Energy Intake:
Maintaining the Nutritional Balance

Our bodies use food energy (calories) to fuel muscle movements and metabolic processes. Too few calories sap the body of needed strength for day-to-day activities and, over time, can be a serious threat to health. Too many calories can cause weight gain and related health complications, such as diabetes and heart disease. Rates of overweight and obesity have been rising steadily over the past century. According to the WHO, more than 1 billion adults worldwide are overweight and at least 30 million are clinically obese.

Global Distribution of Underweight. Source: World Health Organization

Nuclear and isotopic techniques can be used to study important parameters of human nutritional status, such as total energy expenditure, lean body mass, and breast milk intake. The results from these studies can help guide nutritionists in customizing nutrition programmes to provide calories and nutrients in a healthy balance to meet specific needs.

Stable isotope techniques used in nutritional studies

Total Energy Expenditure and Body Composition

Doubly labelled water (2H218O) — so-called because both the hydrogen and oxygen components of water are labelled with stable isotopes — is the only technique available to accurately measure how much food energy people use. This technique has gained wide acceptance. It is accurate and can be used under field conditions.

The (2H218O) doubly-labelled-water method for measuring total body water, body composition and energy expenditure
	Total Body Water — Person drinks a dose of water containing stable isotopes of both hydrogen and oxygen (2H218O) — Isotopes of both H and O mix with normal H and O in the body water within a few hours — The ratio of 2H:1H is measured in urine or saliva to calculate the body water volume
Body Composition — Total body weight is used to quantify fat-free mass. Fat mass is calculated as the arithmetic difference between body weight and fat-free mass.
	Total Energy Expenditure — The body produces carbon dioxide (CO2) and water (H2O). CO2 is lost from the body via the breath and H2O is lost through breath, skin, and urine — 18O is contained in both CO2 and H2O and will be lost from the body faster than 2H. The rate of loss is measured by changes in urine over the next 7–12 days — The difference between the rate of loss of 18O and 2H is used to calculate CO2 production, which in turn is used to calculate energy expenditure

The results of investigations on energy expenditure of young children in Cuba are being used by the FAO/WHO/UNU to establish new energy recommendations. Prior to this project, no data existed for developing countries that could provide a scientific basis for food programmes suited to the local needs and conditions. The data indicate that the existing values actually overestimate the energy needs in children below 7 years old and may be contributing to growing pattern of obesity being observed in children in the region.

Lean Body Mass

Determining lean body mass and fat mass can help identify risks for over-nutrition. While it is a well recognized problem in developed countries, obesity is an increasing problem in countries considered to be in “nutrition transition”.

A dose of labelled water (2H or 18O) is given and allowed to equilibrate for a few hours. The extent of dilution of the tracer in body water is measured by sampling saliva, urine or plasma. Total body water (TBW) is then calculated using a specific equation. TBW is used to quantify lean body mass. Fat mass is the difference between body weight and free fat mass.

Breast Milk Intake

Senegal and Ghana are studying how supplementation programmes for nursing mothers are affecting their breast milk. The IAEA is providing technical support to use stable isotopes as part of this research, also supported by the World Bank, World Food Programme, and the German Kreditanstalt für Wiederaufbau. The results showed that breast milk output was not influenced by supplementation 60 days before delivery, however the quality of the milk, particularly the lactose, total protein, and zinc contents, increased significantly in supplemented mothers.

Using Stable Isotopes to Measure Breast Milk Intake
	Stable isotopes are used to measure how much breast milk a baby receives when nursing. — 2H2O is given by mouth to the mother — 2H enters the baby via breast milk and mixes with all the water in the baby’s body — 2H is measured in the baby’s urine or saliva — The rate of 2H appearance in the baby, measured in spot urine or saliva collections, is proportional to breast milk intake