FitBeyond AI

Sources

Last updated: 26 August 2026

FitBeyond AI turns a few facts about your body and your goal into a daily calorie target, macro targets, and a plan with a date on it. This page lists every recommendation the app makes, the exact number it uses, and the published source behind it.

Some of these numbers are settled science. Others are practical rules of thumb that the research qualifies or contradicts. We have listed both kinds here, and where our number and the evidence disagree, we say so rather than leaving it out.

FitBeyond AI is a general wellness app, not a medical service. The targets it calculates are estimates meant to guide you, not medical, dietary, or clinical advice, and the app is not intended to diagnose, treat, cure, or prevent any condition. Talk to a qualified healthcare professional before making significant changes to your diet or exercise, and especially if you are pregnant, breastfeeding, or managing a medical condition.

Your daily calorie target

Energy at rest

The app estimates the energy your body uses at rest with the Mifflin-St Jeor equation, from your weight, height, age, and gender. It calculates 10 × weight in kg, plus 6.25 × height in cm, minus 5 × age, then adds 5 for men and subtracts 161 for women. If you would rather not specify, it uses 78, the midpoint of the two.

Frankenfield and colleagues compared the four equations in common clinical use and found Mifflin-St Jeor landed within 10 percent of measured resting metabolism more often than any of the others, with the narrowest error range. They also found meaningful error at the level of an individual person, which is why the app treats the result as a starting point you can edit rather than a measurement of you.

Your activity level

That resting figure is multiplied by an activity factor drawn from how often you train each week. The app offers three bands, and they map to 1.375 for nought to two workouts, 1.55 for three to five, and 1.725 for six or more.

Multiplying resting energy by a physical activity level is the method set out by the FAO, WHO, and UNU expert consultation on human energy requirements. Our factors follow the convention used across clinical calculators rather than that report's own bands. Worth knowing: the report's sedentary category starts at 1.40, so the 1.375 we apply to the lowest band is a shade below its lowest published figure, and our top factor of 1.725 sits inside its active range rather than above it.

Turning a goal into a daily deficit

To convert a weekly pace into a daily calorie change, the app uses roughly 7,700 kcal per kilogram of body mass, divided across seven days. That figure traces back to Wishnofsky's 1958 estimate of the energy stored in a pound of body fat, about 3,500 kcal, and it is the arithmetic behind nearly every calorie tracker.

It is a planning approximation, not a promise. Your metabolism adapts as you lose weight, so real loss runs slower than this straight-line rule predicts, and the gap widens the longer you diet. Hall and colleagues showed body weight responds to a change in intake with a half-time closer to a year, and Thomas and colleagues found the rule over-predicted 24-week weight loss by around 11 kg where a validated dynamic model was off by 2.2 kg. We use the simple rule because it makes a plan concrete on day one. We would rather tell you its limits than imply a precision it does not have. If you want a model that accounts for adaptation, the NIH Body Weight Planner is free and public.

How fast the app lets you go, and how low it lets you eat

The app caps your weekly pace at 1 percent of your body weight, and never more than 1 kg a week whatever you weigh. The Recommended badge points at half of that, 0.5 percent a week. Ask for a faster pace and the app labels it as faster than recommended rather than hiding it. Whatever pace you choose, it never prescribes fewer than 1,200 calories a day, and the same floor applies when you edit a target by hand.

Clinical guidance puts a sustainable rate at 1 to 2 pounds, roughly 0.45 to 0.9 kg, per week, produced by a deficit of 500 to 1,000 calories a day, with an initial goal of losing about 10 percent of starting weight. The same guidance places structured low-calorie diets at 1,000 to 1,200 calories for women and 1,200 to 1,500 for men, under professional supervision. Our floor sits at the top of that supervised range, so the app on its own will never take you below it.

Body mass index

The BMI scale in the app uses the World Health Organization's adult bands: below 18.5 is underweight, 18.5 to 24.9 is healthy, 25 to 29.9 is overweight, and 30 or above is obese. That same 18.5 threshold is the floor the app clamps a goal weight to, so you cannot set a target that would place you in the underweight range for your height.

BMI is a rough guide. WHO treats it as a surrogate marker of body fatness rather than a diagnosis, and it cannot tell muscle from fat, so it can mislead for athletes, during pregnancy, and in other cases. A health professional can give you the full picture.

Protein, fat, and carbohydrate

Protein scales with your goal and with how often you train, rather than with body weight alone. Losing weight and rarely training is set at 1.4 grams per kilogram; losing weight and training six or more times a week is set at 2.0. Maintaining lowers the figure and gaining sits between the two. A deficit raises it at every activity level because eating less, not training, is what makes protein most worth protecting: it is a large part of what decides whether the weight you lose comes off as fat or as muscle.

