Fat loss is one equation: energy balance, deficit size and patience

Guides · Fat loss

Strip away the diets with names, the "one weird trick" videos and the supplement aisle, and fat loss comes down to a single idea: over time, you have to spend a little more energy than you take in. That's it — that's the whole equation. Everything else in this guide is really about the three things that decide whether that simple equation actually works for you: how big a gap to run, how fast to expect the scale to move, and how much patience the process needs when the numbers stall.

This is a deliberately deep, evergreen guide rather than a marketing piece. We'll build up the science carefully, correct the myths that make people quit a plan that was working, and finish with a worked example that puts a sensible deficit and a crash diet side by side so you can see exactly what you trade away by rushing. Where a number is genuinely textbook-established we'll give it — always as an estimate, never as a promise — and where the honest answer is "it varies," we'll say so.

1. The one equation: energy balance

Fat loss is governed by energy balance — the difference between the energy you take in from food and drink and the energy your body spends over a period of time. Run a gap on the spending side often enough (a deficit), and your body makes up the shortfall from its own stores. Over weeks and months, and with the right supporting habits, most of what it draws down is body fat. This is an application of a physical law — energy is conserved — and it is not seriously disputed in the physiology literature. Every genuinely effective approach to fat loss, whatever it's called, works by creating this gap; the names and rules are just different routes to the same equation.

What "energy out" is actually made of

"Burn" isn't one number — it's the sum of several, and knowing the parts explains almost everything that follows:

• Resting metabolic rate (RMR, sometimes BMR) — the energy just to keep you alive at rest. For most people this is the largest slice of the total (commonly cited as roughly 60–75%, approximate and dependent on body size and composition). It's closely tied to how much fat-free mass you carry.
• The thermic effect of food (TEF) — the energy spent digesting and processing what you eat, averaging around 10% of the calories consumed on a mixed diet.
• Movement — both deliberate exercise and NEAT, the non-exercise activity of ordinary life: walking, standing, fidgeting, posture. NEAT is the most variable component, differing hugely between people and within one person day to day.

Two consequences matter for fat loss. First, because RMR dominates, most of your daily burn happens whether or not you train — exercise adds to the total and brings large independent health benefits, but you cannot reliably out-train an intake that's simply too high. Second, "energy out" is not a fixed dial: it drifts with your body size, your activity and how your body responds to eating less. That's why the equation is simple but the arithmetic is slippery — a theme we return to in section 4.

2. How big a deficit to run

Once you accept that a deficit is the lever, the obvious question is how big. To size one, it helps to have a rough conversion between energy and body mass. The textbook rule of thumb is that about 7,700 kcal is roughly 1 kg of body-weight change (about 3,500 kcal per pound). It comes from the energy density of adipose tissue — which isn't pure fat, since it contains water and protein, so the figure describes tissue, not oil.

Treat that number as a planning estimate, not a law. It's a static model: it assumes your burn stays constant as you shrink, when in reality RMR falls as body mass falls and the body adapts (see section 4), so it tends to overpredict long-term loss. Use it to point yourself in a direction and set a pace — then let your actual weight trend, not the arithmetic, tell you what's really happening.

A sensible range, not a single answer

Rather than picking a fixed calorie cut, it's more robust to size the deficit relative to your own maintenance level. A widely used, sensible range is roughly 15–25% below maintenance, with leaner people and those with less to lose usually staying toward the smaller end, and people with more body fat able to tolerate the larger end more comfortably. In everyday terms, a moderate deficit often lands somewhere around 300–500 kcal per day for many people — enough to move the scale steadily without wrecking training, recovery or adherence.

The single most important principle here is counter-intuitive: the best deficit is the one you can actually sustain, not the biggest one you can bear for a fortnight. A moderate gap you can hold for months, while training and recovering well, routinely produces more total fat loss than an aggressive one you abandon — a point we make concrete in the worked example.

3. A realistic rate of loss

With a sensible deficit set, what should the scale actually do? A widely cited, evidence-informed target for fat loss is roughly 0.5–1% of body weight per week. That deliberately scales to the person: a heavier body can lose more in absolute terms at the same percentage, while a lighter or leaner person should aim toward the gentler end to protect muscle.

These are approximate ranges to aim around, not a schedule to hold yourself to week by week — and the leaner you get, the more it makes sense to sit at the slower end.

The first week or two lie to you

Almost everyone loses faster at the start, and it feels wonderful — but a large share of that early drop is water and glycogen, not fat. Your body stores carbohydrate as glycogen, and each gram of glycogen holds roughly 3 grams of water (a textbook approximation). Start a deficit or cut carbs, and you shed glycogen and its bound water fast, producing a dramatic first-week number that overstates true fat loss. Re-feed and some of it returns — which is why a single high-carb day can look like a "gain" when no fat was added. Judge your rate on the trend over several weeks once that initial water shift has settled, not on the exciting first reading.

