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Food · Animal Foods · Fish
food source · 3
EPA and DHA are first made by marine microalgae, and sardines feed on plankton, taking them close to the source. Canned sardines have about 1.4–1.5 g of EPA and DHA per 100 g, among the highest of common foods.
abdelhamid-2020-cochrane-omega3Mechanism · At the source of the food chain
EPA and DHA are first made by marine microalgae. Sardines feed on plankton all their lives and take these fats straight from the source, instead of through several links of the food chain as large predatory fish do; they are also small and short-lived, so they build up little mercury.
Evidence · A can of fish and a few capsules
Gram for gram, canned sardines have about 1.4–1.5 g of EPA and DHA per 100 g, so one can roughly matches several standard fish-oil capsules and also brings protein, vitamin D, calcium and B12. That does not mean the evidence for eating fish is stronger than for capsules: people who eat more fatty fish have lower cardiovascular risk, but that is an association from observational studies, while randomized trials of fish-oil capsules in the general population have mostly been negative. The two kinds of study answer different questions.
abdelhamid-2020-cochrane-omega3Canned sardines come with soft, edible bones and have about 300–380 mg of calcium per 100 g, close to a glass of milk, which makes them a rare non-dairy calcium source for people who do not drink milk.
Mechanism · Chew it, then let acid dissolve it
The calcium in soft bones is locked in a crystal called hydroxyapatite. It has to be chewed apart and then soaked in stomach acid before the crystal surface releases calcium ions that can be absorbed; swallowed whole, too little surface is exposed to the acid. The active route that carries calcium across the gut wall is controlled by vitamin D, and sardines carry vitamin D themselves.
Numbers · How much calcium a can holds
Canned sardines have about 300–380 mg of calcium per 100 g, close to a glass of milk. The can is already salty, so do not add much more salt when you serve them.
Oily fish is one of the few natural food sources of vitamin D. Sardines eaten with their bones put calcium and the vitamin D that helps it into the blood in the same bite.
Mechanism · Vitamin D opens the calcium route
Vitamin D from food is processed first in the liver and then in the kidney before it becomes active. The active form makes the lining cells of the duodenum build more calcium channels, calcium-binding protein and calcium pumps, opening the route that carries calcium into the blood. Without it, most of the bone calcium you chew never gets in.
In practice · Calcium and vitamin D in one bite
Most foods supply one end of this chain: natural milk mainly supplies calcium, and sunlight supplies vitamin D. A can of bone-in sardines puts both into the same bite. Canned sardines also have about 1.4–1.5 g of EPA and DHA per 100 g, which is their other side as an omega-3 source.