Weight, calories, hydration and heart rate: reading your health numbers

Body Mass Index (BMI) is a numerical measure that relates a person's body weight to their height. It was developed in the 19th century by Belgian mathematician Adolphe Quetelet and adopted by the World Health Organization as the standard indicator for classifying the weight status of the adult population.

How is BMI calculated?

The formula is straightforward: BMI = weight (kg) ÷ height² (m). For example, a person weighing 75 kg and 1.75 m tall has a BMI of 75 ÷ (1.75 × 1.75) = 24.5. This value falls in the normal weight range according to the WHO.

WHO categories

• •Underweight: BMI < 18.5 — may indicate malnutrition or underlying health problems.
• •Normal weight: 18.5 – 24.9 — associated with the lowest risk of weight-related disease.
• •Overweight: 25 – 29.9 — increases the risk of hypertension, type 2 diabetes and cardiovascular disease.
• •Obesity class I: 30 – 34.9 — moderate-to-high risk.
• •Obesity class II: 35 – 39.9 — high risk.
• •Obesity class III (morbid): ≥ 40 — associated with the highest health risk under the WHO classification; an individualised medical assessment is advisable.

These categories come from the World Health Organization classification and describe statistical associations at population level, not an individual diagnosis: BMI is a screening tool and each person's real risk depends on other factors (body composition, waist circumference, age, sex and metabolic status).

Limitations of BMI

BMI does not distinguish between muscle mass and body fat. An athlete with a lot of muscle may have a BMI of 27 (overweight according to the table) yet enjoy excellent metabolic health. Equally, a person with a normal BMI but low muscle mass and high visceral fat may have an elevated cardiovascular risk. BMI also fails to account for age, sex or fat distribution — abdominal fat is more dangerous than peripheral fat.

What other indicators complement BMI?

• •Waist circumference: elevated risk above 88 cm in women or 102 cm in men.
• •Waist-to-hip ratio (WHR): values > 0.85 in women and > 0.90 in men indicate cardiovascular risk.
• •Body fat percentage: measured by bioelectrical impedance or skinfold calipers.
• •Blood tests: blood glucose, cholesterol, triglycerides.

When should I see a doctor?

If your BMI is below 18.5 or above 30, it is advisable to speak with your doctor for a full assessment. A BMI in the normal range does not guarantee good health if other risk factors are present. Remember that BMI is a population screening tool, not an individual diagnosis.

Calories and basal metabolism

Calories are units of energy. Everything your body does — from breathing to running a marathon — burns calories. The key is finding the balance between what you consume and what you burn, and that balance depends on two fundamental concepts: BMR and TDEE.

What is Basal Metabolic Rate (BMR)?

BMR is the minimum energy your body needs at complete rest to sustain vital functions: heartbeat, breathing, temperature regulation, brain and organ function. It accounts for 60–75% of your total daily calorie expenditure. It depends mainly on weight, height, age and biological sex.

What is TDEE?

TDEE (Total Daily Energy Expenditure) is your actual energy expenditure once physical activity is added. It is calculated by multiplying BMR by an activity factor: 1.2 for sedentary people, up to 1.9 for athletes with demanding physical jobs. TDEE is the number of calories you would need to eat to maintain your current weight.

The Mifflin-St Jeor formula

Published in 1990 and validated across multiple studies, this is the most accurate formula for the general population. For men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5. For women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161. It is more accurate than the classic Harris-Benedict formula because it was calibrated using indirect calorimetry on a representative sample.

How to adjust calories for your goal?

• •Lose weight: reduce TDEE by 300–500 kcal/day. This is not a one-size-fits-all figure: the right deficit depends on your starting point, and aggressive deficits increase muscle loss and the risk of giving up. A moderate deficit equates to losing around 0.3-0.5 kg per week.
• •Maintain weight: eat approximately the same number of calories as your TDEE.
• •Gain muscle: add 200–300 kcal above TDEE (moderate surplus) to minimise fat gain.

Why don't calculators always get it right?

Formulas are statistical estimates. Your actual metabolism may differ by ±10–15% due to factors such as hormonal status (hypothyroidism, menopause), gut microbiome, sleep quality or metabolic adaptation after periods of dieting. If you see no changes after 3–4 weeks, adjust your calories by ±100 kcal and reassess.

Macronutrients matter

Not all calories are equal in terms of satiety and body composition. In general, increasing protein intake (1.6–2.2 g/kg of bodyweight) helps preserve muscle during a deficit and controls appetite. Carbohydrates are the primary fuel for exercise, and fats are essential for hormone production and absorption of fat-soluble vitamins.

