Science & Evidence
Where the numbers come from.
Zealo puts numbers in front of you: calorie targets, macro splits, readiness scores. This page explains where each of those comes from, what it can and cannot tell you, and the rule we use to keep numbers we cannot justify out of the app.
The rule
A number needs a source before it ships
Every numeric threshold in the app has to cite a published source before it can be used. That constraint lives in the codebase, not in a style guide: thresholds are referenced by identifier against a literature gate, and a change that introduces a new number without one does not get merged.
The point is not rigour for its own sake. It is that fitness software is full of numbers that sound authoritative and came from nowhere. Requiring a citation makes the difference visible, including to us.
A worked example
What we removed, and why
An earlier version of Zealo adjusted nutrition by menstrual cycle phase: roughly +120 kcal in the luteal phase, a 10% carbohydrate increase, and a reduction in prescribed training intensity.
When those figures were checked against the literature, the evidence would not carry them. Cycle phase is a real physiological signal, but the case for systematically moving someone's calorie target or capping their training intensity because of it was not there. So the modulation was removed. Calorie and macro targets now stay fixed across phases by design.
What survived is what the evidence does support, and it is categorical rather than numeric: iron emphasis during menstruation, hydration prompts in the ovulatory and luteal phases, and phase-appropriate food suggestions. You still get cycle-aware guidance. You do not get invented numbers.
This is the honest version of “science-backed”: not a badge, but a record of a feature being cut because it could not be justified.
The maths
The equations actually in use
Resting energy expenditure uses a Modified Mifflin-St Jeor formula. When fat-free mass is known it resolves to RMR = 19.7 × FFM + 413, which is validated against clinical DEXA reference; otherwise it falls back to the standard Mifflin-St Jeor equation. Activity multipliers and METs come from the Compendium of Physical Activities.
These are estimates. A predictive equation is not a metabolic cart, and the error bars on any population equation applied to one person are real. They are good enough to plan against and adjust from, which is how the app treats them.
Limits
What this is not
Zealo is a fitness and wellness app. It does not diagnose, treat, or monitor any medical condition, and nothing it shows you is medical advice.
Biomarker reference ranges shown in the app are population-level and indicative. Real decision ranges depend on the issuing lab, the assay, your age and sex, cycle phase for reproductive hormones, pregnancy status and medications. Interpret results with the report from the lab that issued them and a qualified clinician.
If you are pregnant, managing a health condition, or working with a clinician or dietitian, their guidance takes precedence over anything in this app.
Sources
What we cite
Energy and metabolism
- Mifflin-St Jeor: a new predictive equation for resting energy expenditure (1990)
Mifflin et al., American Journal of Clinical Nutrition. The validated equation underlying every RMR estimate in the app.
- Compendium of Physical Activities (Ainsworth et al.)
Standardised MET values used to weight the activity multipliers in a TDEE estimate.
- USDA Dietary Reference Intakes for Energy
National Academies / USDA reference for daily energy requirements by age, sex and activity.
- NIDDK: energy and metabolism
NIH overview of basal metabolism and energy balance.
Activity and training
- WHO physical activity fact sheet
Population-level activity guidance the app's weekly targets are checked against.
- CDC physical activity guidance
US public-health guidance on activity volume and intensity.
- American College of Sports Medicine
Professional guidance on exercise prescription and progression.
Cycle and reproductive health
- ACOG: the menstrual cycle as a vital sign
Committee opinion underpinning how cycle phase is treated as a health signal rather than a training dial.
- NICHD: menstruation
NIH reference for cycle physiology and normal variation.
Biomarker reference ranges
- ADLM / Lab Tests Online
Basis for the general-population ranges shown on biomarker cards.
- MedlinePlus: lab tests
NIH National Library of Medicine reference for individual test interpretation.
- American Thyroid Association
Reference ranges and guidance for TSH, Free T4, Free T3 and thyroid antibodies.
- American Heart Association cholesterol guidelines
Lipid panel reference for total, LDL, HDL, triglycerides, non-HDL and ApoB.
- American Diabetes Association Standards of Care
Reference for fasting glucose, HbA1c and dysglycemia thresholds.
Further reading
The same topics, in depth
See how it works in the app.
Every metabolic figure in Zealo shows its own method and sources alongside it, in the app, not just here.