The Gym Geek mobile app uses evidence-based methodology, drawing on scientific research and advice from public health authorities, to provide accurate daily calorie goals, suitable weight management goals and reasonable safeguards against extreme rates of weight loss or weight gain.
This document tells you how our mobile app works, and contains references to the research and public health authority advice we use.
Not intended to diagnose, treat, monitor or prevent any disease or medical condition
The Gym Geek mobile app allows users to set up a weight management goal, track calories and physical activity and track their weight progression over time. None of the information in our app is advice and you should not treat it as such.
Our app is not intended to diagnose, treat, monitor or prevent any disease or medical condition, including underweight, overweight or obesity. If you have underweight, overweight or obesity, you should always consult a doctor or another suitably qualified medical professional about your weight.
If you are pregnant, have an eating disorder or suffer from any other medical or physical condition or disability, you should not use the Gym Geek mobile app and you should not make any changes to your diet, nutrition, lifestyle, activities or exercise programmes based on information in our mobile app.
Terminology
The references our methodology is based on may use technical or scientific terms and complex language to describe weight management concepts. Our mobile app avoids using technical terms, instead choosing simpler terms that are more accessible and likely to be understood by a general audience.
Our methodology uses the following terms:
- Resting energy expenditure (REE) – The number of calories a person expends each day, without taking into account calories from digestion or physical activity. We do not refer to this concept in our mobile app.
- Total daily energy expenditure (TDEE) – The number of calories a person expends each day from all sources. In our mobile app, we refer to this as “maintenance calories”. We also refer to the concept of “metabolism” to talk about energy expenditure in general terms.
- Adaptive TDEE (“Smart calorie adjustments”) – A methodology using a statistical model to combine an initial calorie goal with information about the user’s calorie intake and weight data. In our app, we refer to this concept as “Smart calorie adjustments”.
- Standard activity factor (SAF) – A way to classify a person’s level of physical activity, from sedentary to extra active. Our app refers to the terms “typical activity level” and “daily activity level” when talking about this concept. It also provides additional context and descriptors to help users select an appropriate level.
Resting energy expenditure
When a user sets up the mobile app, they will be asked a series of questions about their age, gender, height, current weight and activity level. This information is used to calculate the user’s resting energy expenditure (REE).
REE estimates the baseline number of calories that the individual expends each day, but doesn’t take into account the impact of digestion or physical activity.
Our app estimates REE using the Mifflin-St Jeor equation, which came out of a 1990 study of 498 people across both genders, all age groups and normal weight and obese persons. The study introduced a formula based on a person’s weight, height, age and gender.
For men, the formula is 9.99 x Weight + 6.25 x Height – 4.92 x Age + 5.
For women, the formula is 9.99 x Weight + 6.25 x Height – 4.92 x Age – 161.
Mifflin-St Jeor is fairly accurate for most people, but it does not account for specific factors like muscle mass. Specifically, if you have a high muscle mass, you may find that Mifflin-St Jeor underestimates your REE. This is because muscle burns more calories than fat cells at rest. If you are overweight or obese, it’s likely that Mifflin-St Jeor will overestimate your REE.
A study published in the Journal of the American Dietetic Association concluded that the Mifflin-St Jeor equation was more likely to estimate calorie needs to within 10% compared to other equations, which is why our methodology uses this equation over others.
References:
Fabio Comana, M. S., & PES, C. Comparing Energy Expenditure Prediction Equations.
Mifflin, M. D., St Jeor, S. T., Hill, L. A., Scott, B. J., Daugherty, S. A., & Koh, Y. O. (1990). A new predictive equation for resting energy expenditure in healthy individuals. The American journal of clinical nutrition, 51(2), 241-247.
Frankenfield, D., Roth-Yousey, L., Compher, C., & Evidence Analysis Working Group. (2005). Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic association, 105(5), 775-789.
