New Zealand Total Diet Study (NZTDS)
Every 5 years, the NZTDS samples and tests the typical diet of average New Zealanders to measure the presence of residues, contaminants and elements in food.
For the latest results from the NZTDS, see the ‘Documents’ page in the menu to the left.
The NZTDS was previously called the New Zealand Total Diet Survey.
Purpose and aims
The NZTDS assesses the New Zealand population’s exposure to chemical residues, contaminant elements and some nutrients in foods that are regularly eaten in New Zealand. This valuable information helps to develop and improve food safety standards.
The NZTDS also contributes to international food safety commitments and obligations. These include:
- the World Health Organization Global Environmental Monitoring Systems Food programme (WHO GEMS/Food)
- Codex Alimentarius
- WHO/FAO Joint Expert Committee on Food Additives (JECFA)
- WHO/FAO Joint Meeting on Pesticide Residues (JMPR).
You can find out more about some of these organisations in the section about Codex.
In some cases, the NZTDS also contributes to the review of Maximum Permissible Concentrations (MPCs) in food, which is overseen by Food Standards Australia New Zealand.
The NZTDS has been undertaken 7 times since the first study in the mid-1970s. The most recent study was in 2009. The first 5 were carried out by the Ministry of Health, but when the New Zealand Food Safety Authority (NZFSA) was established in 2002, responsibility for the survey was transferred. The NZTDS is now the responsibility of MPI. Because carrying out such a study is a relatively large and complex project, most countries undertake them intermittently.
Since the 1990s, the NZTDS has been undertaken approximately every 5 years and it is expected that this will continue in the future.
Who takes part
Different population groups are chosen for the NZTDS. Over time, the number of population groups identified in the NZTDS has grown to 8.
The diets of the following groups were tested in both the 2003–04 and 2009 studies:
- adult males over 25 years
- adult females over 25 years
- 19–24 year old males
- 11–14 year old adolescent boys
- 11–14 year old adolescent girls
- 5–6 year old children
- 1–3 year old infants
- 6–12 month old infants.
Choosing foods to test
Foods for the NZTDS are selected based on information from nutrition studies – where individuals are asked to record or recall what they have eaten – and from retail sales data. Usually only the most commonly consumed foods are on a TDS food list. This means about 100 foods are tested, rather than the thousands of foods that a nutrition study will identify.
A simulated diet is developed from the listed foods for each of the population groups identified for testing. The NZTDS tests each diet for exposure to selected contaminants, residues, elements and nutrients. Also included are a small number of foods that are known to be high sources of contamination and several foods that are favourites with infants and children.
Foods are also categorised as one of the following:
- a national food, meaning that it can be bought anywhere in New Zealand
- a regional food, meaning the food varies around the country.
Foods tested in the TDS are purchased from retail outlets – the supermarket, greengrocer, butcher, fishshop or café – to ensure variety. National foods are purchased in one city.
How food is tested
A TDS is undertaken by purchasing samples of foods consumed by a ‘typical’ person in one of the 8 population groups. The food is then prepared for eating before it is tested. For example, meat is cooked and a banana is peeled before testing takes place. Each food is sampled twice over a year to allow for seasonal variations. The sampling and analysis is managed in 4 testing periods, each lasting about 6 weeks.
Testing undertaken in each NZTDS is made up of 2 components:
- core components – those specific tests that are always included, unless there is a specific decision to make a change
- add-ons or additional components – specific tests that are included every second or third survey or on a one-off basis.
In this way, trends can be monitored over time and emerging or specific issues identified or monitored.
The core components for the NZTDS are:
- 2 agricultural compound residues screens – a screen is a single test that looks for lots of compounds at the same time:
- A multi-residue screen for agricultural compound, such as herbicides, insecticides, fungicides, veterinary medicines used to control ectoparasites and plant growth regulators. This screen includes compounds that are no longer used, but which persist in the environment, such as DDT.
- A screen for Dithiocarbamate fungicides, which analyses for 8 compounds by testing for carbon disulphide (CS2).
- nutrient elements: iodine and selenium
- contaminant elements: arsenic, lead, cadmium and mercury.
Add-ons in recent studies have included sodium and additional analysis for iodine (both of which have been included several times), as well as iron and an acid-herbicide screen, which were included in 2003-04 NZTDS.
Reporting the results
While the NZTDS is under way, once the results of each sampling round have been confirmed, they are made public. MPI responds quickly to any unusual or expected results.
Previous results and documents relating to NZTDS are available in the Documents page in the left-hand menu or in the Food science programmes pages of the Food science & research section of the website. Refer to the link in the Related information panel at the right of the screen.