The Scientific Formula: Mifflin-St Jeor
At the core of how MyFitnessPal calculates calorie goals is the Mifflin-St Jeor equation, a widely respected formula for estimating a person's Basal Metabolic Rate (BMR). BMR represents the number of calories your body burns at complete rest, simply to perform essential functions like breathing, circulation, and cell production. The Mifflin-St Jeor formula is considered more accurate than older equations for estimating BMR in many populations.
The Mifflin-St Jeor Equations are:
- For Men: BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) + 5
- For Women: BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) - 161
Factors MyFitnessPal Collects
To perform its initial calculation, the app prompts you for several pieces of personal information when you first set up your account. This data directly feeds into the BMR calculation and subsequent adjustments.
Key User Inputs:
- Age: Metabolism naturally changes with age, a factor built into the equation.
- Sex: Men and women have different metabolic rates, which is reflected in the separate formulas.
- Height & Weight: These are essential metrics that determine your body mass and are central to the Mifflin-St Jeor equation.
- Activity Level: MyFitnessPal asks you to select an activity level, from 'Not Very Active' to 'Very Active'. This serves as a multiplier on your BMR to estimate your Total Daily Energy Expenditure (TDEE).
- Weight Goal: You specify a goal for weight loss, gain, or maintenance. This is the final step in the initial calculation to create a calorie deficit or surplus.
Adjusting for Your Weight Goal
After determining your estimated TDEE, MyFitnessPal adjusts your calories based on your stated weekly weight goal. The app uses the standard calculation that a pound of fat is roughly equivalent to 3,500 calories.
Here is how the adjustment works:
- Weight Loss: To lose one pound per week, the app creates a daily deficit of approximately 500 calories (3,500 calories per week / 7 days). It subtracts this amount from your TDEE.
- Weight Gain: To gain one pound per week, the app adds a daily surplus of around 500 calories to your TDEE.
- Maintenance: The calorie goal is set to match your TDEE, with no deficit or surplus added.
Exercise and Dynamic Adjustments
An important feature is MyFitnessPal's dynamic calorie budget. When you log exercise or sync a fitness tracker, the app adds those burned calories back to your daily budget. This allows for a more flexible diet, as you can consume more food on days you are more active without compromising your weight goal. This concept is referred to as 'Net Calories'.
Comparison Table: Standard vs. MyFitnessPal Calorie Calculations
| Feature | Standard Manual Calculation | MyFitnessPal Calculation |
|---|---|---|
| BMR Formula | Can use various formulas (e.g., Harris-Benedict, Mifflin-St Jeor). | Specifically uses the Mifflin-St Jeor equation for higher accuracy. |
| TDEE Calculation | Manually calculated by applying a static activity multiplier to BMR. | Starts with an initial TDEE based on user input, but dynamically updates with daily exercise. |
| Daily Adjustments | Static; the daily calorie goal remains the same unless manually recalculated. | Dynamic; the calorie budget increases in real-time as exercise is logged or synced. |
| Tracking Accuracy | Dependent on the user's diligence and accurate logging. | Dependent on user input, database accuracy (which can vary), and proper portion sizing. |
| Weight Goal | Requires manual calculation of daily deficit/surplus. | Automates the deficit or surplus calculation based on the user's weekly weight goal. |
Best Practices for MyFitnessPal
For the most accurate experience, users should adhere to a few best practices:
- Log Consistently: Even small snacks and drinks should be logged to get a full picture of your intake.
- Use Precise Measurements: Weighing food with a kitchen scale is more accurate than estimating portion sizes.
- Sync Devices: Linking fitness trackers allows for automatic exercise calorie adjustments, providing a more accurate net calorie budget.
- Double-Check Entries: Community-added food items can sometimes be inaccurate, so verify entries against nutrition labels when possible.
- Update Your Weight: Regularly updating your current weight is crucial, as the app will automatically recalculate your BMR and goals over time.
Conclusion
MyFitnessPal's system for calculating calorie goals is a robust, formula-driven process that relies on a user's health profile, activity level, and weight aspirations. By utilizing the respected Mifflin-St Jeor equation and offering dynamic adjustments for exercise, the app provides a highly personalized and adaptable framework for calorie tracking. While no digital tool can be perfectly precise, MyFitnessPal offers a powerful starting point and valuable insights for anyone pursuing fitness or nutrition goals. By understanding the method behind the calculations and logging diligently, you can maximize the effectiveness of the app. For additional nutritional information, consult the resource provided by the Cleveland Clinic on calculating your BMR.