Total daily energy expenditure
While REE estimates a person’s baseline calorie expenditure, it doesn’t take into account the calories expended from digestion or physical activity. In our methodology, we multiply a user’s REE by a Standard Activity Factor (SAF) to estimate their Total Daily Energy Expenditure (TDEE).
The Standard Activity Factors are an established way to classify a person’s activity level. We use the SAF scale defined in the 1996 book Exercise Physiology by McArdle et al. In this scale, there are 5 Standard Activity Factors, each representing a greater level of physical activity. Each level corresponds to a multiplier from x1.2 to x1.9.
When the user sets up the mobile app, they will be asked to select a default activity level, corresponding to one of the 5 SAFs. This level is used to calculate the initial TDEE for each day. The user can override their activity level on each specific day from their diary. When this happens, the app recalculates the TDEE based on the selected activity level.
References:
Frankenfield, D. C., Muth, E. R., & Rowe, W. A. (1998). The Harris-Benedict studies of human basal metabolism: history and limitations. Journal of the American Dietetic Association, 98(4), 439-445.
McArdle, W., Katch, F., & Katch, V. (1996). Exercise Physiology 4th edition Baltimore: Williams and Watkins.
Initial calorie goal
When a user sets up the app for the first time, the app will show the initial calorie goal. This goal is either a single number of calories to eat, or a range of calories, depending on the user’s chosen weight goal.
If the user chooses to maintain their current weight, the initial calorie goal is a single number set to the user’s TDEE.
If the user chooses to lose weight, a range of calories is shown. The upper end of this range is the user’s TDEE. The lower end of this range is between 278 and 1,112 calories below the upper end, corresponding to 0.25 kg to 1 kg per week of weight loss; or 250 and 1,000 calories below, if the user enters their target weight in pounds (lb) or stone and pounds (st lb), corresponding to 0.5 lb to 2 lb per week of weight loss.
If the user chooses to gain weight, a range of calories is also shown. The lower end of this range is the user’s TDEE. The upper end of this range is between 278 and 1,112 calories above the lower end, corresponding to 0.25 kg to 1 kg per week of weight gain; or 250 and 1,000 calories above, if the user enters their target weight in pounds (lb) or stone and pounds (st lb), corresponding to 0.5 lb to 2 lb per week of weight gain.
The calorie deficits and surpluses come from the Wishnofsky study which found “[with an adequate protein intake,] the energy equivalent of 1 lb. bodyweight gained or lost is 3500 Cal”. So for example, to lose 1 lb per week, the daily deficit is 3500/7 = 500.
If the calorie goal or range is below 1,200 calories per day for women or 1,500 calories per day for men, the goal is narrowed to ensure it is always above these safe minimums. This is inline with the Healthy Eating Plan recommendations from the National Heart, Lung and Blood Institute.
References:
Wishnofsky, M. (1958). Caloric equivalents of gained or lost weight.
Koliaki, C., Spinos, T., Spinou, Μ., Brinia, Μ. E., Mitsopoulou, D., & Katsilambros, N. (2018, September). Defining the optimal dietary approach for safe, effective and sustainable weight loss in overweight and obese adults. In Healthcare (Vol. 6, No. 3, p. 73). Multidisciplinary Digital Publishing Institute.
National Heart, Lung, and Blood Institute. (2019). Healthy Eating Plan. Nih.gov. https://www.nhlbi.nih.gov/health/educational/lose_wt/eat/calories.htm
Phase in
Our app supports a feature called “phase in” which allows the user to gradually increase their weight goal over 1, 2 or 4 weeks. The resulting calorie goal is a range between lower and upper, that varies on each day of the phase in period.
If the user chooses to lose weight, on day 1 lower is set to the user’s TDEE. In other words, they will not attempt to lose weight on day 1. Then over 7, 14 or 28 days, lower is reduced by the same amount each day until it equals the initial calorie goal.
If the user chooses to gain weight, on day 1 upper is set to the user’s TDEE. In other words, they will not attempt to gain weight on day 1. Then over 7, 14 or 28 days, upper is increased by the same amount each day until it equals the initial calorie goal.
