MuscleTiger research notes · No. 2

How a daily calorie target should be set: what is supported, what is convention

November 2026 · 12 min read · ZeroKnots Technologies

Every calorie calculator ends with one number. Behind that number are several separate decisions: how much you burn, how fast you should lose weight, how low is too low, and how to split the calories into protein, fat and carbohydrate. Some of these decisions are well supported by research. Others are somebody's convention.

Looking at the pieces one by one is useful, because their quality is very different. The equation for resting energy is solid. The multipliers for activity are ranges, not exact values. The recommended pace of weight loss comes mostly from studies of lean athletes. The lowest "safe" calorie levels are a tradition. Protein is well studied, while fat and carbohydrate are given as wide ranges.

In this note we go through these decisions in the order a calculator makes them, say what the evidence supports and where we made a choice ourselves, and explain why macros need limits at low calorie goals. A table near the end summarises how well each decision is supported. This is the second note in the series; the first one, on weight forecasts, explains why a calorie target should be treated as a starting point rather than a promise. Weights are in kilograms with pounds in brackets.

1. How much you burn

Resting energy

Most calculators start with the Mifflin–St Jeor equation, derived from measurements on 498 healthy adults aged 19 to 78 ^[1]. In its simplified form, resting energy expenditure is

where $m$ is body weight in kg, $h$ is height in cm, $a$ is age in years, and $s$ is a constant that depends on sex. The authors found that this simplification did not reduce the accuracy of the full fit ^[1].

A systematic review by Frankenfield et al. ^[2] compared the common equations and found that Mifflin–St Jeor predicted resting metabolic rate within 10% of the measured value in more non-obese and obese people than any other equation. The same review warns that errors for individual people can still be noticeable. In the first note we put such errors in perspective: a few per cent of daily expenditure is about a slice or two of bread.

From resting to total energy

Total daily expenditure is resting energy multiplied by a physical activity level (PAL):

The FAO/WHO/UNU expert consultation ^[3] gives ranges for typical lifestyles: roughly 1.40 to 1.69 for sedentary or light activity, 1.70 to 1.99 for active, and 2.00 to 2.40 for vigorous, noting that values above 2.40 are difficult to maintain over a long period. Our activity levels lie inside these bands; the exact value within each band is our own choice, and in the app it is derived from your step history when Apple Health is connected.

A small correction while dieting

Expenditure measured during a diet tends to come out lower than a formula predicts. Leibel et al. ^[8] found that after people kept their weight at least 10% below their usual level, total expenditure fell by about 6 kcal per kilogram of fat-free mass per day in people who had never been obese, more than the loss of mass alone explains. While the goal is weight loss we therefore reduce the estimate by a few per cent. The size of the correction comes from a commercial tracker's analysis of its own users, not from a controlled study, and the sources disagree on how such adaptation develops. Roberts et al. ^[4], citing Leibel, report that non-exercise activity falls by about 262 kcal/day after a 10% loss and by about 500 kcal/day after 20%, which depends on how much you have lost rather than how fast. Breit et al. ^[9], using the gold-standard method, measured only about 4% lower expenditure in the first month, partly recovering later. Our constant correction is closer to the second picture.

2. How fast to lose weight

The common clinical advice is a deficit of 500 to 1,000 kcal per day, which corresponds to roughly 0.45 to 0.9 kg (1 to 2 lb) per week ^[10]. Athletes who want to keep muscle are told to go slower. Roberts et al. ^[4] review the evidence for physique athletes in contest preparation and conclude that rates of 0.5% of body mass per week or less are preferable for limiting the loss of fat-free mass.