How much water you need each day

Water is the main component of the human body — it accounts for 55–70% of body mass — and is essential for almost every physiological process: nutrient transport, temperature regulation, joint lubrication, elimination of metabolic waste and nervous system function.

How much water does an adult need?

The most widely accepted recommendation among nutritionists is 30–40 ml of water per kilogram of body weight per day. For a 70 kg person in a temperate climate with light activity, that works out to about 2.1 litres per day. The WHO sets a minimum of 1.5 litres for adults, but this figure does not account for physical activity or climate.

Factors that increase your fluid needs

• •Intense physical activity: exercise can increase fluid losses by 0.5–2 litres per hour depending on intensity and temperature.
• •Hot or humid climate: sweating increases to regulate body temperature. Add at least 500 ml extra on hot days.
• •High altitude: faster breathing and increased urination raise water losses.
• •Pregnancy and breastfeeding: needs increase by an additional 300–700 ml per day.
• •Fever, vomiting or diarrhoea: extraordinary losses that must be replenished with particular care.

Signs of dehydration

• •Dark yellow or amber urine (the goal is pale yellow or near-clear).
• •Persistent thirst or a dry mouth.
• •Fatigue, difficulty concentrating or headache.
• •Dizziness or light-headedness when standing up.
• •With dehydration > 2% of body weight: drop in physical and cognitive performance.

Does water from food count?

Yes. Approximately 20–30% of total fluid intake comes from solid foods. Fruits and vegetables are especially water-rich: watermelon contains 92%, cucumber 96% and lettuce 95%. However, the calculator refers exclusively to water and drinks you consume, as this is the easiest part to consciously track.

Practical tips for drinking more water

• •Start the day with a glass of water when you wake up, before coffee or breakfast.
• •Always carry a reusable water bottle visible on your desk or in your bag.
• •Link drinking water to habits you already have: before each meal, when you arrive home, etc.
• •Add lemon slices, cucumber or herbs to vary the flavour without adding sugar.
• •Set phone reminders if you tend to forget to drink between meals.

Maximum heart rate and training zones

The heart is the engine of physical exercise. The more beats per minute, the more oxygen reaches the muscles and the more energy can be produced. Maximum heart rate (MHR) is the upper limit of that engine: the number of beats it can reach at maximum exertion. Knowing it is the first step to training intelligently and efficiently.

Why is it important to know your MHR?

Training without a heart rate reference is like driving without a speedometer: you might be going too fast without knowing it (overtraining) or too slowly to see results (understimulation). With MHR as a reference, you can calculate your training zones and ensure that each session meets its physiological objective.

The Tanaka formula: more accurate than 220 − age

The classic '220 − age' formula is a simplification that can under- or overestimate MHR by ±12 bpm. The Tanaka formula (2001), validated in a meta-analysis of more than 18,000 people, is more accurate: MHR = 208 − (0.7 × age). For a 40-year-old, both formulas give 180 bpm, but at extreme ages the difference can be 10–15 bpm.

The 5 training zones

• •Zone 1 (50-60% MHR) — Active recovery: warm-up, cool-down and recovery days. Very low intensity.
• •Zone 2 (60-70% MHR) — Base aerobic: fat burning as primary fuel, improved aerobic endurance. The most important zone for endurance runners.
• •Zone 3 (70-80% MHR) — Upper aerobic: cardiovascular improvement, VO₂max increase. Conversation possible but with effort.
• •Zone 4 (80-90% MHR) — Anaerobic threshold: improves power and performance in 20–60 minute efforts. Difficult to hold a conversation.
• •Zone 5 (90-100% MHR) — Maximum effort: develops peak power and VO₂max. Short sprints only. Not sustainable for more than 1–3 minutes.

How to distribute training across zones?

Sports science researchers recommend the polarised model: 80% of volume in Zones 1–2 (low intensity) and 20% in Zones 4–5 (high intensity), avoiding Zone 3 which generates fatigue without the benefits of either extreme. This model is used by most elite endurance athletes.

Is it safe to train near MHR?

For healthy people with no cardiovascular risk factors, Zone 4–5 training is safe once a solid aerobic base (Zones 1–3) has been built over at least 8–12 weeks. If you are over 40, have been sedentary for a long time, or have a history of heart disease, consult your doctor before undertaking high-intensity exercise.