The recommended dietary allowance is 0.8 g/kg, the amount that avoids deficiency rather than the amount that supports a goal, and no target we calculate goes below it. Sports nutrition guidance supports 1.4 to 2.0 g/kg for people who exercise, and a meta-analysis of 49 trials found gains in lean mass plateau near 1.6 g/kg with resistance training, which is why only the most active bands reach the top of that range.

Two guards sit on top. The coefficient is applied to a weight inside the healthy BMI range for your height rather than to your current weight, so the target scales with the body rather than with body fat. And protein never claims more than 35 percent of your calories, so a low target cannot end up as a plan that is mostly meat. From age 65 the figure is held at a minimum of 1.2 g/kg, since holding on to muscle gets harder with age.

An earlier version of the app set protein at a flat 2 g/kg of current body weight for everybody. That number came from the top of a range this same page scopes to people who exercise, and applying it to someone who does not train, on their current weight rather than a healthy-range one, produced targets well above anything the research here supports. It has been replaced by the bands above.

Fat is set at 25 percent of your calories, inside the 20 to 35 percent range the Institute of Medicine gives for adults, and carbohydrate takes whatever calories remain. Grams become calories with the Atwater factors: 4 kcal per gram of protein and of carbohydrate, 9 kcal per gram of fat. These general factors were derived for mixed diets and are less accurate applied to a single high-fiber food.

Fiber, sugar, and sodium

Fiber is set at 14 grams per 1,000 calories, the Institute of Medicine's adequate intake, and never shown as less than 20 grams a day.

The added sugar limit is 10 percent of your calories. WHO's strong recommendation is to keep free sugars below 10 percent of energy, with a conditional recommendation that going below 5 percent brings further benefit. One distinction worth knowing: WHO's "free sugars" is broader than "added sugars", because it also counts the sugars naturally present in honey, syrups, and fruit juice.

Here our number and our data do not line up, and it is worth saying so. The guideline is about free sugars, but the food databases we read report total sugars, which also include the sugar in whole fruit and the lactose in milk. Those are not what the guideline is aimed at. The practical effect is that a day built on fruit and dairy can read as over the limit while being a perfectly good day, so treat this one as the softest number on the page. The app says the same thing where it shows the figure.

Sodium is a flat 2,300 mg a day, the Daily Value used on nutrition labels and the limit in the Dietary Guidelines for Americans. For context, average intake in the United States runs closer to 3,400 mg, and more than 70 percent of it comes from packaged and prepared food rather than the salt shaker.

Water

The app sets a daily water target of about 35 ml per kilogram of body weight, held between 1.5 and 4 litres. This is a practical scaling rule, and we want to be straightforward that it is a heuristic rather than a clinical requirement.

The reference figures are flat daily volumes, not per-kilogram ones: the Institute of Medicine's adequate intakes are 3.7 litres a day for men and 2.7 litres for women, counting all drinks plus the water in food, of which drinks supply roughly 80 percent. There is no established per-kilogram figure for adults. Real needs move with activity, heat, and what you eat, and for healthy people thirst remains the primary signal.

Calories burned in a workout

When you log exercise, the app estimates the burn as the activity's MET value, scaled by how hard it felt, multiplied by your body weight and the duration in hours. One MET is defined as an energy cost of 3.5 ml of oxygen per kg per minute, which is about 1 kcal per kg per hour, and that definition is exactly the arithmetic the app uses. The per-activity values it starts from are the Compendium's representative figures for moderate effort, for example 3.5 for walking, 7.5 for cycling, and 9.8 for running.

The Compendium's authors are explicit that MET values do not produce a precise number for an individual, because resting metabolism varies with age, sex, size, and body composition. Treat a logged burn as an estimate, and edit it if you know better. The app always lets you type your own figure over ours.

Why the app asks you to log

Reminders, streaks, and the nudge to log a meal are not decoration. Keeping a record of what you eat is one of the better-supported behaviours in weight management. A systematic review of the literature found dietary self-monitoring consistently associated with weight loss, and a later study of electronic logging found that how often people logged, rather than how long they spent doing it, was what tracked with success. Both are observational findings, so read them as a strong association rather than a guarantee, and the CDC recommends a food diary on the same basis.

Corrections

If you think we have cited something wrongly, drawn too much from a source, or missed research that contradicts a number above, please tell us at support@fitbeyond.app. We would rather fix it than defend it.