The best-known supporting study is Garthe et al. ^[5]: 24 elite athletes who trained with weights four times a week were put on either a slower or a faster diet. The slower group planned to lose 0.7% of body mass per week (achieved 0.7 ± 0.8%) and gained about 2.1% in lean mass. The faster group planned 1.4% per week (achieved 1.0 ± 0.4%) and did not gain lean mass (−0.2%). Both groups lost about the same amount of weight, around 5.5% of body mass; the slower diet simply took longer (8.5 weeks against 5.3). Slower is not less effective, it is a trade of time for muscle.

We express the pace as a share of body mass per week, between about 0.5% and 1%, with a faster pace allowed when body mass index is higher and there is more fat to lose. The evidence behind this is from lean athletes, so applying it to the general population, and tying the rate to body mass index, is our extrapolation. The daily deficit follows directly from the rate and the energy density of weight change from the first note:

Here $\rho_{\text{loss}}$ is the target weekly loss as a share of body mass, $m$ is body weight (kg), and $\varepsilon = 9{,}100$ kcal/kg (4,100 kcal/lb) is the effective energy density of a change in body weight; dividing by 7 converts a weekly amount into a daily one. For example, a man weighing 80 kg (176 lb) aiming at 0.5% per week loses 0.4 kg (0.9 lb) a week, which needs a deficit of about 0.4 × 9,100 / 7 ≈ 520 kcal per day.

That example is worth a remark. Murphy and Koehler ^[6], in a meta-analysis of people doing resistance training, advise avoiding deficits larger than about 500 kcal per day if the aim is to preserve lean mass. At the lower end of our pace range we are right at that edge, and the slower you go, the better for muscle. If you lift weights and want to protect muscle, the lower end of the range, or below it, is the sensible choice.

The same logic runs in the other direction for people who want to gain weight. We add a modest surplus of a few hundred kcal above maintenance (roughly a tenth of it). This is our choice, not a measured optimum: the evidence for the best surplus is limited, and a larger one mostly adds fat rather than speeding up muscle gain.

3. How low is too low

Two limits keep a calorie target from becoming extreme. The first is a cap on the daily deficit, which in our case is 1,000 kcal. It matches the upper end of the general clinical advice ^[10], and it is twice the limit that the meta-analysis above suggests for preserving lean mass ^[6]. We regard it as a safety limit, not as an optimum.

The second is an absolute floor on the daily calorie goal. The clinical guideline reproduced in ^[10] lists 1,000 to 1,200 kcal per day for women and 1,200 to 1,500 kcal per day for men as acceptable low-calorie diets. We use 1,200 and 1,500 kcal, the upper end of these ranges, and in addition never go below the resting energy of a person of normal weight for your height. These floors are a convention and may not suit every body size.

A fixed floor has one weakness: it ignores how big you are and how much you train. Researchers use a measure called energy availability: the calories you eat minus the calories you spend on training, divided by your fat-free mass (everything in the body except fat). Roberts et al. ^[4] note that for female athletes it should stay at about 30 kcal per kilogram of fat-free mass per day (14 kcal per lb) or more, to avoid menstrual disturbances. As an illustration, a woman who weighs 60 kg (132 lb) and carries about 45 kg (99 lb) of fat-free mass needs 45 × 30 = 1,350 kcal a day left over after training. If training burns 400 kcal, she needs to eat about 1,750 kcal, well above a 1,200 kcal floor.

What are menstrual disturbances, and why do they matter? They mean irregular or missing periods. They are the visible sign that the body, short of energy, has switched off functions it can do without for now, including the reproductive hormones. Roberts et al. ^[4] report that such disturbances are common in female physique athletes during contest preparation. For someone who is dieting, a cycle that becomes irregular or stops is therefore a warning, not a harmless side effect, and the sensible response is to eat more and to talk to a doctor.

