All about protein: the nutrient vital for cell repair, muscle growth and the immune system

Amy Wood - Nutritionist | 06 Nov, 2022

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We place a lot of emphasis on the number of calories in our diets as this is central to our weight loss success, but we mustn't overlook the importance of the nutrients we need as part of a healthy and balanced diet – enter protein.

What is protein, and why is it important?

We can find protein in every cell in our bodies. Proteins are made up of chains of lots of smaller units called amino acids. In humans, there are 20 different types of amino acids. These amino acids join and fold together in a unique order to create each type of protein molecule, all of which are needed for various functions within the body. Protein forms the building blocks for all our tissues, playing a key role in the growth and repair of muscle, skin, bone and organ tissue, as well as the continued growth of our hair and nails. They also play an integral role in activating and suppressing most of our internal chemical reactions, as well as making and deploying hormones that further contribute to signalling around the body.

Of the 20 amino acids found in human proteins, our bodies can make 11 of them. However, we’re unable to produce the remaining nine, known as essential amino acids, so we have to source these through our diet. We’re also unable to store proteins the way we store sugar and fat, so it’s important that we eat protein every day.

Protein

Sources of protein

Protein can be found in most animal-based foods, including:

  • Red meat
  • Poultry
  • Fish
  • Eggs
  • Milk
  • Yoghurt
  • Whey protein supplements

Animal-based proteins are known as ‘complete proteins’, meaning they provide us with all nine essential amino acids.

Meat and dairy foods can also be high in saturated fat as well as protein, so opt for leaner cuts and lower-fat versions if you’re in a calorie deficit or would like to lower your cholesterol levels.

Plant proteins

The truth about plant-based protein

We can also find protein in some plant-based foods. Many people have reservations about eating a more plant-based diet due to fears that they won’t get enough protein, particularly if they're following a higher-protein diet. However, meeting your daily quota for protein through plant foods may actually be easier than you think, and can certainly provide us with all the protein we need if done correctly.

Unlike animal-based foods, plant proteins are typically ‘incomplete’ – they don’t provide us with sufficient quantities of all the essential amino acids. However, providing you mix it up and include different types of plant proteins, you can still achieve a full amino acid profile. For example, rice and grains are lacking in the amino acid lysine, however beans are full of it. The opposite is true of methionine, another amino acid – it’s pretty low in beans and pulses but is found in abundance in grains and seeds [1].

Some plant foods are an exception to this and do count as ‘complete’ protein sources, including:

  • Tofu
  • Tempeh
  • Edamame beans
  • Soy-based milk and yogurt alternatives
  • Soya protein supplements
  • Buckwheat
  • Quinoa
  • Quorn

So providing your plant-based diet doesn’t solely revolve around fruits, potatoes or ultra-processed foods, you’re highly unlikely to be protein-deficient. By making the right choices and eating a variety of plant proteins, you can still achieve a similar intake of dietary quality and balance to that of an omnivorous diet.

Can protein affect weight loss?

In short, yes! There are several reasons why protein intake can impact weight loss.

Protein is a complex molecule and it takes a lot of effort for the body to it break down. In fact, we use between 20 and 30% of the calories supplied by protein just to digest it! This is known as the thermic effect of food. It is worth noting the thermic effect of food only contributes to a very small part of our overall calorie burn, but might be worth bearing in mind if you’re choosing between a processed snack that’s low in protein, or a nutrient-rich handful of nuts with the same calorie content.

As it takes so long to digest, protein is a very satiating nutrient, meaning it keeps us feeling fuller for longer after we eat it. Protein stimulates the release of satiating hormones, which tell our brain we’re full up. So by eating more protein at mealtimes, we can reduce the temptation to reach for snacks, making it easier to stick to a calorie deficit which helps with weight loss.

Eating protein

How much protein should I be eating?

The Reference Nutrient Intake for protein is 0.75g per kilogram of body weight per day. This is the minimum daily intake we should be aiming for to maintain good health and keep all our bodily processes ticking over as they should be. For example, a person weighing 75kg would have a protein target of around 56g per day.

Looking at the recommended breakdown for a healthy balanced diet, it’s advised we get up to 50% of our calories from carbs, up to 35% of our calories from fat, leaving the remaining 15% to come from protein.

Note that the recommendations for carbs and fat are guideline amounts, so there is flexibility here. In the case of fat, this is a maximum recommended intake, so eating less than this is fine. And with carbs, there is more flexibility to eat a little more or less than 50%, without too much concern for health. So this does mean you could consume 45% of your calories from carbs, and 30% of your calories from fat for example, which would leave 25% for protein, and this is still considered a healthy split.

Can we eat too much protein?

While exceeding your protein target a little is fine and can be a good thing depending on your goals, it is possible to have too much.

Firstly, it’s crucial we aim for a balance of nutrients within our diets. By upping our protein intake, we often reduce our intake of other nutrients as a result, such as carbs and fat. Carbohydrates and fat are equally important parts of our diet, so lacking in these nutrients might have consequences for our health.

If we consume more protein than we need, our bodies will absorb the amount we require, and any excess is excreted in our urine. As our kidneys are responsible for filtering and removing excess protein from the blood, consuming too much protein long-term might have a negative impact on kidney function, especially in individuals with a higher risk of developing kidney problems [2]. On top of this, like all excess food, the energy from protein-rich foods can be stored as fat if we exceed our body’s calorie needs.

Although a definitive recommendation for an upper limit on protein intake is still under discussion, and different professional bodies put forward different figures, it’s probably sensible to aim for no more than around 1.8-2g per kilogram of body weight per day as an absolute maximum – to ensure a healthy balance of nutrients across your diet.

Exercise

Should I be eating a high-protein diet?

If you’re currently falling short on your protein, I would certainly recommend trying to incorporate more into your diet for overall health. Aim for a minimum of 15% of your calorie intake to come from protein, especially if you’re eating in a calorie deficit.

Whether or not you choose to eat a higher-protein diet does come down to personal preference, as well as your goals. The filling nature of protein makes it an attractive nutrient for making weight loss easier, plus studies continue to associate a link between higher-protein diets and successful weight loss [3].

Specifically, if you’re looking to gain muscle tissue (which also supports fat loss by increasing your metabolic rate), consuming adequate protein is particularly important. As we mentioned earlier, protein is the key ingredient our bodies need to build and repair muscle tissue. When we work out, the muscle fibres in our muscles form little microtears. Protein is harnessed to repair these and make the muscle stronger than it was before. This mechanism demonstrates why both protein intake and resistance training need to be combined in order to build and tone muscles.

Of course, you don’t need to be training hard in the gym to increase your protein intake. But there are a few considerations if you’re looking to try a high-protein diet:

  1. Make sure you aren’t cutting out other food groups, or sacrificing other nutritious foods, such as fruit, veg, wholegrain carbohydrates and healthy fats. It’s key to strike a balance to continue meeting all your body’s requirements.
  2. Ensure your protein intake isn’t too high. Eating too much of anything isn’t great for overall health. It’s probably best to aim for somewhere between 1-1.6g of protein per kg of your body weight per day depending on your goals.
  3. Spread your protein intake across the day. Our bodies can only absorb around 30-45g of protein in one sitting. So instead of loading up on chicken breasts and protein powders at one meal to meet your target, try to include a source of protein with each meal and snack in between to maximise absorption.

Nutritionist Amy Wood (ANutr), MSci BSc Nutrition has a keen interest in the relationship between diet and health. Having been published in the European Journal of Nutrition, Amy is passionate about making evidence-based nutrition accessible to everyone and helping others to adopt a food-focused approach to taking control of their health.

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