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Sardines are an excellent source of protein. They're really cheap too. If you can't stand the taste, mackeral is great too, as are chicken breast fillets, and eggs and yoghurt. Greek yoghurt has up to 15g of protein per 100g, about 3 times that of regular yoghurt.

It is true that protein from meat and dairy (primary proteins) are more easily absorbed than that from beans, pulses and vegetables like spinach (secondary proteins).

Eggs, milk and meat... But the difference is not that high with other sources.