Four potential hazards
There are four potential hazards associated with eating raw fish, particularly when you're pregnant. These include:
1) Contamination with mercury
Larger game fish like tuna may be contaminated with mercury. The general recommendation of the UK Food Standards Agency for pregnant and breastfeeding women is that they should eat no more than "two tuna steaks (weighing about 170g raw) per week."
It is not likely that a serving of sushi featuring tuna would weigh 170g, so if you eat raw tuna as part of a sushi meal during pregnancy, this should not be a problem – provided you do not have it more than about once or twice a month to be on the safe side.
2) Contamination with dioxins and PCBs
The UK Food Standards Agency has also recommended that pregnant and breastfeeding women should avoid having more than two portions of salmon per week to avoid contamination with dioxins and PCBs (polychlorinated biphenyls), which are compounds that are harmful to unborn babies.
It would, therefore, be prudent not to eat salmon sushi more than once or twice a month. Rather safe than sorry!
3) Infestation with worms
Eating raw fish is also associated with the risk of worm infestation. Fish can be infested with fish tape worms (Diphyllobothriasis latum), which may pose a health risk, particularly during pregnancy.
However, if fish is frozen, the freezing process will kill the worms. Always ask the staff of your sushi restaurant if the fish they are using has been thoroughly frozen (not just preserved on ice!) at -20 degrees Celsius for at least 24 hours.
If they are using freshly caught fish, which has not been frozen, don't eat it if you're pregnant or breastfeeding. In fact, it may be a good idea for all sushi lovers, pregnant or not, to check on this and to avoid eating freshly caught raw fish.
4) Anti-thiamine enzymes
Thiamine or vitamin B1 is one of the essential B vitamins we need for health and normal growth and development. Some foods, particularly raw fish and shellfish, contain so-called thiaminases, or thiamine-degrading enzymes.
Having one or two servings of raw fish a month during pregnancy will not deplete your thiamine status, but if you start having sushi regularly it is possible that this may affect your total thiamine intake and lead to a thiamine deficiency.
Once again, it is evident that moderate consumption of raw fish in the form of sushi once or twice a month, will not be harmful to your thiamine intake or that of your baby, but repeated ingestion of raw fish could pose a problem.
Other forms of raw fish
Sushi is not the only way in which fish is served raw. Raw herring and raw oysters are also popular. It would be a good idea for pregnant and breastfeeding women to take the above-mentioned precautions when eating these foods.
The bottom line
During pregnancy and breastfeeding, you need to limit your intake of raw fish and seafood. You also need to make sure that you buy fish at a good restaurant that uses frozen supplies.
Eating sushi once or twice a month should be safe, provided the sushi is made with fish that has been frozen at -20 degrees Celsius for at least 24 hours.
If you are pregnant or breastfeeding, you need to eat fish to ensure a good omega-3 intake, but you also need to regulate how often you eat raw tuna, salmon or seafood.
By taking the necessary precautions, you can ensure that your unborn baby is not exposed to unacceptable levels of mercury, dioxins, PCBs, or fish tape worms, and that you don't deplete your thiamine reserves.
- (Dr I.V. van Heerden, registered dietician)
(Arbor Clinical Nutrition Updates, 2007. Mercury, fish and health. Issue 274, May. www.eatwell.gov.uk/asksam /healthydiet/fishandshellfishq/)