Phase in improves the user experience, avoiding the need for the user to manually increase or decrease their weight goal over time. Because the resulting calorie range is always smaller than the initial calorie goal, it’s use is supported by the same methodology.
Calorie goal behaviors
When the user first sets up the app, they can select a goal from losing 0.25 kg – 1 kg (0.5 lb – 2 lb) per week, maintaining their current weight or gaining 0.25kg – 1 kg (0.5 lb – 2 lb) per week.
This weight goal is used to calculate the user’s initial calorie goal and adjusted calorie goal. However, when the user records a new weight record, the behavior of the calorie goal may change in the following circumstances:
- BMI falls below 18.5 – If user selected a weight gain goal, the new calorie goal is calculated based on this goal. However, if the user selected a weight loss or maintenance goal, the new calorie goal is calculated on gaining 0.25 kg (0.5 lb) per week. This is communicated to the user.
- Reached goal – If the user reaches their weight loss or weight gain goal, or is within 1.5 kg of their target weight, the new calorie goal is calculated based on maintaining their existing weight.
- Overshot goal – If the user loses more weight than expected, after a 1.5 kg tolerance, the new calorie goal switches to gain 0.25 kg (0.5 lb) per week. If the user instead has a weight gain goal and gains more than expected, the new calorie goal switches to lose 0.25 kg (0.5 lb) per week.
- Not maintaining – If the user selected a maintenance goal, and is more than 1.5 kg above or below their target weight, the new calorie goal switches to lose/gain 0.25 kg (0.5 lb) per week. Once the user’s weight comes back within tolerance, the new calorie goal switches back to maintenance.
Adaptive TDEE (“Smart calorie adjustments”)
Adaptive TDEE is a method of using a person’s actual recorded weight and daily calorie intake data to estimate their TDEE. Based on Wishnofsky’s rule, a person’s TDEE can be estimated using the formula (Total calorie intake) – 7700 * (Difference in weight). Most adaptive TDEE methodologies use this formula.
Using this method directly requires a large amount of user data. In situations where there is a limited amount of user data (i.e. just after onboarding), this method would be vastly less accurate than the Mifflin-St Jeor estimate.
Gym Geek uses a statistical model to combine the Mifflin-St Jeor equation with the user data. In this model, the initial calorie goal is weighted by K calories and the adaptive TDEE estimate is weighted by the total number of calories tracked.
To improve the user experience, the data is also divided into multiple time periods, with weightings exponentially decaying by a factor γ. This ensures that the most recent user data is taken into account.
K and γ are chosen by Gym Geek empirically, based on real-world user data, to account for the statistical variation in the estimate.
The adaptive TDEE calculation is performed whenever the user records a new weight.
References:
Wishnofsky, M. (1958). Caloric equivalents of gained or lost weight.
Body mass index (BMI)
When setting a target weight, our app calculates the user’s Body Mass Index (BMI). If the BMI is below 18.5, the app displays an error message to the user and will not allow the user to save the target weight.
The app will allow a user to enter a current weight with a BMI below 18.5. When the user first sets up the app, they will be unable to select a weight loss or maintenance goal if their BMI is below 18.5. They must select a weight gain goal of at least 0.25 kg per week or 0.5 lb per week to proceed.
If the user sets up a weight loss or maintenance goal, and their BMI subsequently falls below 18.5, the app displays a message telling the user that their BMI has fallen below the healthy range. The app forcibly changes the calorie goal to gain 0.25 kg per week or 0.5 lb per week, until the user’s weight returns to the healthy range.
BMI is calculated using the formula: BMI = Weight (kg) / [Height (m)]2
The BMI categories for underweight are defined by the World Health Organization’s Healthy Lifestyle Recommendations. The 18.5 threshold for underweight is adopted by most public health bodies.
References:
World Health Organization. (2010). A healthy lifestyle – WHO recommendations. World Health Organisation. https://www.who.int/europe/news-room/fact-sheets/item/a-healthy-lifestyle—who-recommendations
