What is a calorie deficit?

How does energy balance work?

Energy balance means the change in the body’s stored energy equals the energy you take in minus the energy you spend. A consensus panel, writing in The American Journal of Clinical Nutrition (2012), states it as one equation that follows the first law of thermodynamics: the rate of change in energy stores equals the rate of intake minus the rate of expenditure.

The equation is not the hard part. The hard part is that none of its terms is measured directly in everyday life.

Four numbers that all get called calories

People often talk about these as one number. They are related, and they are not the same thing.

• 1Calories on a label or in a logPrinted

  A Nutrition Facts panel rounds calories to the nearest 5 up to 50 and to the nearest 10 above that. Under 21 CFR 101.9, a food is treated as misbranded when its calories run more than 20% above the declared value. A label is a regulated estimate, not a laboratory reading of the package in your hand.

• 2What you actually ateReported

  Self-reports tend to come in low. Pooling five large validation studies, American Journal of Epidemiology (2014) found energy intake under-reported by 28% on average with a food frequency questionnaire and 15% with a single 24-hour recall.

• 3What you spendCalculated

  Expenditure is usually a resting-rate equation multiplied by an activity level. A systematic review in Journal of the American Dietetic Association (2005) found Mifflin-St Jeor the most reliable of the common equations. It predicted resting rate within 10% of the measured value in more people than any other. The authors still note that “noteworthy errors and limitations exist when it is applied to individuals.”

• 4Your weight trendMeasured

  The weight trend is what the scale shows across weeks, and the only one of the four measured on you. Scale weight mixes changes in stored energy with water, glycogen and digestive contents. That mix is why it is read as a trend rather than a daily result.

How the expenditure side is estimated is covered in detail in BMR and TDEE.

Why doesn’t one day of arithmetic predict tomorrow’s weight?

One day of calorie arithmetic cannot predict the next morning’s weight, because body weight moves within and between days for reasons unrelated to energy stores. The same consensus panel names changes in hydration and in the contents of the digestive tract as the primary contributors to typical day-to-day swings of 1 to 2 pounds.

1. WaterPhysiological Reports (2017), citing earlier work, notes that total body water can shift by up to about 5% a day. The authors also measured 46 adults living normally for two weeks. In that group, 84% of the change in body weight was fat-free mass rather than fat.
2. GlycogenGlycogen is stored with three to four parts water, so filling or emptying those stores changes weight by several times the glycogen’s own mass (The American Journal of Clinical Nutrition, 1992).
3. Digestive contentsFood and drink still in the digestive tract weigh something until they are absorbed or excreted.
4. How you weighA different time of day, a recent meal or different clothes change the reading without any change in stored energy.
5. Expenditure that adaptsOver longer periods, spending moves too. In The New England Journal of Medicine (1995), people holding their weight 10% or more below their usual weight spent 6 to 8 kcal less per kilogram of fat-free mass per day. The authors describe that change as compensatory. Six years after a televised weight-loss competition, Obesity (2016) found resting expenditure in 14 former contestants about 500 kcal a day below what their body composition and age predicted.

Put together, a daily deficit is better understood as a long-run average than as a daily forecast. The model described in The Lancet (2011) found that body weight responds slowly to a lasting change in intake, with a half-time of about a year. For a static rule that ignores this, see the 3,500-calorie rule. The National Institute of Diabetes and Digestive and Kidney Diseases publishes a free Body Weight Planner. The planner estimates the calories needed to reach and then maintain a goal weight, including the effect of a change in physical activity.

What is a calorie target for?

A target is an estimate that gives you something consistent to compare against. The useful question is rarely whether the number is exactly right. It is what your own trend has done over the last few weeks, and what changed.

A deficit size is not something a guide can suggest. How large a deficit is appropriate, if any, depends on your health, your medications and your circumstances. That is a conversation for a clinician or a registered dietitian. To see how an estimate is built, the BMR and TDEE calculator shows the arithmetic in your browser.

What energy-balance research shows, and where it stops