Calories, macronutrients, and how to read what's in your food
Part A · the calorie confusion — kcal vs cal
The calorie confusion — kcal vs cal
Food "calories" are actually kilocalories (kcal). 1 food calorie = 1,000 physics calories. When a label says "250 calories," it means 250 kcal. This is one of the most widespread unit confusions in everyday life. From here on, "calorie" means food calorie = kcal.
Part B · macronutrients — what's in your food
Human daily energy need
~2,000 kcal
Average adult. ~2,500 for men, ~2,000 for women. Athletes can need 3,500–5,000 kcal.
In watts (power output)
~80 W
A resting human radiates about as much heat as a standard incandescent light bulb. Your body is literally a heater.
1 gram of fat
9 kcal
More than double carbs or protein (4 kcal/g each). Fat is the body's most dense energy store.
1 gram of carbohydrate
4 kcal
Same as protein. Sugar = carbohydrate. 1 teaspoon of sugar (~4g) = 16 kcal.
The three macronutrients — what they actually do
Primary role: Fast energy. The brain alone uses ~120g of glucose per day — about half your carb intake. Carbs are the body's preferred fuel for high-intensity work.
Storage: Excess carbs convert to glycogen (stored in liver and muscle, ~400–500g total) or, once glycogen is full, to fat. Glycogen is why you can run a 10km without eating — you're burning stored carbs.
Primary role: Long-term energy storage, cell membrane construction, fat-soluble vitamin transport (A, D, E, K), hormone production. Fat is not the enemy — it's essential.
Types matter: Unsaturated fats (olive oil, avocado, nuts) are associated with better cardiovascular outcomes. Saturated fats (butter, red meat) should be moderate. Trans fats (partially hydrogenated oils) are the ones to genuinely avoid — largely banned in many countries now.
Density insight: At 9 kcal/g, fat is the most energy-dense macronutrient — 2.25× more than carbs or protein. A tablespoon of olive oil (~14g) has ~125 kcal. This is why cooking oils add calories faster than people expect.
Primary role: Building and repairing tissue — muscle, skin, enzymes, antibodies, hormones. The body cannot store protein the way it stores fat or glycogen, so it needs a daily supply.
Complete vs incomplete: Proteins are made of 20 amino acids; 9 are "essential" (must come from food). Animal proteins (meat, eggs, dairy) are "complete" — they contain all 9. Most plant proteins are incomplete, but combining sources (e.g. rice + beans) covers all 9.
Satiety superpower: Protein is the most satiating macronutrient — gram for gram, it keeps you full longer than carbs or fat. This is why high-protein breakfasts reduce total daily calorie intake in studies. The mechanism: protein raises GLP-1 and PYY (satiety hormones) more than other macros.
Part C · reading real food — what's actually inside
Protein in 200g of cooked chicken breast
Protein~46 g
Fat~5 g
Total weight200 g
Rule of thumb: cooked chicken breast = ~23% protein by weight. So 200g gives you about 46g of protein — roughly what a 75kg person needs in an entire day.
A Snickers bar (standard, ~52 g)
Sugar~27 g
Fat~13 g
Protein~4 g
Total bar weight52 g
That 27g of sugar = about 6–7 teaspoons. The WHO recommends ≤25g free sugar per day for an adult. One Snickers already exceeds it.
How to read a nutrition label — annotated
Nutrition Facts
8 servings per container Serving size2/3 cup (55g)
Amount per serving
Calories
230
% Daily Value*
Total Fat 8g10%
Saturated Fat 1g5%
Trans Fat 0g
Cholesterol 0mg0%
Sodium 160mg7%
Total Carbohydrate 37g13%
Dietary Fiber 4g14%
Total Sugars 12g
Includes 10g Added Sugars20%
Protein 3g
Vitamin D 2mcg10%
Calcium 260mg20%
Iron 8mg45%
Potassium 235mg6%
* % Daily Values based on a 2,000 kcal diet. Your daily values may be higher or lower depending on your calorie needs.
① Serving size is the key trap. Everything on this label is per serving. This container has 8 servings. Eat the whole box = multiply everything by 8. Most people don't.
② Calories = the headline number. 230 kcal per serving. Quick mental check: is this a meal or a snack? 230 kcal is a light snack for most adults.
