Weight loss · 8 min read

Why weight loss slows down, and what actually helps

The 3,500-calorie rule says your loss should be a straight line. Your body never read it. Here’s what really happens as you lose weight, with the numbers.

Matt StevensonFounder of CutWise

Running shoes resting on a sunlit stone staircase that levels out into a wide landing

Almost every diet plan you’ve seen is built on one sum. A pound of fat holds about 3,500 kcal (7,700 per kilo), so eat 500 kcal a day less than you need and you’ll lose a pound a week, every week, for as long as you keep going. It’s tidy, it’s easy to remember, and it’s wrong in a way that causes a lot of people to give up just as things are working.

What the 3,500-calorie rule gets wrong

The number itself is fine. What’s wrong is the assumption that your body stays the same while you lose weight. It doesn’t. As you get lighter, you need less energy: a smaller body costs less to run and less to move around. So the deficit that worked at the start gets smaller as you go, even if you eat exactly the same.

Researchers led by Kevin Hall at the US National Institutes of Health built a model of this from real metabolic data. Their rule of thumb for an average overweight adult: every 24 kcal a day you cut from your intake eventually lowers your weight by about a kilo, and it takes roughly a year to get halfway there.1 Commentaries in the International Journal of Obesity have since shown how far the old straight-line rule overestimates real weight loss.2

Fig. 1A 500 kcal daily deficit, predicted two ways

0−10 kg−20 kg−30 kgstart6 months1 year18 months2 yearsthe 3,500-calorie ruleallowing for a lighter bodyabout −10 kg at one yearrule says −24 kgsame 500 kcal a day less than you started out needing
The 3,500-calorie rule predicts about 24 kg lost in a year and never stops. Allowing for a lighter body needing less predicts about 10 kg in the first year, levelling off towards 21 kg. Simplified from Hall et al.’s model; your first few weeks will drop faster than either line, from water.
Slowing down isn’t a sign that something broke. It’s what successful weight loss looks like on a graph.

Three things that look like a plateau

When the scale stalls, it’s almost always one of three things, and they need different fixes.

1. It isn’t a plateau yet

Daily weight swings by a kilo or more from water, salt, carbohydrate and your cycle. A real loss of half a kilo a week can hide inside that noise for a fortnight. If it’s been less than three weeks, look at the trend, not the readings (we explain why in why your weight jumps around).

2. Logging has drifted

This is the most common one, and nobody notices it happening. Portions creep up. The weekend gets looser. Cooking oil, milk in coffee and the bites while you cook stop making it into the log. In one well-known study, people who were sure they were eating very little were in fact under-reporting by nearly half.4 Nobody was lying; this is just how memory works.

3. You’ve reached a new maintenance

If you’ve lost a fair amount of weight, your body now genuinely needs less. Using Hall’s figure, being 10 kg lighter means needing roughly 240 kcal a day less, so a deficit of 500 at the start is now about 260. On top of that, many people unconsciously move less when they eat less: fewer steps, less fidgeting, more sitting.

Fig. 2Same food, smaller deficit

At the startneeds 2,600 kcaleats 2,100deficit 50010 kg lighterneeds 2,360 kcaleats 2,100deficit 260
An illustrative person who started out needing 2,600 kcal a day. Ten kilos later they need about 240 kcal less, so the same 2,100 kcal of food creates roughly half the deficit it used to.

That second effect, the unconscious movement, varies enormously between people. When researchers at the Mayo Clinic overfed volunteers by 1,000 kcal a day for eight weeks, some burned off a large part of the surplus by moving more without meaning to, while others barely changed, and fat gain varied about tenfold between them.3 The same system runs in reverse when you diet, which is why some people stall sooner than others on paper identical plans.

What actually helps

  1. Wait for the trend. Give any stall three to four weeks of consistent logging before you call it one.
  2. Audit a week honestly. Scan packaged food, weigh a few of your usual portions, and log every oil, drink and snack for seven days. Compare the result with what you thought you were eating. This alone breaks most plateaus.
  3. Recalculate for the body you have now. Your target should come from your current weight, not your starting weight. Recalculate after every 5 kg or so.
  4. Protect your steps. Everyday movement is the part of your energy use that drops quietly when you diet, and it’s the easiest to keep up deliberately. Track your daily steps and keep them where they were when you started.
  5. Keep protein up and keep lifting. Muscle is what you want to keep. Plenty of protein and some resistance training help the weight you lose come from fat.
  6. Consider a planned break. In the MATADOR trial, men who alternated two weeks of dieting with two weeks at maintenance lost more fat over the same total time spent dieting, and kept more of it off, than men who dieted continuously.5 It’s one trial, in men, but a planned fortnight at maintenance is a reasonable tool, and much easier to stick to than grinding on.

Sometimes the plateau is the win

At some point the most useful thing you can do is stop losing on purpose. Spend a few weeks eating at your new maintenance. You learn what that amount of food looks like, the hunger settles, and you prove to yourself you can hold the weight you’ve lost, which is the part most diets never practise.

If you want to see what your own numbers might look like, the NIDDK’s Body Weight Planner uses the same model as figure 1.6 And our calorie calculator gives you a fresh maintenance estimate for the weight you are today.

Sources

  1. Hall KD et al. (2011). Quantification of the effect of energy imbalance on bodyweight. Lancet 378(9793):826–837.
  2. Thomas DM et al. (2013). Can a weight loss of one pound a week be achieved with a 3500-kcal deficit? Int J Obes 37:1611–1613; and Hall KD, Chow CC (2013). Why is the 3500 kcal per pound weight loss rule wrong? Int J Obes 37:1614.
  3. Levine JA, Eberhardt NL, Jensen MD (1999). Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science 283:212–214.
  4. Lichtman SW et al. (1992). Discrepancy between self-reported and actual caloric intake and exercise in obese subjects. N Engl J Med 327:1893–1898.
  5. Byrne NM et al. (2018). Intermittent energy restriction improves weight loss efficiency in obese men: the MATADOR study. Int J Obes 42:129–138.
  6. NIDDK Body Weight Planner, based on Hall et al.’s model.

General information, not medical advice. Speak to your GP or a registered dietitian about your own needs, especially if you have a health condition.

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