Fish Intake, Mercury, and Your Health

Today (November 2nd) is World Vitamin D Day, so allow me to be the first to wish you a happy one!

From my perspective as a dietitian, vitamin D is equal parts elusive and intriguing.

Elusive, because fish is essentially the only significant dietary source available to us.

Intriguing, because vitamin D is increasingly associated with so many aspects of human health.

In fact, close to 20% of Americans have inadequate circulating levels of vitamin D in their system with dietary intake and supplement use being the key determinants of their status. 

So what happens when a unique vitamin meets a unique food group?

Today’s article happens!

Let’s first explore what I mean when I say fish is a “unique” food group.

Here are a few reasons why:

1. Fish, particularly certain varieties, are among the only rich dietary sources of vitamin D.
2. Fish, particularly certain varieties, are among the only rich dietary sources of the long-chain omega-3 fatty acids DHA/EPA.
3. Fish, particularly certain varieties, contain varying amounts of mercury which may be a cause of attention from a public health perspective.

The key theme here?

It depends on the variety.

The goal of today’s article is to help you better understand the unique nutritional value of fish and how to navigate potential concerns around mercury to land on the best consumption strategy possible for your health – assuming you do indeed eat fish as part of your routine.

Fish Intake & Vitamin D Status

Given the unique nutritional composition of fish, the FDA advises Americans to consume at least 8 oz per week.

This makes sense on a number of levels, especially considering that studies have demonstrated fish intake to be an effective way to improve one’s vitamin D status.

With some caveats of course:

• Fatty fish increase circulating vitamin D levels by about 3x as much as lean fish
• Longer term fish consumption (6 months vs 6 weeks) leads to a 2x greater increase in D levels
• Fish consumption at recommended levels may be inadequate to optimize vitamin D status

These caveats help explain why many of the top-selling supplements on Amazon.com are indeed vitamin D-featuring products, but also leave us more questions to answer.

Which Fish Are “Fatty” Fish?

Common fatty fish varieties include various types of salmon, sardines, trout, herring, and mackerel – among others.

While not necessarily a revelatory point, it is worth noting for those to whom it is readily available that wild-caught salmon tends to have up to 4x the vitamin D content of farmed. 

These selections are termed “fatty” because they contain moderate to large amounts of dietary fat including in the form of the unique and beneficial omega-3 fatty acids EPA/DHA which are not found in other food sources.

But omega-3 fatty acids aren’t the only unique component found in fish, unfortunately, we also have mercury to contend with.

Fish & Mercury – Risk Vs Benefit

It has been well established that fish and seafood contain varying levels of methylmercury which is of no benefit to human health and certainly at higher levels potentially harmful – especially among certain demographics such as pregnant women.

According to FDA data many of the fish highest in omega-3 and vitamin D content is also among the lowest in mercury as well.

These include various types of salmon, sardines, and Atlantic mackerel.

Whereas, on the other end of the spectrum, species such as swordfish, shark, and various types of tuna ( such as bigeye, albacore, and yellowfin) tend to be higher in mercury content.

So we can see a clear trend here where the fish with the most useful characteristics (high vit D, omega-3) also tend to be lower in mercury content.

These realities explain in large part why major journal papers, including JAMA, clearly conclude that the benefit of fish intake for human health outweighs the risk.

This is particularly true of strategic fish intake emphasizing lower mercury and higher vitamin D/omega-3 varieties, as discussed.

As per a recently conducted umbrella review on fish intake and health outcomes out of Advances in Nutrition, some of the broader health benefits of higher fish intake may include a reduced risk of cardiovascular disease, depression, and all-cause mortality.

It is certainly an added bonus that, as compared to farm beef, the environmental impact of aquaculture (ie: farmed fish) is far less extensive. 

Some food for thought.

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