③ % Daily Value is based on 2,000 kcal/day. 5% DV = low. 20% DV = high. Use it to compare products, not as gospel — your needs differ.
④ Added Sugars vs Total Sugars. "Total Sugars" includes naturally occurring sugar (fruit, milk). "Added Sugars" is the dangerous one — 10g here = 40% of the WHO daily max in one serving.
⑤ Protein has no % DV. Regulators consider deficiency rare in Western diets. For active people, aim for body weight in kg × 1.2–2.0g daily.
Sugar in teaspoons — a visual gut-check
Each cube = 1 teaspoon (~4g) of sugar. WHO daily max for adults = 6 teaspoons (25g).
Part D · how far does 100 kcal go?
How far does 100 kcal go? (exercise to burn it off)
Running (fast)
~8 min
Cycling
~15 min
Brisk walking
~25 min
Sitting at a desk
~75 min
100 kcal = roughly a small apple, a plain rice cake with peanut butter, or 3 squares of dark chocolate. Exercise burns less than most people think — that croissant (250 kcal) needs 30 min of running to offset.
Part E · energy in the world — from food to nuclear weapons
Energy released — logarithmic scale (in joules, J)
Each step up is ~1,000× more energy. 1 food calorie = 4,184 joules.
Lifting 1 kg by 1 metre
~10 J
1 food calorie (1 kcal)
~4,200 J
Human daily food (2,000 kcal)
~8.4 MJ
1 litre of petrol (burned)
~34 MJ
Lightning bolt
~250 MJ–1 GJ
Full tank of car petrol (~50L)
~1.7 GJ
Hiroshima bomb (Little Boy)
~63 TJ (63 trillion J)
Largest H-bomb ever (Tsar Bomba)
~210 PJ — 3,300× Hiroshima
Surprising fact: a typical lightning bolt (~250 MJ to ~1 GJ) contains less total energy than a full car petrol tank (~1.7 GJ). The difference is delivery speed — a lightning bolt releases its energy in microseconds. Your petrol tank releases it over hours. Energy rate (power) matters as much as total energy.
Part F · interactive calorie calculator
How long to burn off any food?
Part G · fibre — the forgotten macronutrient
Why fibre deserves its own section
Fibre is a carbohydrate the body cannot digest — it passes through mostly intact, but does critical work on the way. It is not "inert bulk." It feeds gut bacteria (your microbiome), slows glucose absorption, lowers cholesterol, and keeps bowel movements regular. Most adults in wealthy countries eat about half the recommended amount.
Recommended daily fibre
25–30 g
WHO / dietary guidelines. Average Western adult gets ~15g. A 50% shortfall is typical.
Fibre in an apple (medium)
~4.5 g
Apple juice has almost zero — the fibre is in the pulp. Whole fruit, not juice.
Fibre in white bread (slice)
~0.6 g
Vs ~2g in a slice of wholegrain. Processing strips most of the fibre out.
Fibre in lentils (100g cooked)
~8 g
One of the best sources. Legumes, oats, and vegetables are the practical high-fibre staples.
Fibre content per serving — common foods compared
Lentils, 100g cooked
8 g
Black beans, 100g cooked
7.5 g
Oats, 40g dry (1 serving)
4 g
Apple, medium (~182g)
4.5 g
Broccoli, 100g
2.6 g
Wholegrain bread, 1 slice
2 g
White bread, 1 slice
0.6 g
Rule of thumb: ≥5g fibre per serving = "high fibre." ≥3g = "source of fibre" (EU labelling). Legumes beat everything else by a wide margin.
Part H · food label estimation game
Guess the calories — 5 rounds
You'll be shown a real food and a quantity. Pick the closest calorie estimate.
Part I · the glycaemic index — why not all carbs are equal
What is the glycaemic index (GI)?
GI measures how fast a carbohydrate food raises blood glucose, on a scale of 0–100 (pure glucose = 100). High-GI foods cause a rapid spike and subsequent crash. Low-GI foods cause a slower, steadier rise. GI matters most for diabetics and athletes, but it's a useful mental model for everyone: it explains why you're hungry 90 minutes after white toast but not after oats.
