Protein is made up of 20 amino acids; some of these the body can produce itself, but the rest have to come from our food. All meats (from sea or land), as well as dairy foods and eggs contain these essential amino acids, but only two plant foods do. The super-grain quinoa and soy protein are the only vegetarian foods which provide all you'll ever need. So, if you're not a fan of these it's really important you eat a variety of nuts, legumes and grains every day to ensure you're keeping your protein intake at a good level.
When planning your daily intake consider the following levels; 1 cup cooked quinoa 18g, 1 cup tofu 20g, a cup soy milk 7g, 2 Tbs peanut butter 8g, and 100g tempeh (sadly underrated, this fermented soy based "cake" is delicious) contains 18g. An egg gives you 50g. Or 3/4 cup dried, cooked beans about 135g.
As to what you can have at the Sunday roast when your mates are tucking into roast lamb, I'd say you do actually need to be quite imaginative. Serve hummus (high amounts of protein rich chickpeas) on toasted wholegrain (the grains contain protein) as a starter. Make a side salad from roast carrots, tarragon, walnuts and quinoa tossed together, and serve steamed brown rice and barley with the roast as well. A thick slice of pan-fried Zany Zeus halloumi (dairy is high in protein) with pesto (pinenuts and parmesan have protein) would also be a good replacement for the meat.
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