Tuna is a popular, protein-rich fish enjoyed across the UK, but concerns about mercury exposure mean safe consumption limits are important to understand. The UK Food Standards Agency (FSA) and NHS provide clear guidance on how much tuna you can safely eat each week, with specific recommendations for pregnant women, breastfeeding mothers, and children. Mercury levels vary between fresh tuna steaks and canned varieties, and between different tuna species. This article explains the recommended weekly tuna intake, mercury content in different types, NHS guidelines for vulnerable groups, and how to balance the nutritional benefits of tuna against potential risks.

Recommended Weekly Tuna Intake in the UK

The UK Food Standards Agency (FSA) and NHS provide clear guidance on safe tuna consumption to balance nutritional benefits against potential mercury exposure. For pregnant women, those trying to conceive, and breastfeeding mothers, the recommendation is to limit tuna intake to no more than four medium-sized cans per week (with a drained weight of approximately 140g per can) or two fresh tuna steaks per week (approximately 140g cooked weight each).

For other adults, there is no tuna-specific limit, though general fish consumption guidance still applies. The NHS recommends eating at least two portions of fish per week, including one portion of oily fish. It's worth noting that while fresh tuna counts as an oily fish, canned tuna does not due to the processing methods that reduce oil content.

Key points for safe consumption include:

• Spreading tuna meals throughout the week rather than consuming multiple portions in one day

• Varying fish choices to include other options such as salmon, mackerel, and sardines

• Being mindful that restaurant portions may exceed standard serving sizes

• Women of childbearing age should eat no more than two portions of oily fish per week, while other adults can have up to four portions weekly

• Certain fish with particularly high mercury levels (shark, swordfish, and marlin) should be avoided by pregnant women and children, and limited by others

For individuals who regularly consume tuna as part of their diet, maintaining awareness of weekly intake is sensible. Those with specific health conditions or concerns about mercury exposure should consult their GP or a registered dietitian for personalised advice tailored to their circumstances.

Mercury Levels in Different Types of Tuna

Mercury content in tuna varies depending on the species, size, and age of the fish. Understanding these differences can help inform dietary choices. Larger, longer-lived tuna species generally accumulate more mercury through a process called biomagnification, where mercury concentrations increase as it moves up the food chain.

Fresh tuna steaks, typically from larger species such as yellowfin or bigeye tuna, generally contain higher mercury levels than canned varieties. These fish can weigh hundreds of kilograms and live for many years, allowing greater mercury accumulation in their tissues. Bluefin tuna, often used in high-end restaurants, tends to contain higher mercury levels than many other commercially available tuna species.

Canned tuna predominantly comes from smaller species, primarily skipjack and occasionally yellowfin. Skipjack tuna, which comprises the majority of canned tuna in the UK market, generally contains lower mercury levels than larger tuna species. This is because skipjack are smaller fish with shorter lifespans, resulting in less mercury bioaccumulation. Albacore tuna falls between skipjack and fresh tuna steaks in terms of typical mercury content.

The processing method does not influence mercury levels. Canned tuna in spring water or brine does not reduce mercury content, as mercury binds to the protein in fish muscle rather than fat. However, the smaller species used for canning inherently tend to contain less mercury from the outset. When selecting tuna products, checking labels for species information can help consumers make informed choices. Opting for skipjack over albacore or fresh steaks from large species can help reduce mercury exposure whilst maintaining nutritional benefits.

It's important to note that the FSA and NHS limits apply regardless of tuna species, as they are designed to be protective for all types of tuna consumption.

NHS Guidelines for Pregnant Women and Children

The NHS provides specific, more restrictive guidance for vulnerable populations, particularly pregnant women, those trying to conceive, breastfeeding mothers, and young children. These groups require additional precautions because developing nervous systems are more susceptible to mercury's neurotoxic effects.

For pregnant and breastfeeding women, the NHS recommends limiting tuna consumption to no more than two tuna steaks per week (approximately 140g cooked weight each) or four medium-sized cans per week. This restriction aims to protect foetal brain development, as methylmercury can cross the placental barrier and accumulate in foetal tissues. Studies have shown that excessive mercury exposure during pregnancy may affect cognitive development, attention span, and motor skills in children.

Women planning pregnancy should also adhere to these limits, as mercury can remain in the body for several months. The FSA advises that reducing mercury exposure before conception provides additional protection during the critical early weeks of pregnancy when neural tube development occurs.

