The basics of nutrition, without the hype

What you'll learn: what a calorie actually measures, where the energy in food comes from, how the body spends it, and why energy balance is real but less tidy than a calculator suggests.

This course is general education about food and nutrition, not medical advice. If you have a health condition, are pregnant, take medication or are thinking about a big change to how you eat, talk to a doctor or a registered dietitian first.

Meet Marcus

Marcus is 42 and manages projects for an engineering firm. He eats at his desk, feels flat every afternoon around half past three, and has a vague sense that his weekday eating could be better. He is not trying to lose weight. He wants steadier energy, more vegetables in his week, and to stop feeling confused by every new headline about food.

Here is a typical Tuesday for him before this course:

These figures are typical label values rounded for illustration; your own versions of these foods will differ. Over this course, Marcus will look at each part of this day and change it one evidence-based step at a time.

What a calorie is

A calorie is a unit of energy. In nutrition, "calorie" almost always means a kilocalorie (kcal), the energy needed to warm one kilogram of water by one degree Celsius. Many countries also use kilojoules (kJ), the metric unit: one kcal is about 4.2 kJ. Labels in the UK and EU show both; Australia and New Zealand mostly use kilojoules; the US and Canada mostly use calories. So a 250 kcal snack is roughly a 1,050 kJ snack.

Food energy comes from four sources:

Vitamins, minerals and water provide no energy at all. Fat is the most energy-dense, which is why a spoonful of oil adds more than a spoonful of sugar. Alcohol is the one that surprises people: Marcus's two beers carry about as much energy as a small meal.

How the body spends energy

Your body uses energy in three main ways:

1. Basal metabolism. Keeping you alive at rest: heart, brain, breathing, body temperature, repairing cells. For most people this is the largest share, often around 60% or more of daily use.
2. Digesting food. Breaking down and storing what you eat uses roughly 10% of the energy you take in, and more for protein than for fat.
3. Movement. Everything from deliberate exercise to walking to the printer, fidgeting and standing. This is the part that varies most between people and from day to day.

How much energy an adult needs depends on body size, age, sex, muscle mass and activity. Public health bodies publish rough averages, and the 2,000 kcal you see on many labels is a reference figure for comparison, not a personal target.

Energy balance, and why it is untidy

Over time, if you take in more energy than you use, the body stores the surplus, mostly as fat. If you take in less, it draws on stores. That principle is real. But treating the body like a simple bank account, where every calorie in and out is logged precisely, runs into problems:

• Label values are estimates, and regulators allow some margin of error.
• You don't absorb every calorie listed. Some energy in whole nuts and high-fibre foods passes through undigested.
• The body adapts. When people eat less for a long time, their energy use tends to fall and their appetite tends to rise, which partly offsets the change.
• What you eat affects how full you feel, which affects how much you eat next. Two snacks with the same calories can lead to very different appetites an hour later.

So calories matter, but the type of food matters too, both for health and for how much you end up eating. The rest of this course is mostly about that second part.

What Marcus notices

Adding up his Tuesday, Marcus gets roughly 2,580 kcal. He has no reason to aim for a particular number, but two things stand out. First, about 680 kcal came from drinks: juice, latte, cola and beer. Second, a large share of his breakfast energy came from sugar, in the granola and the juice, with only modest protein and fibre, and he is hungry again by mid-morning. Both will come up again. In the next lesson he starts with the nutrient that most affects how full he feels: protein.

Recap

• A food calorie is a kilocalorie; one kcal is about 4.2 kJ, and countries differ in which unit they put first.
• Fat gives about 9 kcal per gram, alcohol 7, protein and carbohydrate about 4, fibre about 2.
• The body spends energy on basal metabolism, digestion and movement, and needs vary a lot between people.
• Energy balance is real, but the body is not a precise bank account: labels, absorption, adaptation and appetite all blur it.
• Marcus's starting point: about a quarter of his energy comes from drinks, and his breakfast leans on sugar, with only modest protein and fibre.

