Nutrition Intake Estimation API

Estimate Intake (from two body scans)

Computes daily energy intake and macro guidance from two body scans using BMR and tissue-energy change.

Endpoint

URLs:

• Development: https://api.rex.fit/[org-id]/dev/v1/estimate-intake
• Production: https://api.rex.fit/[org-id]/prod/v1/estimate-intake

Method: POST
Content-Type: application/json
Authorization: Bearer <GENERIC_WEBHOOK_API_KEY>

Request Fields

Both scan1 and scan2 are required objects with the following fields:

Field	Type	Required	Description	Example
testing_time	string	Yes	Timestamp of the scan. Default parse format %b %d, %Y %H:%M:%S	"Sep 20, 2024 12:00:37"
weight_kg	number	Yes	Weight in kilograms	78.4
height_cm	number	Yes	Height in centimeters	175
age_years	integer	Yes	Age in years	32
sex	string	Yes	Biological sex	"male" or "female"
fat_mass_kg	number	Yes	Fat mass in kilograms	18.2
fat_free_body_weight_kg	number	Yes	Fat-free mass (FFM) in kilograms	60.2

Optional fields (per scan or globally):

Field	Type	Default	Description
activity_level	string	"moderate"	"sedentary", "light", "moderate", "active", "athlete"
time_format	string	%b %d, %Y %H:%M:%S	Format string used to parse testing_time
bmr_method	string	"katch"	Basal Metabolic Rate method: "katch" or "mifflin"
language	string	"en"	Language code for personalized message: "en", "es", "fr", etc.

The following may be provided but are fixed internally by the service today:

• protein_g_per_kg_ffm
• fat_fraction_of_kcal

Example Request

{
  "scan1": {
    "testing_time": "Sep 20, 2024 12:00:37",
    "weight_kg": 78.4,
    "height_cm": 175,
    "age_years": 32,
    "sex": "male",
    "fat_mass_kg": 18.2,
    "fat_free_body_weight_kg": 60.2
  },
  "scan2": {
    "testing_time": "Oct 20, 2024 11:31:02",
    "weight_kg": 77.1,
    "height_cm": 175,
    "age_years": 32,
    "sex": "male",
    "fat_mass_kg": 17.1,
    "fat_free_body_weight_kg": 60.0
  },
  "activity_level": "moderate",
  "bmr_method": "katch",
  "language": "en"
}

Response

Field	Type	Description
days_between_scans	number	Number of days elapsed between scans
TEE_kcal_per_day	number	Total Energy Expenditure (per day)
energy_change_from_tissue_kcal_total	number	Energy change implied by tissue changes
daily_energy_balance_kcal	number	Daily energy balance
intake_kcal_per_day	number	Estimated average daily energy intake
message	string	Personalized analysis message in requested language
protein_min_g_per_day	number	Minimum protein grams per day to support lean mass
protein_max_g_per_day_energy_cap	number	Max protein grams if all intake were protein
fat_g_per_day_range_at_Pmin	[min, max]	Fat grams/day range at protein minimum
carbs_g_per_day_range_at_Pmin	[min, max]	Carbs grams/day range at protein minimum
extremes_at_Pmin	object	Macro extremes holding protein at minimum
assumptions	object	Calculation assumptions and constants

extremes_at_Pmin contains:

{
  "carb_heavy": { "protein_g_per_day": 130, "fat_g_per_day": 40, "carbs_g_per_day": 350 },
  "fat_heavy": { "protein_g_per_day": 130, "fat_g_per_day": 110, "carbs_g_per_day": 120 }
}

assumptions includes:

{
  "activity_level": "moderate",
  "activity_factor": 1.55,
  "bmr_method": "katch",
  "protein_content_of_ffm": 0.22,
  "protein_deposition_efficiency": 0.70,
  "lean_energy_density_kcal_per_kg": 1100,
  "fat_energy_density_kcal_per_kg": 7700
}

Error Responses

Code	Description
400	Missing required scan fields or invalid timestamps (scans must be hours/days apart)
401	Missing/invalid bearer token
500	Internal error