GI — the nuances that matter
GL > GI
Glycaemic Load = GI × carb amount ÷ 100
Watermelon has a high GI (80) but a low GL (~5 per serving) because it's mostly water. GL is a better real-world metric than GI alone.
Context
Fat and protein lower a meal's effective GI
Eating white rice alone spikes blood sugar fast. Eating it with chicken and olive oil slows digestion considerably. This is why isolated GI numbers can mislead.
Cooking
Cooking method changes GI
Al dente pasta has a lower GI than overcooked pasta. Cooled, then reheated rice has a lower GI than freshly cooked (resistant starch forms on cooling).
Part J · food label decoder — interactive
Calculate calories from a label
Enter the macros from any nutrition label and see the calorie breakdown.
Part K · anchor numbers to memorize
2,000 kcal
Average human daily energy need
Fat = 9 kcal/g. Carbs & protein = 4 kcal/g each.
~80 W
Resting human heat output
A human body radiates as much as a standard incandescent light bulb, continuously.
23%
Protein content of cooked chicken breast
200g chicken ≈ 46g protein — nearly a full day's requirement.
25 g
WHO recommended maximum free sugar per day
About 6 teaspoons. One Snickers bar (27g sugar) already exceeds this.
25–30 g
Daily fibre target
Most Western adults get only ~15g. Legumes, oats, and whole vegetables are the practical solutions.
GI <55
Low glycaemic index threshold
Lentils (29), oats (55), brown rice (50). Combines with GL for real-world usefulness.
Part L · test yourself
1. You eat 300g of chicken breast for dinner. How much protein is that?
About 69g of protein (23% of 300g). That's close to a full day's requirement for most adults (roughly 0.8–1g per kg of body weight, so ~60–80g for an average person).
2. A person eats 3 Snickers bars. How many teaspoons of sugar is that?
About 20 teaspoons. Each bar has ~27g sugar, three bars = ~81g. One teaspoon of sugar ≈ 4g, so 81 ÷ 4 ≈ 20 teaspoons. That's more than 3× the WHO daily recommended limit in a single snack.
3. A person eats a 550 kcal Big Mac and then goes for a 30-minute jog. Have they burned it off?
Not quite. Running burns roughly 10–12 kcal per minute at moderate pace, so 30 minutes = ~300–360 kcal. The Big Mac has 550 kcal — so you'd need closer to 50 minutes of running to fully offset it. This is the key insight: it is much faster to consume calories than to burn them. A 2-second bite of a burger takes 4 minutes of running to undo. This is why diet (what you eat) has far more impact on weight than exercise alone.
4. A food label says "per 100g: 15g fat, 45g carbohydrate, 8g protein." Roughly how many kcal per 100g?
About 347 kcal. Fat: 15g × 9 kcal/g = 135 kcal. Carbs: 45g × 4 kcal/g = 180 kcal. Protein: 8g × 4 kcal/g = 32 kcal. Total: 135 + 180 + 32 = 347 kcal per 100g. The fat contributes the most calories despite being only 15g — because fat is 9 kcal/g, more than double carbs or protein. This is the fundamental label-reading skill: multiply and add.
5. You see two cereals. Cereal A: 30g serving, GI 70, 22g carbs. Cereal B: 40g serving, GI 40, 28g carbs. Which has the higher glycaemic load?
Cereal A: GL = 70 × 22 ÷ 100 = 15.4. Cereal B: GL = 40 × 28 ÷ 100 = 11.2. Cereal A has the higher GL despite its smaller serving size. The high GI (70) overwhelms the lower carb quantity. This is why GL is more useful than GI alone — it accounts for how much carbohydrate you're actually eating.
6. A bag of chips lists "per 30g serving: 10g fat, 17g carbs, 2g protein." You eat 90g (the whole bag). How many kcal did you consume?
The label is per 30g, so multiply everything by 3: 30g fat, 51g carbs, 6g protein. Now apply kcal/g: 30 × 9 = 270 kcal (fat) + 51 × 4 = 204 kcal (carbs) + 6 × 4 = 24 kcal (protein) = 498 kcal total. Nearly 500 kcal for a bag of chips most people eat as a snack. The serving size trap is real.