Fish has always played an important role in our diet. Great societies grew up at the intersection of land and water. Seas, rivers and lakes were inexhaustible sources of healthy food.

The current increase in contamination creates doubts and confusion about its consumption. Today you will know the Benefits main fish and you will learn to choose the best.

Fish benefits

Fish is one of the few foods on which there is consensus. Almost all experts agree on its benefits and recommend including it to a greater or lesser extent. Let’s do a quick review of its properties.

It is a concentrated source of nutrients

Although each species has its own nutritional profile, we can highlight general characteristics:

  • Minerals like iodine, selenium, and zinc, difficult to find in land animals.
  • Vitamins A and D, the latter especially in fatty fish such as salmon and herring.
  • Quality protein, easy to digest and highly bioavailable (detail, study).

Some fish have interesting nutritional characteristics. For example, him salmon pink color (and prawns) is due to its high content of astaxanthin, a powerful antioxidant.

Feed your brain

Fish has played an important role in our evolution. Some researchers claim that our advanced brains benefited from the rich fatty acids from fish, especially DHA (study, study).

Fish is associated with less cognitive decline and greater volume of gray matter (study, study), as well as lower rates of depression (study).

It is also important for the development of baby brain (review), hence its importance during pregnancy (detail).

Prevents multiple diseases

A higher consumption of fish is associated with a lower risk of general mortality (meta-analysis, meta-analysis) coronary disease (meta-analysis, meta-analysis, study), cerebrovascular disease (meta-analysis), cancer (study, study, study), macular degeneration (study, study) and diabetes (study).

In minors, higher fish consumption is associated with higher intelligence, even after controlling for socioeconomic status (study).

Higher fish consumption associated with higher IQ. Source: https://www.nature.com/articles/s41598-017-17520-w

In addition to the many observational studies that show correlation, there are clinical trials that demonstrate causality. In this study, eating hake daily (100g) for 8 weeks improved different parameters of metabolic syndrome (better lipid profile, lower waist and reduction in hypertension), compared to the group that followed a diet considered healthy but without fish.

The mechanisms of action are multiple, but reducing low-grade inflammation is undoubtedly one of the most relevant (study).

For all these reasons you should include enough fish in your diet (study), but the risk of toxicity is real if you don’t choose carefully.

Fish and mercury

Like any food produced in the modern world, fish contains undesirable substances, such as dioxins, herbicides, arsenic, and mercury.

Each species has different concentrations of these compounds, hence the importance of rotate fish, without abusing any in particular.

Your body can easily get rid of small amounts of many substances, but a high dose of a particular one can be a problem. Within these, the most worrying is mercury.

How Mercury Impacts Us

Mercury is a heavy metal, like iron and zinc, but unlike these, the body does not need it at all, and above a certain threshold it is toxic.

Fish accumulate methylmercury, more dangerous than inorganic mercury. The larger and the older the more mercury they will accumulate. When the big fish eats the small fish, it also incorporates its load of mercury.

By ingesting mercury our detoxification system tries to get rid of it, doing a good job with small amounts.

If the intake of methylmercury increases, it is gradually deposited in the kidneys and affects the cardiovascular system, but its most perverse effect is exerted on the brain (detail), when crossing the blood-brain barrier. It also easily crosses the placenta, potentially affecting the fetus, hence the special caution during pregnancy.

Maximum recommended amounts

According to the EFSA (European Food Safety Authority), the tolerable weekly methylmercury intake is 1.3 µg / kg (detail). This would correspond to 91 µg (0.091 mg) in a 70 kg person.

Some consider this limit excessively conservative, as it does not take into account the protective effect of selenium (detail), which binds to mercury reducing its toxicity (study, study).

Since fish is an excellent source of selenium, the mercury / selenium ratio should be considered when establishing tolerance limits (study, study).

Selenium and mercury in fish
Source: https://www.ncbi.nlm.nih.gov/pubmed/17916947

But ignoring the potential protective role of selenium, How much fish can we eat without exceeding the threshold defined by EFSA? The answer varies greatly depending on the type of fish.

Tuna “allowed” per week

Tuna has an average of 0.3 mg / Kg of mercury (listing, study). A large can of tuna contains about 80g of product, and therefore approximately 0.024 mg of mercury, and a small can 0.015 mg.

A person weighing 70 kg would therefore exceed the tolerable weekly levels with 4 large cans or 6 small cans.

tuna cans

Although I recommend prioritizing fresh fish, good preserves are a valid alternative, always checking the ingredients. There should be nothing but fish, olive oil and maybe a little salt. Interestingly, canned tuna usually has less mercury than tuna steaks (and therefore you could eat an extra can), probably because they use smaller fish.

How much salmon or sardines can you eat per week?

If we do a similar calculation with salmon or sardines, the result is very different. These fish have about 0.015 mg / kg of methylmercury (listed), and you would have to eat more than 6 Kg (or 75 cans) weekly to exceed thresholds.

Mercury levels also vary by species and fishing area, but the general message is that prioritizing the right fish makes it almost impossible for mercury to pose a real risk.

What fish to eat

The following guide considers the nutritional profile and mercury content of common fish. In summary:

  1. Prioritize the green column by far, being able to eat these fish daily. If you are pregnant it would be wise to limit yourself to this list.
  2. Moderate the consumption of the fish that appear in the orange column (2-3 times a week).
  3. Minimize consumption of the red column. You can do without them or eat them a couple of times a month if you have a predilection for one in particular.

Within each category, the lower the worse, although the differences are not always relevant.

Fish classification

I also include crustaceans and mollusks, recommended for their great contribution of nutrients and low levels of contamination. You can also consider criteria of sustainability (detail).

Preparation matters

As we saw, the scientific evidence clearly supports the benefits of fish, but the results of some studies showed inconsistencies (example).

When delving into these cases, the factor that causes the difference is detected: the way of preparation.

The fried fish does not have the same benefits (study, study). Grilled, oven or steam preparations are much more recommended. Not forgetting the stews or fish broths.

Frying can oxidize the oil (especially if you use vegetable oils like sunflower or corn) and also contribute to more AGEs (advanced glycation end products). If flour is used to coat it, even worse. The Fish & Chips are to the fish what the Big mac is a good sirloin.

Fish and chips are unhealthy
Battered and fried fish is not as healthy

Wild or Farmed

As a general rule, wild is better. The health of any animal (including homo sapiens) depends on its diet and its environment. Processed food and a crowded environment lead to less rich and dirtier fish.

That said, the differences are more or less significant depending on each species and type of farm.

Delving into the salmon, levels of mercury and other metals are similar (study), since they share the same seas.

Metals Wild Salmon Hatchery
Source: https://www.ncbi.nlm.nih.gov/pubmed/15378985

But the overcrowding and perhaps the type of food make farmed salmon have a higher concentration of other potentially toxic compounds (study). In general below the safe limits, but if all your salmon is farmed I would recommend reducing its consumption (<3 times / week).

Wild Salmon Fish Farm
Wild salmon vs Hatchery. Source: https://www.ncbi.nlm.nih.gov/pubmed/14716013

Conclution

Like any food (or exercise), fish has risks and benefits. To minimize the former and take advantage of the latter, it is advisable to prioritize the options with minimal mercury concentrations, leaving the rest for special occasions. Not eating fish is riskier.