4. Why plateaus happen — and what they aren't

People expect a smooth, straight-line descent. Real fat loss is noisy and stepwise — a staircase with flat stretches. Flat spells of one to two weeks, sometimes longer, are normal and do not mean fat loss has stopped. It helps to separate two very different things that both get called a "plateau."

Water plateaus: the fat is still leaving

Most short "plateaus" are really water masking a real deficit. As you lose fat you can simultaneously retain water for unrelated reasons — a salty meal, hard or novel training, stress, poor sleep, or menstrual-cycle hormones. Fat leaves while water arrives, so the scale holds flat even though fat is genuinely dropping; when that water later clears, the scale can fall several hundred grams seemingly overnight. The fat was leaving the whole time — only the water was hiding it. This is exactly why weighing under consistent conditions and following a trend, not a single morning, matters so much. It's also why Strathlon's cycle-aware view exists: a temporary luteal-phase bump should read as water, not lost progress.

Real plateaus: your maintenance moved

A genuine, multi-week plateau usually has a physiological cause worth understanding rather than fearing. Two things quietly shrink the gap you set:

• A lighter body burns less. As body mass falls, RMR falls too — there's simply less tissue to maintain. The 500 kcal deficit you calculated at your starting weight is a slightly smaller deficit at your new, lighter weight, because your maintenance has dropped underneath you.
• Adaptive thermogenesis. Beyond that expected RMR drop, the body can reduce expenditure somewhat more than predicted during sustained restriction — partly by quietly cutting NEAT (you fidget and move less without noticing), partly through hormonal changes. Its magnitude is individually variable and commonly overstated online: it slows loss and makes maintenance harder, but it does not halt fat loss or make a true deficit add fat.

Because your maintenance keeps shifting as you get lighter, Strathlon re-centres your targets on a rolling ~7-day trend of your weight rather than a single morning's reading — so as your real weight drifts down, your calorie and protein targets follow it, tracking genuine change while ignoring day-to-day water noise.

The protectable lever inside all of this is muscle. Because RMR tracks fat-free mass, losing muscle lowers your burn and deepens the plateau, while keeping muscle defends it. Two habits reliably bias your body toward losing fat while sparing muscle: eating enough protein — commonly cited around 1.6–2.4 g/kg of body weight per day when active or dieting — and doing resistance training. Maintaining your lifts in a deficit is one of the best everyday signs that the weight you're losing is fat, not muscle.

5. Diet breaks and maintenance phases

Because a deficit gets harder the longer it runs — adaptation grows, hunger climbs and adherence frays — one of the most useful tools for a longer fat-loss phase is the planned break. A diet break is a deliberate stretch, typically one to two weeks, spent eating at roughly maintenance — not a free-for-all, and not a deeper cut, but a genuine pause at your current calorie balance before resuming the deficit.

Structured diet breaks tend to help in a few honest, overlapping ways: they give a psychological reset that supports long-term adherence, they can ease some of the hormonal and behavioural pressure that builds during a sustained deficit, and by restoring glycogen they often relieve the "flat," under-fuelled feeling that drags down training. Some research — the MATADOR study is the one usually cited — has found intermittent dieting with maintenance breaks encouraging for fat loss and muscle retention compared with a continuous deficit of the same total length. It's worth being even-handed about that evidence: it's promising but not magic, the effect is not guaranteed for everyone, and the clearest benefits tend to concentrate in people with more to lose over a longer diet. A break's biggest, most reliable payoff is simply that it makes a long fat-loss phase easier to finish.

A gentler cousin is the maintenance phase: once you've reached a goal, deliberately eating at maintenance for a while lets your body — and your habits — settle at the new weight before you decide whether to lose more. It's also the natural landing point when a structured plan ends, rather than snapping straight back to old eating. If you want the app's take on that transition, see what happens when your plan ends.

6. Why crash diets backfire

A crash diet — a very large deficit, often paired with very low protein and little or no resistance training — is the mirror image of everything above. It looks efficient because the early scale drop is steep, but that speed is bought with costs that tend to make it slower, or worse, over any real timeframe:

• A bigger share of the loss is muscle and water, not fat. Very aggressive deficits, especially without enough protein and resistance training, increase the fraction of weight lost as fat-free mass. You end up lighter but softer — and with less muscle, your RMR is now lower too.
• Your maintenance drops harder. Losing muscle and provoking stronger adaptation both push your burn down, so the very approach that felt fast quietly makes future loss and maintenance harder.
• Training, recovery and mood suffer. Chronic under-fuelling tends to sap strength, endurance and recovery — undermining the training that would have protected your muscle in the first place — and it's simply miserable to sustain.
• Rebound and weight cycling. Intense restriction ramps up hunger hormones (ghrelin up, leptin down) and is hard to hold, so it often ends in a rebound. Repeated cycles of aggressive loss and regain — "weight cycling" — tend to leave people no better off, and often with a less favourable body composition than before.
• Genuine clinical risk at the extremes. A chronic large shortfall relative to training load is associated with low energy availability and, in athletes, Relative Energy Deficiency in Sport (RED-S), which can affect hormones, bone and menstrual function. This is a real reason to be cautious, and a point where professional guidance matters most.

