Health Articles

Ideal Body Weight

The Devine Ideal Weight Formula Came From a Drug Dosing Paper — Here's What Actual Health Outcomes Research Shows Instead

The Devine ideal weight formula appeared in a 1974 drug dosing paper — it was never derived from health outcome research. Here's where each major formula came from (insurance actuarial tables, drug pharmacokinetics, military studies), what mortality outcome data actually shows about "healthy" weight (lowest mortality at BMI 22-25 but the curve is flat 20-27), and why the obesity paradox in clinical populations complicates the "lower is healthier" framing.

Height Converter

Dutch Men Grew 18 cm Taller in 130 Years Without Changing Genes — The Secular Height Trend Explained

Dutch men grew 18 cm taller in 130 years — genetics don't change that fast, so the entire increase came from nutrition and disease improvements. North and South Koreans are genetically identical but differ by ~9 cm in average height from 50 years of divergent nutrition. Here's the secular height trend, why "80% heritable" doesn't mean what people think it means, and why contemporary height differences between populations are mostly environmental history, not genetics.

Period Calculator

Perimenopause's First Sign Is Cycle Variability, Not Hot Flushes — What to Track and When to See a GP

Perimenopause's first sign isn't hot flushes — it's cycle length variability, with reliably predictable cycles becoming erratic over 2-10 years before the final period. Here's how declining ovarian reserve drives follicular phase variability, why anovulatory cycles cause heavier periods, the specific tracking patterns that indicate perimenopause vs thyroid issues or PCOS, and why fertility doesn't reach zero until 12 months after the final period.

Due Date Calculator

Why Your 40-Week Due Date Is Actually 38 Weeks of Pregnancy — Ultrasound Dating, Naegele's Rule, and When Induction Is Recommended

A 40-week pregnancy is actually ~38 weeks of fetal development — gestational age is counted from 2 weeks before fertilisation. First-trimester ultrasound (±5 days accuracy) is more accurate than LMP dating for anyone with irregular cycles. Here's why the due date is a median with an asymmetric range, what the ARRIVE trial says about 39-week induction, and why gestational age accuracy matters most in preterm delivery decisions.

Calories Burned

Why Your Exercise Calorie Estimates Are Systematically Too High — NEAT Compensation, Dose-Response, and the Muscle Mass Advantage

Adding up exercise calorie estimates to calculate weekly expenditure overstates the benefit by 30-50% — because NEAT drops on exercise days, appetite partially compensates, and training efficiency improves. Here's the dose-response relationship between volume and actual burn, why MET values have a ±20-30% accuracy ceiling, the long-term resting metabolism boost from added muscle (13-22 kcal/kg/day), and why fasted exercise doesn't reliably burn more fat over 24 hours.

Calorie Intake

Why 100 Calories of Protein Doesn't Equal 100 Calories of Carbs — The Thermic Effect of Food and What It Means for Your Calorie Target

Protein burns 20-30% of its own calories just in digestion — eat 100 calories of protein and you net roughly 70-80 calories. Fat burns only 0-3%. This "thermic effect of food" means macronutrient composition changes your effective calorie intake even if the label says the same total. Here's the full TEF breakdown, why it matters for calorie targets, and how high-protein diets exploit it.

BMR Calculator

Brown Fat Was "Only in Babies" Until 2009 — The Thermogenesis Biology That Changes How We Understand BMR

Brown adipose tissue was thought to exist only in infants until 2009, when PET/CT scans revealed metabolically active brown fat in adults — concentrated around the neck and collarbones, inversely correlated with BMI. Here's the white vs brown vs beige fat distinction (storage vs heat generation vs metabolic plasticity), how cold activates UCP1 thermogenesis adding 100-300 kcal/day, and why thyroid hormone is the primary BMR regulator that calculators completely ignore.

BMI Calculator

Metabolic Syndrome Predicts Heart Disease Better Than BMI — Here's What the Five Criteria Are and Why They Matter More

Metabolic syndrome — any three of five criteria (waist circumference, triglycerides, HDL, blood pressure, fasting glucose) — predicts cardiovascular disease better than BMI alone, and "TOFI" individuals (normal BMI, high visceral fat) are misclassified as healthy by BMI. Here's the complete metabolic syndrome criteria, A Body Shape Index (ABSI) as the next-generation abdominal obesity measure, the obesity paradox in clinical populations, and why WHtR is emerging as the recommended companion to BMI.

Water Intake Calculator

Why "Drink to Thirst" Fails at Altitude — and the Specific Contexts Where You Need More Water Than General Guidance Suggests

Altitude dehydration is real because respiratory rate increases and inspired air is drier — but thirst doesn't compensate fully, making trekkers a specific population where "drink to thirst" systematically underestimates needs. Here's the quantified altitude respiratory water loss (up to 1,200 mL/day at high altitude), the fever rule of thumb (each 1°C above normal adds ~20-25% water loss), why coffee's diuretic effect is smaller than its fluid content, and the specific kidney stone prevention fluid target (2.5 litres urine output daily).

Age Calculator

Processing Speed Peaks at 25, Vocabulary at 65 — Why "Peak Age" Depends Entirely on What You're Measuring

Cognitive processing speed peaks at 18-25, but vocabulary peaks in the late 60s, emotional regulation improves into the 70s, and crystallized intelligence doesn't decline until the 70s-80s. Here's the multiple-peaks framework for human performance, why processing speed loss doesn't translate to overall cognitive decline (strategy, knowledge, metacognition compensate), why marathon runners peak later than sprinters, and the relative age effect that makes September-born UK children more likely to be identified as gifted.

Steps to Calories

Walking Burns as Many Calories Per Kilometre as Running — Here's the Biomechanics That Explains Why

Walking burns approximately the same calories per kilometre as running — because running is twice as fast but twice as many calories per minute, netting out the same per distance. The pendulum mechanics of walking and spring-bounce mechanics of running are fundamentally different energy systems that happen to converge at similar efficiency per unit distance. Here's the biomechanics, why brisk walking is the most efficient gait, the NEAT advantage of higher step count, and why fitness trackers give different calorie estimates for the same steps.

Sleep Calculator

Why the First Half and Last Half of Your Night Do Different Things — Sleep Stages, Memory, and Nap Strategy

Sleep stages don't repeat in equal proportions across the night — slow-wave deep sleep dominates the first third, REM dominates the last third. Cutting sleep short primarily loses REM; shifting sleep later primarily loses SWS. Here's what each stage does (SWS for physical restoration and declarative memory, N2 spindles for procedural memory, REM for emotional processing), why the "coffee nap" works, and how nap duration determines which stages you reach.