We have built this check into the calorie goal. For women, the goal cannot fall below

where $\mathrm{FFM}$ is fat-free mass in kg and $E_{\text{training}}$ is the average daily energy of your training. We estimate fat-free mass from height and weight with the Boer equation ^[11], because the app does not measure body composition, and we estimate training energy from your onboarding answers (sessions per week and their length). The goal is never pushed above maintenance, so the floor can only remove a deficit, never create a surplus. For most people the floor does not bind, because their goal is already above it. It matters for women who are tall or heavy, who train a lot, or whose pace of loss is high: for them it reduces the deficit. We apply it to women because the 30 kcal/kg figure comes from studies in women.

4. Protein, fat and carbohydrate

Protein

Protein has the strongest evidence of the three. Roberts et al. ^[4] support 1.8 to 2.7 g per kilogram per day (0.8 to 1.2 g per lb) during contest preparation, and describe a converging range of 1.6 to 2.7 g/kg/day across research groups. Morton et al. ^[7] pooled 49 trials with 1,863 participants and found that intakes above about 1.6 g/kg/day did not add further to gains in fat-free mass from resistance training. During a deficit a higher intake helps to retain lean mass, which is why the recommendations for dieting athletes sit higher. In a four-week study with a 40% energy deficit, 2.4 g/kg preserved more lean mass than 1.2 g/kg (Longland et al., as reported in ^[4]).

We use about 2 g/kg (0.9 g/lb), slightly lower when the goal is maintenance or gaining weight. For people far above their target weight we base it on the target weight rather than the current one, and we limit protein to a share of total calories, so that at low calorie goals it cannot crowd everything else out.

Fat and carbohydrate

For fat and carbohydrate the evidence gives ranges rather than numbers. For contest preparation Roberts et al. ^[4] suggest fat at 10 to 25% of calories (athletes in general are observed at 20 to 35%) and carbohydrate at a realistic 2 to 5 g/kg (0.9 to 2.3 g/lb), noting that very low carbohydrate during a deficit can impair training. They also caution against very low fat intakes. The calories left after protein and fat go to carbohydrate:

where $G$ is the calorie goal, $P$ and $F$ are protein and fat in grams, and the factors 4 and 9 are the calories per gram of protein or carbohydrate and of fat.

Why macros need limits

Rules in grams per kilogram work well at normal calorie levels and break at low ones. At a low goal, fixed amounts of protein and fat can use up almost the whole budget and leave nothing for carbohydrate, or even add up to more than the goal. So it is safer to set protein in grams per kilogram, limit protein and fat to a share of the calories, and keep a floor under carbohydrate of about 2 g per kilogram (0.9 g/lb), taking the calories from fat when needed. The sum of the macros then never exceeds the calorie goal. The exact limits are our choice within the published ranges.

Results

Conclusions

• The resting-energy equation is well supported, and its error is small in daily terms. Treat the calorie goal as a starting point and adjust it from your own weigh-ins.
• If you lift weights, slower is better for muscle. The evidence points to about 0.5% of body mass per week or less, and to deficits of around 500 kcal per day or less.
• The lowest calorie levels are a convention. They are reasonable, but they do not replace judgement about your body, training and health.
• Protein is the part with the strongest evidence. About 2 g/kg (0.9 g/lb) in a deficit is well within the supported range.
• Do not let carbohydrate fall too low while you train hard in a deficit. A floor of about 2 g/kg (0.9 g/lb) is the lower end of the range that the research treats as realistic.
• Women who train should not let the goal fall below their energy availability. That is about 30 kcal per kg of fat-free mass plus the energy of training. Irregular or missing periods are a warning to eat more and see a doctor.

Limits of this note. Most of the evidence on pace and macros comes from lean athletes and from short studies, and was applied to a general population by us. Individual needs differ. This is general information, not medical advice; if you have a medical condition, are pregnant, or have a history of disordered eating, talk to a doctor before restricting calories.

Sources

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2. Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc 2005. doi:10.1016/j.jada.2005.02.005
3. FAO/WHO/UNU. Human energy requirements: report of a joint expert consultation. FAO Food and Nutrition Technical Report Series 1, 2004; chapter 5, Table 5.3. fao.org
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