Macros explained: protein, carbs and fat

What are macronutrients?

Macronutrients are the nutrients your body needs in relatively large amounts to function: protein, carbohydrate and fat. Unlike micronutrients (vitamins and minerals), macros provide calories—the energy your body uses for everything from breathing to exercising.

Each gram of macronutrient provides a fixed number of calories:

• Protein: 4 calories per gram
• Carbohydrate: 4 calories per gram
• Fat: 9 calories per gram

This is why fat is more calorie-dense than protein or carbs, gram for gram. Alcohol, sometimes considered alongside macros, provides 7 calories per gram but isn't a nutrient the body needs.

What each macro does

Protein is made of amino acids, which your body uses to build and repair muscle, skin, enzymes and hormones. It also tends to be the most filling macro, which is one reason it's often given priority when setting targets, particularly for people trying to manage their weight or maintain muscle.

Carbohydrate is your body's preferred quick energy source, particularly for the brain and during moderate-to-high intensity exercise. Carbs include starches, sugars and fibre, found in foods like grains, fruit, vegetables and legumes.

Fat supports hormone production, helps absorb fat-soluble vitamins (A, D, E and K), and provides long-lasting energy. It's also essential for cell structure. Despite being more calorie-dense, fat shouldn't be feared or minimised unnecessarily—it's a required nutrient, not an optional extra.

Why protein often gets priority

When people set macro targets, protein is usually calculated first, with carbs and fat filling in the remaining calories. There are a few reasons for this:

• Protein supports the maintenance (and, with resistance training, growth) of muscle mass, which becomes increasingly important as we age
• It has a higher satiety value than carbs or fat, which can help manage hunger, particularly during periods of reduced calorie intake
• Many people—especially those cutting calories, following plant-based diets, or older adults—may not naturally eat enough protein without a specific target

A useful starting point for many active adults is around 1.6–2.2g of protein per kilogram of body weight, though needs vary by goal, age and activity. A protein calculator can help you find a more personalised figure based on your details.

Setting a macro split by goal

There's no universally 'correct' macro split—the right balance depends on your goals, training, preferences and how different ratios make you feel. That said, here are some general starting patterns people often use:

• General health/maintenance: a balanced spread, often around 30% protein, 40% carbs, 30% fat, adjusted to preference
• Muscle building: higher protein (around 25-30%) with adequate carbs to fuel training
• Fat loss: protein is often kept relatively high to preserve muscle and support fullness, with carbs and fat adjusted to create a calorie deficit
• Endurance training: higher carbohydrate intake to support glycogen stores and performance

These are starting points, not rules. A macro calculator can generate a tailored split based on your goals, activity level and body stats, which you can then adjust based on real-world experience.

Don't forget fibre

Fibre is a type of carbohydrate that your body can't fully digest. It doesn't usually get its own macro target, but it's worth paying attention to separately. Adequate fibre intake—generally around 25-30g per day for most adults in the UK—supports digestive health, helps you feel full, and is linked to a lower risk of several chronic conditions.

When tracking carbs, it can help to note fibre alongside total carbohydrate, since fibre-rich carbs (vegetables, wholegrains, legumes, fruit) tend to be more filling and nutritious than refined, low-fibre alternatives.

How to track your macros

Tracking macros simply means logging the protein, carbs and fat in the foods you eat, usually via a food tracking app, and comparing your totals against a target. Most apps calculate this automatically once you log a food, so you don't need to do maths by hand.

A few practical tips:

• Start by setting a target using a macro calculator based on your goal, then adjust after a few weeks based on progress and how you feel
• Weighing food, at least initially, gives more accurate logging than estimating portions by eye
• Aim for consistency over perfection—hitting your targets most days matters more than being exact every single day
• Revisit your targets periodically, especially if your weight, activity level or goals change