 million children under five years of age with excess weight, 31 million are in developing countries. Children who are overweight and obese are likely to stay obese into adulthood and are more likely to develop non-communicable diseases (NCDs) such as diabetes, cardiovascular diseases, and some types of cancer, at a younger age. Changes in lifestyle result in physical inactivity and poor diet quality, which are risk factors for obesity and related NCDs. In many low- and middle-income countries under-nutrition and obesity exist together in the same family; sometimes in the same person. This is referred to as the double burden of malnutrition.

The IAEA supports the application of stable isotopes to assess the effect of lifestyle changes on body composition (lean mass and fat mass) and total energy expenditure in order to inform the design and improvement of activities aimed at prevention and control of obesity and related health risks. The IAEA supports the United Nations' Sustainable Development Goals (SDGs) 3.4 and 2.2., aiming to reduce NCD-related pre-mature mortality by 2030 in the former and end all forms of malnutrition by 2025 in the latter. Monitoring body composition is important, because changes in body composition are associated with physiological changes in the body that can lead to NCDs. Total energy expenditure, which is measured using the doubly labelled water method, can be used to validate measurements of physical activity. It also gives an estimate of energy requirements and can be used to validate dietary assessment tools in different age groups.

Looked at simply, a person’s body weight comprises: fat mass (FM) and fat free mass (FFM). FFM is composed of mainly water with some proteins and minerals. It is assumed that FM has no water and that is not the same as adipose tissue. Total body water can be measured by isotope dilution so the FM can be calculated. FM is used as a proxy indicator for the risk of obesity. Here is how:

• A person drinks an accurately weighed amount of water labelled with deuterium (²H₂O) or oxygen-18 (H₂¹⁸O), stable isotopes of hydrogen and oxygen.
• The labelled water mixes with the water in their body. After a few hours the isotope is evenly spread throughout the body water, which can be sampled in the form of saliva or urine.
• The enrichment of deuterium or oxygen-18 in saliva or urine is measured.
• Total body water is calculated from the measured isotope enrichment and the weight and enrichment of the labelled water consumed. From this we can estimate the FFM.
• The hydration of the FFM changes with age. In new-born babies the FFM is 80 per cent water, but by the time they are adults it has reduced to 73 per cent. FFM is calculated using an appropriate hydration factor.
• The fat mass is the difference between body weight and fat-free mass. The results can be expressed as a percentage of body weight.

Total energy expenditure can be assessed using the doubly labelled water technique. Here is how:

• A person drinks a dose of water containing two stable, non-radioactive, isotopes (deuterium (²H₂O) and oxygen-18 (²H¹⁸O).
• The dose mixes with the water in the body. The isotopes leave the body in urine, sweat and breath. Urine samples are collected before administration of the dose and for 7-14 days afterwards. Deuterium leaves the body only in body-water, whereas oxygen-18 leaves the body faster in both water and carbon dioxide.
• The difference in the elimination rates of deuterium and oxygen-18 is a measure of the carbon dioxide production rate, from which energy expenditure can be calculated. It is the only way to accurately measure the amount of energy a person uses each day under normal, daily living conditions.