A healthy diet should include at least two portions of fish a week, because fish and shellfish are good sources of many vitamins and minerals, says NHS Choices. But while it’s undoubtedly good for you, eating seafood is not without its risks. Maximum recommended amounts of oily fish, some types of white fish, and crab, have been set because of the pollutants they can contain. And while shellfish is low in fat and a good source of selenium, zinc, iodine and copper, if it is not fresh, has not been stored and prepared hygienically, or thoroughly cooked, it can cause food poisoning.
A Food Standards Agency (FSA) study found that over three-quarters of oysters from UK harvesting beds contained norovirus, although it was only at low levels in half of those affected. This risk from oysters is compounded by the fact that they are often eaten raw. As a result, the FSA advises that older people, pregnant women, very young children and people who are unwell should avoid eating raw or lightly cooked shellfish. It was exactly this source of contamination that forced Heston Blumenthal’s Fat Duck restaurant to close in 2009 after 500 people became ill with norovirus.
But shellfish can also be contaminated with biotoxins, which are largely heat stable and so aren’t broken down by cooking. The four types of shellfish poisoning, mainly associated with bivalve molluscs such as mussels, clams, oysters and scallops, are: amnesiac shellfish poisoning (ASP), diarrhoeal shellfish poisoning (DSP), neurotoxic shellfish poisoning (NSP) and paralytic shellfish poisoning (PSP). These can variously cause symptoms such as vomiting, diarrhoea, anaesthesia, memory loss, disorientation and breathing difficulties.
It is bivalves’ effectiveness as filter feeders that makes them likely to accumulate toxins. A single oyster, for example, can filter over 220 litres of water in a day. The toxins they accumulate are produced by algae in the water, such as dinoflagellates and diatoms, and by cyanobacteria.
If you still can’t resist them, there is some truth in the myth that oysters are only safe to eat during months with the letter ‘r’ in their names. In the Northern hemisphere, oysters are more likely to spoil in May, June, July and August. Their reputation as an aphrodisiac also has some scientific basis. Bivalves are rich in amino acids that trigger increased levels of sex hormones, and their zinc content aids testosterone production.
But for those who enjoy an element of risk with their seafood, Japanese fugu must be hard to beat. Pufferfish contains the neurotoxin tetrodotoxin and its consumption is responsible for an average of up to six deaths a year in Japan. In 1958 alone, 176 people died from eating fugu. Chefs must study for up to three years to earn a licence to prepare this dangerous dish.