What Is Protein? Types, Functions, Amino Acids and Best Food Sources
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Last updated: 3 October 2026 · By D8 Nutrition, UAE
Quick answer: Protein is a macronutrient made of chains of amino acids. Your body uses 20 amino acids to build its proteins, and nine of them are essential, meaning they must come from food. Protein builds and repairs muscle, skin and other tissues, and makes the enzymes, hormones and antibodies that keep you alive. It provides about 4 kcal per gram.
Key takeaways
- Protein is one of the three macronutrients, alongside carbohydrate and fat.
- Proteins are made of 20 amino acids: 9 essential (from food) and 11 non-essential (made by your body).
- A complete protein contains adequate amounts of all nine essential amino acids. Eggs, meat, fish, dairy and soy are complete proteins.
- Most plant proteins are incomplete on their own, but a varied diet covers the gaps across the day.
- Collagen is a protein, but it is not a complete protein.
- The RDA for protein is 0.8g per kg of body weight per day, which is a minimum and not an ideal target.
What is protein?
A protein is a large molecule built from smaller units called amino acids, joined together in a long chain. Think of amino acids as letters and a protein as a word or a sentence: the same small alphabet, arranged in different orders, produces thousands of different results.
Your body uses 20 different amino acids to build its proteins, and a single protein can contain hundreds of them. Once the chain is built, it folds into a specific three-dimensional shape, and that shape decides what job the protein can do.
In food, protein is one of the three macronutrients, together with carbohydrate and fat. Each gram of protein provides about 4 kcal.
What does protein do in the body?
Protein is in every cell you have. Building muscle is the best-known function of protein, but it is only one of several:
| Type of protein | Function | Example |
|---|---|---|
| Structural proteins | Give shape and strength to cells and tissues | Collagen (skin, tendons, cartilage), keratin (hair, nails) |
| Enzymes | Speed up chemical reactions, including digestion | Pepsin (digests protein), lipase (digests fat) |
| Transport proteins | Carry substances around the body and across cell walls | Haemoglobin, which carries oxygen in your blood |
| Defence proteins | Recognise and neutralise bacteria and viruses | Antibodies |
| Messenger proteins | Carry signals between cells and organs | Some hormones, such as insulin |
| Motor proteins | Create movement | The proteins that make your muscles contract |
So when you eat protein, you are not only feeding your muscles. You are supplying the raw material for all of the above.
What are the 9 essential amino acids?
Of the 20 amino acids, your body can make 11 on its own. These are called non-essential amino acids: you still need them, you just don't have to eat them.
The other nine are essential amino acids. Your body cannot make them, so they must come from food. The nine essential amino acids are:
- Histidine
- Isoleucine
- Leucine
- Lysine
- Methionine
- Phenylalanine
- Threonine
- Tryptophan
- Valine
What is the difference between complete and incomplete protein?
A complete protein contains adequate amounts of all nine essential amino acids. An incomplete protein is low in one or more of them.
- Complete protein foods: eggs, chicken, fish, meat, milk, curd, paneer, and whole soy foods such as tofu.
- Incomplete protein foods: dal and other legumes, nuts, seeds, whole grains and vegetables.
"Incomplete" does not mean "bad." Different plant foods are low in different amino acids, so they cover each other's gaps. Dal with rice or roti, or hummus with bread, are everyday combinations that do exactly this.
You also don't need to combine them in the same meal. What matters is a balance of essential amino acids across the whole day.
What are the main types of protein in food, powders and protein bars?
If you read the ingredient list on a protein bar or protein powder, these are the protein sources you are most likely to see:
| Protein type | Source | Complete protein? | Good to know |
|---|---|---|---|
| Whey protein | Milk | Yes | Digests quickly |
| Casein | Milk | Yes | Digests slowly |
| Milk (dairy) protein | Milk | Yes | Contains both casein and whey |
| Soy protein | Soybeans | Yes | A complete plant protein |
| Pea protein | Yellow peas | Low in methionine | Often blended with other plant proteins |
| Collagen peptides | Animal connective tissue | No, lacks tryptophan | A structural protein for skin and joints |
Is collagen a complete protein?
No. Collagen is not a complete protein, because it lacks the essential amino acid tryptophan. Collagen is a structural protein: your body uses it to build skin, cartilage, tendons and connective tissue.
