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Calorie deficit: how much less to eat to lose weight

· 5 min read

You lose weight when you take in less energy than your body uses. The gap between the two is your calorie deficit. Every diet that works for weight loss, low-carb, low-fat, fasting or anything else, works by making that gap.

This article covers how big to make the gap, what the old rule of thumb says, and why your weight loss slows even when you change nothing.

How big should the deficit be?

The NHS advises people who want to lose weight to aim for 0.5 to 1 kg a week, which is 1 to 2 lb. To turn that into calories, most people use one old rule:

1 pound of body weight ≈ 3,500 kcal
1 kilogram of body weight ≈ 7,700 kcal

The rule comes from a 1958 paper by Max Wishnofsky. On that rule, eating 500 kcal a day less than you burn makes a deficit of 3,500 kcal a week. That is about 0.45 kg, or 1 lb, a week. A deficit of 1,000 kcal a day doubles it.

So a sensible starting point is this: work out roughly how many calories you burn in a day, then eat about 500 kcal less. Our article on how many calories you burn a day shows how to estimate the first number.

What the rule gets wrong

Wishnofsky wrote that one pound lost is worth approximately 3,500 kcal, and only under certain conditions. His paper says the figure is markedly lower when a person is fasting. Kevin Hall, a researcher at the US National Institutes of Health, checked the rule in 2008. He found that it roughly holds for people with more than about 30 kg of body fat. For leaner people it overestimates the deficit you need for each kilo.

There is a bigger problem. The rule treats your body as if it stays the same while you diet. It does not. A smaller body burns less energy. So the meal plan that gave you a 500 kcal deficit at 90 kg gives you a smaller deficit at 85 kg.

In 2011, Hall and his colleagues published a model of how weight responds when people change how much they eat. They found that the response is slow, with a half time of about a year. In plain words: after you change your diet and keep it changed, your weight takes about a year to move half of the way to where it will finally settle.

This is why the rule predicts too much weight loss over the long run. The first weeks often go faster than the rule says, because stored carbohydrate goes, and the water stored with it goes too. Later months go slower than the rule says, because the deficit has shrunk.

How to set your number

  1. Estimate the calories you burn in a day. An equation such as Mifflin-St Jeor gives you a starting figure.
  2. Eat about 500 kcal less than that each day.
  3. Weigh yourself daily and follow your trend weight, not single days.
  4. After three or four weeks, compare the trend with what you expected. If it falls more slowly than you want, cut a little more, or move a little more.
  5. As you lose weight, work out your daily burn again with your new weight.

Downtrend does the comparison in step 4 for you. From your profile and your trend, it estimates what you burn at rest and with activity, and the daily deficit your weight change points to.

If you want a tool that allows for the slowdown, try the free Body Weight Planner from the US National Institute of Diabetes and Digestive and Kidney Diseases. It is built on Hall’s 2011 model. You enter your details and your goal, and it gives the calories to reach the goal and to stay there. NIH calls it a research tool, not medical advice, so treat its answer as an estimate.

Why the deficit beats the diet name

Which foods you cut matters for your health and for how full you feel. For weight, the deficit is what counts. Two well-known trials compared popular diets and found small differences in weight loss between them. In one of them, how well people kept to their plan predicted how much they lost, and the type of diet did not. We cover those trials in which diet is best for weight loss.

Sources

  1. NHS. 12 tips to help you lose weight. The aim of 0.5 to 1 kg a week.
  2. Wishnofsky M. Caloric equivalents of gained or lost weight. American Journal of Clinical Nutrition, 1958.
  3. Hall KD. What is the required energy deficit per unit weight loss? International Journal of Obesity, 2008.
  4. Hall KD and others. Quantification of the effect of energy imbalance on bodyweight. The Lancet, 2011.
  5. Thomas DM and others. Time to correctly predict the amount of weight loss with dieting. Journal of the Academy of Nutrition and Dietetics, 2014.
  6. National Institute of Diabetes and Digestive and Kidney Diseases. Body Weight Planner.
  7. Kreitzman SN, Coxon AY, Szaz KF. Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. American Journal of Clinical Nutrition, 1992.

This article is general information, not medical advice. Talk to a doctor before a big change to what you eat or how you train, and especially if you are pregnant, under 18 or have a health condition.