Undesirable substances and tolerable intake

Chemical pollutants in relation to seafood safety have received increased attention in the last decade. Pollutants can occur naturally (such as algal toxins and metals) or be a result of human activity such as industrial discharges of metals and organic contaminants. Certain pollutants such as methyl mercury can bioaccumulate in organisms and biomagnify up the food chain, consequently large predatory fish may contain considerably higher mercury levels than small fish lower in the food chain. Organic pollutants which are lipid soluble (such as dioxin) are stored in fatty tissue and are therefore mainly found in oily fish and in lipid-rich tissue such as fish liver.

Risk assessment of food

Toxicological studies and epidemiology form the basis for assessing the risk associated with consuming food containing various additives and contaminants. Risk assessments should answer the following questions: How much of the substance in question can humans tolerate in the course of a lifetime? How much of the substance is found in our diet? How much of each foodstuff is eaten? Risk assessments are conducted internationally by expert committees, such as the Joint FAO/WHO Expert Committee on Food Additives and Contaminants (JECFA) and the European Food Safety Authority (EFSA) and result in the establishment of acceptable daily intakes (ADIs) for food additives and tolerable weekly intakes (TWIs) for contaminants.

ADI - for substances used in food production

The Acceptable Daily Intake (ADI) is the amount of a substance that a person can eat every day throughout their life without posing a health risk. ADIs are established by risk assessment bodies (e.g. EFSA and JECFA) for authorised food and are usually given in mg per kg of body weight.

TWI - for undesirable substances that are present in food

The Tolerable Weekly Intake (TWI) is the amount of a substance that a person can eat every week throughout their life without posing a health risk. It is not “acceptable” that our food contains pollutants, it must however be “tolerated” since low levels of pollutants are present in our food. TWIs are established for undesirable substances that are present in food. If there is a lack of toxicological data available for a given substance, a PTWI is established which stands for ”Provisional Tolerable Weekly Intake”. The ”provisional” indicates that the assessment will be reassessed when more data are available.

How much can we tolerate?

In order to determine an acceptable or tolerable intake of a substance, the no observable adverse effect level (NOAEL) is established, usually using animal trials. To establish the ADI or TWI, i.e. amount of a substance that can be safely eaten by humans the NOAEL is divided by a safety factor which, for example, may be 100 to account for variability among individuals, and variability in sensitivity between species. The intake of a substance that exceeds the TWI for a short period does not pose a health risk but reduces the safety margin during this period.

A comprehensive assessment of fish and other seafood in the Norwegian diet

The Norwegian Scientific Committee for Food Safety (VKM) has assessed the nutritional benefits to be gained by eating fish and other seafood compared with the health risk associated with the intake of contaminants that may be present in these food products. The results of this assessment are available in the report ”A comprehensive assessment of fish and other seafood in the Norwegian diet” (2006). VKM  concludes that the health benefits of eating fish are considerable and that Norwegians should eat more fish and seafood and that the type should be varied. Pregnant women are also encouraged to eat more fish particularly since marine omega-3 fatty acids are important for brain development, however they should follow consumption advisories for certain species of fish and fish liver to protect the foetus.

Monitoring of contaminants in fish

NIFES conducts surveillance programmes for the National Food Safety Authority to ensure that the level of contaminants in fish and other seafood are below legal maximum limits. At http://www.nifes.no you can search in the database ”Seafood Data” to find information about the presence of a number of contaminants and also nutrients in different feed resources, species of fish and other seafood.