What you'll learn: what protein does in the body, how much most adults need, which foods supply it, and why spreading it across the day may matter more than piling it up.

Why start with protein

Marcus is hungry by 10:30 every morning. His breakfast is mostly sugary granola and juice, and although the milk adds some protein, it is not much of a meal. Protein is a sensible first thing to look at, not because he is short of it, but because of what it does for fullness and for the rest of his day.

What protein is

Proteins are long chains of smaller units called amino acids. Your body uses about 20 different amino acids. It can make some of them from others, but nine it cannot make, so they must come from food. These are called essential amino acids.

When you eat protein, your gut breaks it down into amino acids, and your cells reassemble them into the proteins you need: muscle fibres, enzymes, hormones, antibodies, the structure of skin and hair. It is like a box of building bricks. A sandwich and a lentil soup arrive as finished models, the body takes them apart brick by brick, and then builds what it needs from the pieces. The body has no dedicated store of spare protein, so it relies on a steady supply.

How much is enough

Many health authorities set an adult reference intake of around 0.75 to 0.8 grams of protein per kilogram of body weight per day. That figure is designed to cover the needs of nearly all healthy adults.

For Marcus, who weighs about 80 kg, that is roughly 60 to 64 g a day. Adding up his Tuesday from lesson 1:

Like most adults in high-income countries, Marcus already meets the reference intake. He doesn't need protein shakes or high-protein bars. The question for him is not how much, but how it is spread and what comes with it.

Some groups may benefit from more than the reference figure. Expert groups have suggested somewhat higher intakes for older adults, to help preserve muscle, and for people doing a lot of strength or endurance training, though recommendations vary between countries and organisations. People with kidney disease have different needs and should follow medical advice.

Protein and fullness

In many studies, protein has been found to be the most filling of the three macronutrients, calorie for calorie. Meals with a reasonable amount of protein tend to keep people satisfied for longer than meals of the same energy made mostly of sugar or refined starch. This is a well-replicated finding, though the size of the effect varies between people and meals.

There is also some evidence, mostly from research on muscle, that spreading protein across meals rather than eating most of it at dinner helps the body use it for maintenance and repair, particularly in older adults. The evidence is stronger for muscle outcomes than for anything else, and it is not settled how much it matters for a healthy 42-year-old. Still, Marcus's pattern (a modest breakfast, a protein-heavy dinner) is the one this advice targets.

Where protein comes from

Protein is found in a wide range of foods, not just meat. Figures below are typical and vary by brand and recipe.

Two points from this table. First, plant-based drinks are not interchangeable: soya drink has protein similar to milk, but oat and almond drinks have little. Second, plant proteins can easily meet needs. Individual plant foods tend to be lower in one or more essential amino acids, but eating a variety across the day (beans, lentils, grains, nuts, seeds, soya) covers them. You don't need to combine them carefully at every meal, an older idea that has largely been dropped.

What comes with the protein

Protein foods differ in what else they bring. Lentils and beans bring fibre; oily fish brings omega-3 fats; processed meats such as ham and bacon bring salt and, in large amounts over the long term, are linked with a higher risk of bowel cancer. We will look at how to read that kind of finding in lesson 8.

Marcus's change

Marcus tries a different breakfast on three weekdays: plain Greek-style yogurt with a handful of oats, some berries and a spoonful of nuts. It has roughly 20 g of protein and noticeably less sugar than his granola and juice. On the other two days he has two eggs on wholemeal toast. He keeps the latte. After two weeks he reports that he rarely thinks about food before lunch. That is one person's experience, not a trial, but it fits what the research suggests about protein and fullness.

Recap

• Proteins are chains of amino acids; nine are essential and must come from food.
• A common adult reference intake is around 0.75 to 0.8 g per kilogram of body weight a day, and most people in high-income countries already meet it.
• Protein tends to be the most filling macronutrient per calorie, and spreading it across meals may help, especially for older adults.
• Plant foods can meet protein needs when eaten in variety; plant drinks vary widely in protein.
• Look at what comes with the protein: fibre and healthy fats, or salt and processed meat.