That doesn't make collagen useless. It means collagen works best alongside a complete protein rather than as your only protein source. We look at the research in collagen in protein bars: what research says about skin and joint health.
Which foods are high in protein?
Approximate protein content of common everyday foods in the UAE:
| Food | Serving | Protein |
|---|---|---|
| Chicken breast (cooked) | 100g | ~31g |
| D8 protein bar | 1 bar (60g) | 24g |
| Paneer | 100g | 14–18g |
| Milk | 1 glass (250ml) | ~8g |
| Egg (boiled) | 1 large | ~6g |
| Toor dal (cooked) | 1 katori | 4–9g |
Figures for everyday foods are approximate and vary by brand, preparation and portion size. For a full day worked out meal by meal, see how much protein you're already getting from normal food.
How much protein do you need per day?
The Recommended Dietary Allowance (RDA) for protein is 0.8g per kg of body weight per day. For a 70kg adult, that is 56g of protein a day. The RDA is the minimum needed to prevent deficiency, not an ideal target.
- Active people: 1.4–2.0g per kg per day (ISSN position stand)
- Adults aged 65+: 1.0–1.2g per kg per day (PROT-AGE guidance)
Our full guide explains how to calculate your own number: how much protein do you need per day?
What happens if you don't get enough protein?
According to Cleveland Clinic, symptoms of protein deficiency can include muscle loss, swelling (oedema), a weakened immune system, skin and hair changes, and anaemia. True protein deficiency is uncommon in people who eat enough food overall, and is mostly linked to low total food intake or a chronic health condition.
Falling short of an optimal intake is far more common than true deficiency, especially on rushed days with skipped meals.
How does your body digest protein?
Your body does not use the protein in food as it arrives. Digestion breaks protein down into individual amino acids, which are absorbed and then reassembled into whichever proteins your body needs. Unlike fat and carbohydrate, your body has no dedicated store for protein, which is one reason a steady intake across the day is more useful than one very large serving.
More detail here: how your body actually absorbs protein.
Is a protein bar a good source of protein?
It can be. A protein bar is a good source of protein when it delivers a meaningful amount of complete protein without a lot of added sugar. Three things to check on the label:
- Protein per bar and per 100g. See how to compare protein density.
- The protein source. Look for a complete protein such as dairy or soy protein in the blend.
- Added sugar. See our guide to no added sugar protein bars in the UAE.
Where D8 fits
A D8 protein bar contains 24g of protein in a 60g bar: 18g of dairy protein, which is a complete protein, and 6g of collagen peptides. The dairy protein supplies all nine essential amino acids, and the collagen sits alongside it rather than replacing it. D8 bars have no added sugar, are gluten-free, and are Halal-certified.
It's a convenient way to top up a day that falls short. It isn't a meal replacement, and it doesn't replace a varied diet.
Shop D8 Double Chocolate & Peanut Salted Caramel →
Frequently asked questions about protein
Is protein a macronutrient?
Yes. Protein is one of the three macronutrients, along with carbohydrate and fat.
How many calories are in a gram of protein?
One gram of protein provides about 4 kcal.
How many amino acids are there?
Your body uses 20 amino acids to build proteins. Nine are essential and must come from food; your body makes the other 11.
Is plant protein as good as animal protein?
It can be, if you eat a variety. Most single plant proteins are incomplete, but a mixed diet of legumes, grains, nuts and seeds provides all nine essential amino acids across the day. Soy is a complete plant protein.
Is protein only important if you go to the gym?
No. Everyone needs protein for tissue repair, enzymes, hormones and immunity. Exercise increases how much protein you need; it doesn't create the need.
Can you eat too much protein?
More is not always better. Beyond what your body needs, extra protein is simply extra calories, and people with kidney disease may need to limit their intake. Ask your doctor if you are unsure.
Sources
- Cleveland Clinic: What Are Proteins? Definition, Types & Examples
- Cleveland Clinic: Complete vs. Incomplete Proteins and Examples
- Cleveland Clinic: Symptoms of Protein Deficiency
This article shares general nutrition information and isn't medical advice. If you have kidney disease, are pregnant, or have another condition affecting protein needs, consult your doctor or a registered dietitian before changing your intake.