What is the dark muscle in fish, and is it safe to eat

You bring home a fresh fish, ready to fry or curry, and notice a strip of darker flesh running near the spine or along the sides. It looks reddish brown or gray, very different from the flaky white meat you are used to. This is what is commonly called dark muscle in fish. But is it safe to eat, or does it indicate the fish has gone bad?

The answer is more nuanced than many think. Scientists studying fish muscle structure have found that dark muscle is a normal part of a fish’s anatomy. In fact, a study on lipid oxidation in dark and ordinary fish muscles found that dark muscle contains higher fat and oxygen-binding proteins. This makes it nutritionally rich but also more prone to spoilage compared to white flesh. Understanding these factors helps you enjoy fish safely without unnecessary waste.

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What is the dark muscle in fish
Dark muscle in fish refers to the darker-coloured sections of flesh, usually located near the backbone or close to the ribs. Unlike the white, flaky muscle most people are familiar with, dark muscle has:

• Higher myoglobin levels: Myoglobin stores oxygen, giving the muscle its darker colour.
• Higher fat content and mitochondria: This muscle powers continuous swimming for the fish.
• Distinct taste and texture: Some find it richer in flavour, while others find it stronger or more “fishy.”
• Dark muscle is a natural part of the fish and not a defect or contamination.

Why does dark muscle appear in the fish

Fish use different muscles for different movements. The lighter white muscle is for short bursts of speed, like escaping predators. Dark muscle supports steady long-distance swimming. Fish species such as tuna, mackerel, and salmon have more prominent dark muscle because they swim long distances, while smaller or less active species show less noticeable dark muscle.



Nutritional value of dark muscle in fish
Dark muscle in fish can be more nutritious than white flesh. Studies show that dark muscle contains:

• Higher omega-3 fatty acids, which support heart and brain health.
• Greater concentrations of vitamins and minerals compared to white muscle.
• Energy reserves due to higher fat content.
• Fresh, properly stored dark muscle is both safe and nutritious.

Is the dark muscle in fish safe to eat

Yes, dark muscle in fish is generally safe to eat. However, its higher fat content makes it more prone to spoilage. Research shows that dark muscle is the first part to undergo lipid oxidation, which can lead to off-flavours, rancid smells, and nutrient degradation.

If the fish is fresh and properly stored, dark muscle is safe. Poor storage or prolonged time increases the chance of spoilage in dark muscle before the rest of the flesh.



Signs of dark muscle in fish may not be safe
Watch for the following warning signs:

• Strong unpleasant odours such as sour, ammonia, or overly fishy smell.
• Slimy texture around the dark flesh.
• Colour changes beyond normal dark brown, such as greenish or gray patches.
• Extended storage without proper refrigeration.
• If any of these appear, trim the dark muscle or discard the fish.

How to safely consume fish with dark muscle

• Buy fresh fish from trusted sources with proper cold storage.
• Check freshness through smell and texture before cooking.
• Trim dark muscle if it looks questionable.
• Cook thoroughly to reduce microbial risks, with an internal temperature of at least 63 °C (145 °F).
• Consume fish quickly within 1–2 days or freeze promptly for longer storage.

Dark muscle in fish is a normal and natural part of fish anatomy. It is richer in fats and nutrients but spoils faster than white flesh. Fresh, well-stored fish with dark muscle is safe to eat and can even be more nutritious.

The next time you see a dark strip in your fillet, check its freshness, cook it well, and enjoy the health benefits. Always trust your senses: if it looks off, smells odd, or feels slimy, discard it to stay safe.



Disclaimer: This article is for general informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider regarding any medical condition or lifestyle change.




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