If you've ever stared at a nutrition label wondering how any of it applies to you, you're not alone. Most people start a fitness or health journey by guessing - a little less rice here, a bit more protein there, without ever knowing the actual numbers behind their body's needs.
This post breaks down the science in plain language, so you can walk away knowing exactly what your body needs and why.
Start With BMR: Your Body's Idle Speed
Your Basal Metabolic Rate (BMR) is the number of calories your body burns just to stay alive - breathing, circulating blood, repairing cells - if you did nothing but lie in bed all day. It's not a random guess; it's calculated from your age, sex, height, and weight using an equation called the Mifflin-St Jeor formula, which has become the standard used by dietitians.
Either way, BMR is your starting point.
The Mifflin-St Jeor formula:
- Men: BMR = 10 x wt (kg) + 6.25 (height in cm) - 5 x age + 5
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Women: BMR = 10 x wt (kg) + 6.25 (height in cm) - 5 x age - 161
TDEE: Adding Real Life to the Equation
Your Total Daily Energy Expenditure (TDEE) takes your BMR and multiplies it by an activity factor based on how much you actually move in a week:
Sedentary (desk job, little exercise): BMR × 1.2
Lightly active (exercise 1–3x/week): BMR × 1.375
Moderately active (exercise 3–5x/week): BMR × 1.55
Very active (exercise 6–7x/week): BMR × 1.725
Extra active (athlete, physical job + daily training): BMR × 1.9
This is the number that roughly represents what it takes to maintain your current weight. Eat consistently below it -> you'll lose weight over time;
Eat above it -> you'll gain.
It's not exact to the calorie, nobody's metabolism is that predictable, but it's a useful starting estimate.
Macros: Where the Calories Actually Go
Once you know your total calorie target, the next question is: what should those calories be made of? This is where macronutrients - protein, carbohydrates, and fat - come in, along with fiber, which doesn't provide calories in the same way but plays a huge role in digestion and satiety.
Protein is the one most people under-eat. The bare minimum to avoid deficiency is around 0.8g per kg of body weight, but that number is a floor for sedentary people, not a target for anyone training.
Active individuals generally benefit from somewhere between 1.2 and 2.2 grams per kg bodyweight, scaling up with training volume, according to position statements from the International Society of Sports Nutrition. Protein supports muscle repair, keeps you fuller for longer, and has the highest "thermic effect" of any macro, meaning your body burns more calories just digesting it.
Fat typically makes up 20–35% of total calories. It's essential for hormone production, vitamin absorption, and cell function - cutting it too low is a common mistake for people chasing fast weight loss, and it can backfire on energy and hormone health.
Carbohydrates are your body's preferred fuel source, especially for anything moderate-to-high intensity. Active people, particularly those training hard multiple times a week, generally need more carbs than sedentary people , despite what a lot of diet trends suggest.
Fiber is the quiet workhorse of the group. A commonly cited target is about 14 grams per 1,000 calories consumed, which works out to roughly 25g for most women and 38g for most men. It supports digestion, blood sugar stability, and importantly helps you actually feel satisfied after a meal.
Putting It Together
None of these numbers are meant to be followed rigidly forever. They're a starting baseline - a way to stop guessing and start with something grounded in real data about your body. From there, the best approach is simple: track how you feel and how your body responds over a few weeks, and adjust from there.