Health News: 27.Feb.03

Mercury in the fish causes concern

Last week the Food Standards Agency advised pregnant women, and even women who intend to become pregnant, to cut swordfish, shark and marlin from their diet and to reduce their consumption of tuna to no more than two medium- sized tins or one tuna steak a week. This advice about tuna does not apply to older children or adults, but all infants and children under 16 are still advised to avoid eating shark, swordfish or marlin.

The poisonous nature of mercury has been recognised since the days of Hippocrates. He was the first doctor to write about the colic that it can cause, and Pliny wrote about its effect on the health of slaves who worked with this metal. In the past, chemists, laboratory technicians, dentists and farmers — who used mercury compounds for seed dressings — suffered outbreaks of mercury poisoning.

In 1953 the first account of poisoning by eating fish which had lived in water contaminated by mercury effluent, was reported from fishermen around Minamata Bay in Japan. In recent years mercury pollution of the sea and rivers and the subsequent contamination of fish has been a constant concern to public health physicians who are anxious about its potential effects.

The greatest concern is for the unborn child and breast-fed babies — in both cases excessive quantities of mercury can damage the developing brain and central nervous system of the child. In adult life patients who have absorbed mercury or inhaled mercury vapour lose their appetite, are emotionally unstable, sleepless, poorly co-ordinated and shaky, so that their handwriting makes that of the most dyspractic child look neat. As well as the tremor, they develop gastrointestinal symptoms, sore gums and dribble excessively. Older children become irritable and develop cold, pink and swollen extremities.

The problem with eating predatory fish is that when they devour smaller fish they concentrate the mercury derived from them in their own flesh. No harm will befall anyone eating the meat of stewed hare, but a swordfish steak may have levels of mercury five to seven times higher than is found in tinned tuna.

Tuna is particularly rich in the fish oils which are important for the optimum development of the developing foetus or baby’s nervous system, so that the withdrawal of such a ready source needs to be made up elsewhere.

A survey in 1998 carried out by the then Ministry of Agriculture, Fisheries and Food revealed that herring and mackerel, and less oily fish such as cod and haddock, showed low levels of mercury which did not exceed a tolerable intake.

There is a good case for drinking milk in which the formula includes additional long-chain polyunsaturated fatty acids (LCPs), better known to most people as omega 3 and omega 6 fatty acids.

Pregnancy tends to deplete a woman’s level of these essential fatty acids just when her developing baby’s nervous system is developing fast and it needs a high level

(1). Breast-feeding also lowers a mother’s level of omega 3 and omega 6 and it may lead to postparum depression

(2). Evidence has been growing that women who have low levels of LCPs tend to be more likely to have preterm delivery

(3) and to have children who have smaller heads and a slower intellectual development

(4). Likewise, some infants who do not receive enough omega 3 and omega 6 in early life do not develop a mature eyesight as quickly as their contemporaries. When older, it has been shown that these babies are not as fast at solving the problems that psychologists set for toddlers and small children.

In a well-nourished woman, breast milk should give the child enough of the LCP fatty acids, but in those who are not being breast-fed it is as well to use an artificial milk such as Milupa to which they have been added. Women who doubt the quality of their nourishment might be advised to take additional fish oils to supplement their diet.

The Times Comments

Literature

1. Oatridge A, Holdcroft A, Saeed N et al. change in brain size during and after pregnancy: study in healthy women and women with preeclampsia. Am J Neuroradiol 2002; 23: 19-26. (Medline)

2. Hibbeln JR. Seafood consumption, the DHA content of mothers' milk and prevalence rates of postpartum depression: a cross-national, ecological analysis. J Affect Disord. 2002 May;69(1-3):15-29. (Medline)

3. Olsen SF, Secher NJ. Low consumption of seafood in early pregnancy as a risk factor for preterm delivery: prospective cohort study. British Medical Journal 2002 Feb 23;324(7335):447. (Medline)

4. Bhutta AT, Cleves MA, Casey PH et al. Cognitive and Behavioral Outcomes of School-Aged Children Who Were Born Preterm: A Meta-analysis. JAMA. 2002 Aug 14;288(6):728-37. (Medline)