None of this means fast loss is always harmful for everyone — under medical supervision, some people do use faster approaches for specific reasons. But as a default, self-directed strategy, a crash diet trades away muscle, performance, adherence and durability for a fast start that rarely lasts.

7. Worked example: 500 vs 1,000 kcal a day

This is the heart of the guide, because it makes the trade-off concrete. Picture two people at the same starting weight, both wanting to lose fat. One runs a sensible ~500 kcal/day deficit; the other decides to "get it over with" and runs a ~1,000 kcal/day crash deficit. Using the planning estimate of ~7,700 kcal per kilogram:

Read the arithmetic honestly and the crash plan looks tempting: on paper it loses about twice as much per week. Here's the catch the paper hides — doubling the deficit doesn't double the fat loss. The extra weight the crash dieter drops is disproportionately muscle and water, not fat. The ~500 kcal dieter's roughly half-a-kilo a week is mostly fat, and because they're eating enough to train, their muscle — and therefore their RMR — is largely protected.

Now play it forward. The sensible dieter can hold their plan for months, keeps their strength, and finishes leaner with their metabolism intact — and because the approach is livable, they're far more likely to keep the result. The crash dieter loses fast, then hits stronger hunger, worse training, a lower burn from lost muscle, and — very often — a rebound that gives much of it back. Over any horizon that matters, the tortoise routinely beats the hare: the smaller deficit tends to deliver more fat lost, better kept.

8. The misconceptions, corrected

Most fat-loss frustration comes from a handful of persistent myths. Here they are, plainly corrected:

• "Eat as little as possible to lose faster." Beyond a moderate deficit you mostly buy muscle loss, worse training, stronger hunger and a higher chance of quitting. The best deficit is the sustainable one — usually around 15–25% below maintenance.
• "Fast loss now, worry about keeping it off later." The way you lose weight shapes whether you keep it. Crash approaches tend to shed muscle and provoke rebound, so "later" often means regaining it. Losing at a livable pace, with protein and training, is what makes the result stick.
• "The scale went up overnight, so I gained fat." Gaining a meaningful amount of fat overnight from one meal isn't physiologically plausible; a same-day rise is essentially always water and food weight. Judge the multi-week trend, not yesterday versus today.
• "I've stalled for a few days — my metabolism is damaged." A few days is water noise, and adaptation is a partial offset that slows loss, not a broken metabolism. A true deficit still mobilises fat.
• "Any break from the diet ruins my progress." A planned week or two at maintenance is a legitimate, evidence-informed tool that often makes a long fat-loss phase more successful, not less.
• "Some foods have negative calories, so they burn more than they contain." No food does; the energy cost of digesting food (TEF) is always a fraction of the food's energy, never more than it.

9. The bottom line, and how Strathlon helps

• It's one equation. Spend a little more energy than you take in, consistently, over time. Everything else is a detail of how you run that gap.
• Run a moderate deficit. Roughly 15–25% below maintenance — often around 300–500 kcal a day — sized to how much you have to lose, not to how impatient you feel.
• Expect a modest, steady rate. Around 0.5–1% of body weight per week once the early water drop settles. Judge it on the multi-week trend.
• Protect muscle. Enough protein (commonly ~1.6–2.4 g/kg when active) plus resistance training keeps your loss biased toward fat and defends your metabolism.
• Treat plateaus as information. Short ones are water; long ones mean your maintenance moved. Neither means the equation stopped working.
• Use breaks on purpose. A week or two at maintenance can make a long phase easier to finish — and easier to keep.
• Choose patience over the crash. The smaller, sustainable deficit tends to lose more fat, and keep it, than the aggressive one.

Where it's honest to say so, Strathlon already turns much of this into numbers you can glance at rather than calculate. It sets your targets around a moderate deficit by default rather than an extreme one, and re-centres them on a rolling ~7-day trend of your weight rather than a single morning's reading, so your calories and protein follow real change and not daily noise; it shows your energy balance and weight trend together, so you learn to read the multi-week signal instead of panicking at a single morning; it gives your protein target a number behind the "protect your muscle" advice; and its cycle-aware view helps a temporary water bump read as what it is. The app is there to make patience easier — to show you the equation is working even on the days the scale pretends it isn't. But the fundamentals on this page hold with or without any app: a moderate gap, protein and training, and time.

This is general educational information, not medical, dietetic or psychological advice. Individual variation is large, and figures such as the ~7,700 kcal ≈ 1 kg estimate, the 15–25% deficit range, the 0.5–1% per week rate and the 1.6–2.4 g/kg protein guide are approximate, evidence-based estimates rather than exact prescriptions. Anyone with a medical condition (for example a thyroid condition, PCOS, diabetes or heart disease), anyone who is pregnant or breastfeeding, anyone with a history of disordered eating, and any athlete concerned about under-fuelling or RED-S should consult a qualified professional before changing how they eat or train. See our Terms for more.

← All guides · Eating for results · Energy balance · Understanding your stats · User guide · Home