For children, the NHS advises parents to ensure their children eat at least two portions of fish a week, including one of oily fish. Children should avoid eating shark, swordfish and marlin due to higher mercury levels. For tuna specifically, children should follow portion sizes appropriate for their age and body weight.

Practical advice for families includes:

• Offering a variety of lower-mercury fish such as salmon, trout, and sardines

• Raw fish, including tuna, is safe during pregnancy if it has been frozen first and prepared hygienically (though raw shellfish should be avoided)

• Consulting midwives or health visitors if uncertain about appropriate portion sizes

• Remembering that these restrictions apply specifically to tuna and certain other high-mercury fish, not all fish species

Health Benefits and Risks of Regular Tuna Consumption

Tuna offers substantial nutritional benefits that make it a valuable component of a balanced diet when consumed within recommended limits. Tuna is an excellent source of high-quality protein, providing all essential amino acids necessary for tissue repair, immune function, and muscle maintenance. A typical 100g serving of tuna contains approximately 25-30g of protein whilst being relatively low in calories and saturated fat.

Fresh tuna is considered an oily fish and provides omega-3 fatty acids, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These essential fatty acids support cardiovascular health by reducing triglyceride levels, decreasing blood pressure, and potentially reducing the risk of coronary heart disease. Omega-3s also play crucial roles in brain function, eye health, and reducing inflammation throughout the body. Canned tuna, however, is not classified as an oily fish in UK dietary guidance due to the processing methods that reduce the oil content.

Tuna also contains important micronutrients including vitamin D, vitamin B12, selenium, and iodine. Vitamin D supports bone health and immune function, whilst B12 is essential for red blood cell formation and neurological function. Selenium acts as an antioxidant, and iodine is crucial for thyroid hormone production and metabolic regulation.

However, the primary risk associated with regular tuna consumption is mercury exposure. Methylmercury, the form found in fish, is a neurotoxin that can accumulate in the body with repeated exposure. Chronic high-level mercury exposure may cause neurological symptoms including tremors, memory problems, and sensory disturbances, though such effects are rare with dietary exposure alone when following NHS and FSA guidelines.

Balancing these benefits and risks requires adherence to recommended intake limits. For most adults, moderate tuna consumption within guidelines allows individuals to gain nutritional advantages whilst minimising mercury exposure. Those concerned about mercury levels should consider alternating tuna with lower-mercury fish options such as salmon, mackerel, herring, and sardines, which provide similar nutritional benefits with substantially less mercury content.

Safe Tuna Portions: Fresh vs Canned Varieties

Understanding portion sizes and the differences between fresh and canned tuna is essential for staying within safe consumption limits. Fresh tuna steaks are typically sold in portions ranging from 140g to 250g or more. A standard safe portion is considered to be approximately 140g cooked weight, which is roughly the size of a deck of cards or the palm of an adult hand. Restaurant servings often exceed this amount, sometimes providing 200-300g portions, which should be considered when calculating weekly intake.

Fresh tuna undergoes minimal processing and retains its full mercury content. When preparing fresh tuna at home, weighing portions before cooking helps ensure accuracy, as tuna loses some moisture during cooking but mercury concentration remains unchanged. Grilling, baking, or pan-searing are common preparation methods, none of which reduce mercury levels.

Canned tuna offers more standardised portion control. A medium-sized can typically contains about 140g of drained tuna, making it easier to track consumption. In the UK market, canned tuna is available in several formats:

• Skipjack tuna: generally has lower mercury content, suitable for regular consumption within guidelines

• Yellowfin tuna: intermediate mercury levels

• Albacore tuna: typically higher mercury content than skipjack, requiring more careful portion monitoring

• Tuna in spring water, brine, or oil: the liquid medium does not affect mercury content

Practical portion guidance:

• One medium can (approximately 140g drained weight) = one portion

• One fresh tuna steak (140g cooked) = one portion (though for pregnant/breastfeeding women, this counts as two portions in terms of weekly limits)

• Sushi-grade tuna portions vary widely; two to three pieces of nigiri typically equal approximately 40-60g

For families, dividing canned tuna among multiple people or using it as an ingredient in dishes like pasta or salads can help manage portions effectively. Reading product labels for net weight and species information enables informed choices. Remember that while fresh tuna counts as an oily fish portion in UK dietary guidance, canned tuna does not due to its lower oil content.

If you're pregnant and concerned about having exceeded the recommended limits, or if you experience any unusual symptoms after consuming large amounts of tuna, consult your GP for advice.

Frequently Asked Questions