Building a health or fitness AI application? Accuracy is not optional. When users ask about their BMI, daily calorie needs, or heart rate zones, wrong answers are not just unhelpful; they can be harmful. This guide shows you how to equip your AI with deterministic health calculations via MCP.
Why Accuracy Matters in Health Apps
Health calculations involve formulas that LLMs know but cannot reliably execute. Consider BMI calculation:
BMI = weight (kg) / height (m)^2
Example: 70 kg, 175 cm
BMI = 70 / (1.75)^2 = 70 / 3.0625 = 22.86An LLM might calculate this as 22.9, 23.0, 22.5, or even wildly wrong values. In health contexts, these errors matter:
- BMI categories have precise thresholds: 24.9 vs 25.0 is the difference between "normal" and "overweight"
- Calorie calculations affect real decisions: Wrong TDEE leads to ineffective diet plans
- Users trust health apps: They may not verify calculations independently
BMI Calculations
Body Mass Index is the most commonly requested health calculation. TinyFn provides accurate BMI with category classification:
Tool: health/bmi
Input: {
"weight": 70,
"height": 175,
"unit": "metric"
}
Result: {
"bmi": 22.86,
"category": "Normal weight",
"healthy_range": {
"min_weight": 56.7,
"max_weight": 76.6
}
}BMI Categories
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 - 24.9 | Normal weight |
| 25.0 - 29.9 | Overweight |
| 30.0 and above | Obese |
BMR and TDEE
Basal Metabolic Rate (BMR) and Total Daily Energy Expenditure (TDEE) are foundational for nutrition planning.
BMR Calculation
Tool: health/bmr
Input: {
"weight": 70,
"height": 175,
"age": 30,
"gender": "male",
"formula": "mifflin_st_jeor"
}
Result: {
"bmr": 1680,
"formula": "mifflin_st_jeor",
"unit": "kcal/day"
}TDEE Calculation
Tool: health/tdee
Input: {
"weight": 70,
"height": 175,
"age": 30,
"gender": "male",
"activity_level": "moderate"
}
Result: {
"bmr": 1680,
"tdee": 2604,
"activity_multiplier": 1.55,
"activity_description": "Moderate exercise 3-5 days/week"
}Activity Levels
| Level | Multiplier | Description |
|---|---|---|
| sedentary | 1.2 | Little to no exercise |
| light | 1.375 | Light exercise 1-3 days/week |
| moderate | 1.55 | Moderate exercise 3-5 days/week |
| active | 1.725 | Hard exercise 6-7 days/week |
| very_active | 1.9 | Very hard exercise, physical job |
Calorie and Macro Calculations
Once you have TDEE, calculate calorie targets for weight goals and macro distributions:
Tool: health/calorie-goal
Input: {
"tdee": 2604,
"goal": "lose",
"rate": "moderate"
}
Result: {
"maintenance": 2604,
"target": 2104,
"deficit": 500,
"weekly_change": "0.45 kg loss"
}Macro Distribution
Tool: health/macros
Input: {
"calories": 2104,
"goal": "balanced"
}
Result: {
"protein": { "grams": 158, "calories": 630, "percentage": 30 },
"carbs": { "grams": 236, "calories": 946, "percentage": 45 },
"fat": { "grams": 58, "calories": 526, "percentage": 25 }
}Fitness Metrics
TinyFn also provides tools for fitness tracking and training:
Heart Rate Zones
Tool: health/heart-rate-zones
Input: {
"age": 30,
"resting_hr": 60
}
Result: {
"max_hr": 190,
"zones": {
"zone1": { "name": "Recovery", "min": 114, "max": 133 },
"zone2": { "name": "Aerobic", "min": 133, "max": 152 },
"zone3": { "name": "Tempo", "min": 152, "max": 162 },
"zone4": { "name": "Threshold", "min": 162, "max": 176 },
"zone5": { "name": "Anaerobic", "min": 176, "max": 190 }
}
}Pace Calculator
Tool: health/pace
Input: {
"distance": 5,
"time_minutes": 25,
"unit": "km"
}
Result: {
"pace_per_km": "5:00",
"pace_per_mile": "8:03",
"speed_kmh": 12,
"speed_mph": 7.46
}Building a Health Chatbot
Here's how a health chatbot conversation flows with TinyFn MCP tools:
User: "I'm 175cm tall, weigh 85kg, I'm 35 years old male.
I want to lose weight. What should I eat?"
Agent workflow:
1. Calculate BMI to assess current status
2. Calculate BMR and TDEE
3. Calculate calorie target for weight loss
4. Calculate macro distribution
Tool calls:
- health/bmi { "weight": 85, "height": 175 }
Result: { "bmi": 27.76, "category": "Overweight" }
- health/tdee { "weight": 85, "height": 175, "age": 35, "gender": "male", "activity_level": "light" }
Result: { "bmr": 1769, "tdee": 2432 }
- health/calorie-goal { "tdee": 2432, "goal": "lose", "rate": "moderate" }
Result: { "target": 1932, "deficit": 500 }
- health/macros { "calories": 1932, "goal": "high_protein" }
Result: { "protein": 174g, "carbs": 169g, "fat": 64g }
Agent response:
"Based on your stats, your BMI is 27.8, which is in the overweight range.
Here's your personalized plan:
**Daily Targets:**
- Calories: 1,932 kcal (500 calorie deficit)
- Protein: 174g (helps preserve muscle during weight loss)
- Carbs: 169g
- Fat: 64g
This should result in about 0.45kg (1 lb) weight loss per week.
Your healthy weight range is 56-77kg."MCP Setup
Add TinyFn health tools to your AI application:
{
"mcpServers": {
"tinyfn-health": {
"url": "https://api.tinyfn.io/mcp/health",
"headers": {
"X-API-Key": "your-api-key"
}
}
}
}Available Health Tools
| Tool | Description |
|---|---|
health/bmi |
Calculate Body Mass Index with category |
health/bmr |
Calculate Basal Metabolic Rate |
health/tdee |
Calculate Total Daily Energy Expenditure |
health/ideal-weight |
Calculate ideal body weight range |
health/body-fat |
Estimate body fat percentage |
health/calorie-goal |
Calculate calorie target for goals |
health/macros |
Calculate macro nutrient distribution |
health/heart-rate-zones |
Calculate training heart rate zones |
health/pace |
Calculate running/cycling pace |
Build Your Health AI with Accurate Calculations
Get your free API key and add deterministic health tools to your application.
Get Free API Key