Consultant dietician Sophie Medlin has evaluated popular types of fish to reveal which species offer the best health benefits and which carry mercury risks. Speaking to the Daily Mail, the nutrition expert explained that while fish has long been recommended as a filling, low-fat source of protein, certain species contain elevated levels of toxins while others offer far higher nutritional value.
The National Health Service advises people to eat at least two portions of fish per week to obtain essential omega-3 fatty acids, which support heart and brain health. However, dieticians emphasize that species differ significantly in their fat profiles, calorie counts, and nutrient density, making selection critical for weight loss and overall health.
Some species, such as mackerel, supply exceptionally high amounts of vital nutrients that many people lack, including vitamin B12. Medlin, who is a consultant dietician and founder of City Dieticians, noted that nutritional data drawn from supermarket retailer tesco.com shows clear trade-offs between calorie intake, essential minerals, and potential contaminants across popular seafood choices.
Mackerel leads on omega-3 fatty acids
Mackerel emerged as one of the healthiest fish choices available to consumers. A fast-swimming oily fish native to UK and European coastlines, mackerel belongs to the same Scombridae biological family as tuna. A typical fillet contains 214 calories, 15.7 grams of protein, 16.8 grams of fat, 4.4 grams of saturated fat, and 0.15 grams of salt, earning it a verdict score of 4.5 out of 5.
The star quality of mackerel is its unmatched abundance of omega-3 fatty acids, containing more than any other fish and almost double the amount found in an average salmon fillet. A comprehensive Harvard University study published in the early Noughties demonstrated that consuming two mackerel fillets per week could reduce the risk of heart attack death by 36 per cent. Medlin pointed out that while about a third of its fat content is saturated fat, which can damage arteries, a fillet contains less than a quarter of a woman's daily limit and a fifth of a man's, making the benefits outweigh the risks for most people.
Cod offers lean protein for weight loss
For individuals seeking weight loss on a low-calorie diet, experts recommend cod baked or roasted rather than deep fried. Cod is a bottom-dwelling white fish with flaky meat that is often imported from overseas, predominantly China, Iceland, and Norway. A typical fillet delivers 112 calories, 25.1 grams of protein, 1.3 grams of fat, and 0.35 grams of salt, receiving a verdict rating of 4 out of 5.
"Cod is an excellent lean protein source," said Medlin, noting that it contains roughly twice the filling protein of mackerel to prevent post-dinner hunger. Cod also provides uniquely high levels of iodine, an essential mineral that fuels metabolism-controlling hormones, regulates heart rate, and supports critical brain and nervous system development during pregnancy and early childhood.
"Cod's main disadvantage is that it contains much less omega- 3," Medlin stated, adding that "most people aren't getting enough omega-3 in their diet." Data from the University of California Davis Department of Nutrition indicates that cod contains roughly ten times less omega-3 than an equivalent portion of salmon.
Seabass poses potential mercury toxicity risks
Seabass, native to the North East Atlantic Ocean, Mediterranean Sea, and Black Sea, as well as some freshwater rivers, presents a higher omega-3 profile than standard white fish. A typical fillet packs 149 calories, 17.9 grams of protein, 8.4 grams of fat (almost entirely unsaturated), and 0.11 grams of salt, scoring a 3 out of 5 verdict.
Medlin noted that seabass serves as a strong white fish option for protein and omega-3, stating, "You generally get more omega-3 here than you do with other white fish." However, frequent consumption carries health risks. "Seabass often contains more mercury than other fish such as salmon and cod," Medlin explained. Mercury enters aquatic ecosystems through coal burning, mining, industrial waste, and manufacturing, accumulating in small underwater organisms before building up in larger fish higher up the food chain.
Elevated exposure to mercury is toxic to human health and can cause severe impairments to vision, physical coordination, and memory. "Mercury levels in seabass are usually low enough that you could eat it, say, once every week," Medlin advised, warning against eating the species multiple times per week.
Salmon provides complete nutrient profile
Salmon achieved the highest rating among all evaluated fish, securing a perfect verdict score of 5 out of 5. As an oily fish containing 197 calories, 23.8 grams of protein, 11.3 grams of fat, and 0.12 grams of salt per typical fillet, salmon delivers rich omega-3 fatty acids that lower chronic disease risks and protect brain cells against age-related degeneration. Scientific studies highlight that these health benefits are only delivered when omega-3s are ingested via whole food rather than dietary supplements.
Describing salmon, Medlin said "salmon is an excellent all-rounder," adding: "It is high in omega-3, protein, vitamin D, B12, and selenium." Up to a third of older British adults suffer from vitamin B12 deficiency, which is crucial for healthy blood circulation and energy levels, while vitamin D deficiency remains widespread across the population. Vitamin D is essential for enabling bones to absorb strengthening calcium.
Although salmon contains approximately ten times the fat of fresh tuna, 80 per cent of its fat consists of healthier unsaturated types. While higher in calories than cod or seabass, Medlin emphasized: "But given the additional nutrients it supplies, it's worth the extra calories." Furthermore, salmon protein contains all nine essential amino acids required by the human body for muscle building and energy production.
Tuna and prawns carry specific dietary caveats
Fresh tuna steak, containing 144 calories, 31.9 grams of protein, 1.6 grams of fat, and 0.09 grams of salt per fillet, scored a 3 out of 5 verdict. A large, fast-swimming ocean fish from the mackerel family found across the Indian, Pacific, and Atlantic Oceans, tuna offers dense meat with a mild savoury taste cooked or rich buttery flavor raw. Medlin cautioned: "although a lean protein sauce that's rich in selenium and B12, mercury contamination is a concern."
Medlin added that "Larger tuna species like yellowfin and Atlantic bluefin contain particularly high levels, like so we need to be conscious of how much we consume regularly." Research shows that tinned tuna features significantly lower mercury levels than fresh cuts. "While fresh tuna contains some omega-3, it's far, far less than mackerel and salmon, although slightly more than you'll find in cod," Medlin explained.
King prawns, which are crustaceans rather than fish, recorded 51 calories, 11 grams of protein, 0.3 grams of fat, and 1.19 grams of salt per serving, earning a 3 out of 5 verdict. Found in both freshwater and seawater, prawns carry higher allergy risks, with shellfish allergies affecting between 0.5 per cent and 2.5 per cent of the population. Although low in calories and fat, prawns contain the highest salt concentration among all evaluated options.
While a single serving provides one-sixth of an adult's daily salt limit, totals accumulate rapidly because individuals often consume larger portions to feel satiated. Medlin observed: "But they do contain some omega-3, although much less than oily fish." Addressing cooking methods, Medlin warned: "The problem with prawns is they're often served breaded or fried, which counters some of the health benefits, so make sure they're steamed."
