Thai children enjoying lunch that includes rice fortified with vitamin A, iron and zinc. (Photo: T. Pongcharoen, Thailand)
What is the benefit of using stable isotope techniques to assess vitamin A status?
Administering a dose of labelled vitamin A (Photo: T. Pongcharoen, Thailand)
- Vitamin A is essential for good vision, healthy growth and immune function – the ability of the body to fight infections;
- Vitamin A is unusual, in that it can be toxic if taken in excess;
- Vitamin A is found in the liver of animals and fish, eggs, green leafy vegetables, and orange and yellow fruits and vegetables such as carrots, pumpkins and sweet potatoes;
- Some countries have vitamin A supplementation programmes, or commonly consumed foods, such as oil or sugar, which may be fortified with vitamin A to combat vitamin A deficiency. The World Health Organization has classified vitamin A deficiency as a public health problem in more than half of all countries, affecting over 250 million pre-school children, especially in Africa and South-East Asia;
Vitamin A body stores can be assessed using the retinol isotope dilution technique. This technique can be used to:
- Provide a sensitive assessment of current vitamin A status in the whole body, from deficient to normal to excessive;
- Assess the success of interventions to prevent vitamin A deficiency.
- Traditionally, vitamin A concentrations in the blood have been used to assess deficiency. However this only allows the detecting of concentrations when they are very low and when extreme deficiency has already occurred.
How does it work?
- A dose of vitamin A labelled with a stable, non-radioactive, isotope of hydrogen (2H) or carbon (13C) is administered after a baseline blood sample has been collected;
- The labelled vitamin A mixes with the vitamin A in the body, and after two weeks they are evenly mixed: the dose has equilibrated with the vitamin A body pool;
- A second blood sample is taken and the enrichment of labelled vitamin A is measured by mass spectrometry (A*/A);
- From the dilution of the precisely measured dose of the isotope labelled vitamin A, it is possible to calculate the total vitamin A stores in the body.
Improving the vitamin A status of Thai children
- Vitamin A-fortified rice is a potential intervention strategy to prevent vitamin A deficiency in at-risk populations. Rice was fortified with iron, zinc and vitamin A and its ability to improve vitamin A status was tested in Thai schoolchildren. Over five months, a group of children was served with fortified rice and another (control) group of children was served with unfortified rice, as part of their diets;
- The retinol isotope dilution technique was used to quantify the vitamin A body pools at the beginning and at the end of the feeding period. Children consuming the fortified rice had a significant increase in body vitamin A stores, despite the fact that there was no difference in serum vitamin A (retinol) concentration;
- This demonstrated the benefit of rice fortification with vitamin A for these children. The fortified rice also improved the iron and zinc status of the children;
- Fortified rice is an effective means to provide additional vitamin A to at-risk populations and can be recommended to policy makers and programme planners as an effective strategy to prevent vitamin A, iron and zinc deficiencies in rice-eating populations where the diet is not diversified and largely plant-based.