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The number most calculators use is 7,700 calories. A kilo of body fat tissue stores roughly that much energy, so a kilo a week needs a gap of about 1,100 calories a day between what you eat and what you burn. Half a kilo a week needs about 550. The sum holds up reasonably well for the first few weeks of a diet. Over the months it drifts ahead of the scale, because not everything you lose is fat and a lighter body uses less energy.
A daily deficit of about 1,100 calories matches 1 kg a week on paper. Only larger, active people can reach it without eating too little. A gap of 500–750 calories, closer to half a kilo a week once early water loss has passed, suits most adults much better.
Where does the 7,700-calorie figure come from?
It is the metric version of the old American rule of 3,500 calories per pound. Fat tissue is mostly fat with a little water and protein, and a kilo of it stores about 7,700 calories. If every kilo you lost were fat tissue, and your daily burn never changed, the rule would be exact.
Neither is true. Kevin Hall, a mathematician and physiologist at the US National Institutes of Health, worked out what a kilo of lost weight is really made of. The answer depends on how much fat a person starts with. People with plenty of body fat lose mostly fat, and for them the rule fits well. Leaner people lose a larger share of lean tissue, which holds less energy, so each kilo on the scale takes a smaller deficit.
Hall's model put the dividing line at about 30 kg of starting body fat. Above that, 7,700 per kilo is close. Below it, the rule overestimates what each kilo costs, at least in the early stretch of a diet.
Sources: [1]
What daily deficit does 1 kg a week need?
On paper, 7,700 divided by seven is 1,100 calories a day. Whether that is possible depends on how much your body burns. Take Priya, 35, who works at a desk in Indore, weighs 78 kg and is 162 cm tall. An online energy calculator puts her daily burn at about 1,900 calories. A 1,100 deficit would leave her eating around 800 a day, which is far too little to sustain or to keep her muscle.
Her colleague Rahul is 40, weighs 98 kg, stands 178 cm and spends most of the day on his feet at a warehouse. His estimated burn is closer to 2,900. Cutting 1,100 leaves him about 1,800 calories, three proper meals with room for a snack. For Rahul a kilo a week is reachable for a while. For Priya it is not.
The 2013 American obesity guideline sets a gap of 500–750 calories a day, or about 30% of daily needs, as the usual prescription. In practice that means 1,200–1,500 calories a day for most women and 1,500–1,800 for most men. On the 7,700 arithmetic, 500–750 a day comes to roughly half to three-quarters of a kilo a week.
Sources: [2]
Why does the scale fall faster in the first week?
Most people who start a diet see one or two kilos go in the first week, which seems to prove that a kilo a week is easy. Much of that early drop is water. The body stores carbohydrate in the liver and muscles as glycogen, bound up with water, and a calorie cut uses some of that store first. Less salt and fewer late, heavy dinners take some extra water with them.
Weeks two and three are usually slower, and that is where a lot of people give up. The scale has stopped matching the first week, although fat loss has carried on more or less steadily underneath. Hall's modelling describes weight responding slowly to a change in intake, which is the part that the first week's drop hides.
Judge the deficit from the third week onwards, and by weekly averages rather than single mornings. A period, a salty restaurant meal or constipation can hide fat loss for several days at a time.
Sources: [4]
Why does the same deficit lose less each month?
Diana Thomas and colleagues at Montclair State University tested the 7,700 rule against seven experiments in which food intake was measured rather than reported. In every one, the rule predicted more weight loss than actually happened, and the error grew with time. Their conclusion was that the rule grossly overestimates weight loss over the longer run.
The reason is that a smaller body needs less energy. As Priya loses weight, her daily burn falls, so a deficit that started at 500 shrinks without any change to her plate. Hall's 2011 model in The Lancet gives a better rule of thumb. A lasting change in intake of about 24 calories a day eventually moves weight by about 1 kg. Half that change arrives in roughly the first year and most of it by the third.
By that measure, a steady cut of 500 calories a day leads, in time, to about 20 kg less body weight for an average adult. That is a large change, but it comes slowly and keeps slowing, which is the opposite of the straight line the 7,700 rule draws.
Is 1 kg a week too fast, and does it cost muscle?
The speed of loss seems to matter less for regain than most people assume. Katrina Purcell's trial in Melbourne put 200 adults with obesity on either a rapid 12-week diet or a slower 36-week plan, both aiming at 15% weight loss. About three years later both groups had regained around 71% of what they lost, whatever their pace.
What the weight is made of matters more. In a Maastricht trial, a very-low-calorie diet and a moderate one produced similar weight loss, but the very-low-calorie group lost far more of it as fat-free mass, 8.8% against 1.3%. Across the trial, the people who lost more lean mass regained more weight over the next nine months.
Ina Garthe's work with Norwegian athletes shows how pace and training interact. Athletes losing about 0.7% of body weight a week, with four weight sessions a week, gained lean mass, and those losing twice as fast did not. For an ordinary dieter the lesson is to lift twice a week and stay nearer half a kilo. It is also why Metaboliq's doctor-led plans aim to lose up to 20% of body weight while keeping muscle, starting from a home blood test and a doctor's review that set the pace.
Should the deficit come from food or from exercise?
Mostly food, with exercise protecting the muscle and helping the loss last. The American College of Sports Medicine found that 150–250 minutes a week of moderate activity, on its own, gives only a modest weight change. Above 250 minutes, the effect becomes clinically meaningful. Activity also improved results when people ate moderately less.
A brisk 45-minute walk uses somewhere around 200 calories for most adults. Burning 1,100 a day through exercise alone would take hours. A realistic split for Rahul is 700–800 calories from his plate and 300 from a daily walk. Priya's plan looks like a 400–500 calorie cut in food, a walk after dinner and two strength sessions a week.
Where the calories come from on the plate matters too. Oil, sugar in chai, fried snacks and a second helping of rice are the usual places to find 400 calories in an Indian day. Keeping dal, curd, paneer, eggs or chicken at every meal holds hunger down while the portions shrink.
Sources: [8]
How do you check that your deficit is real?
Weigh yourself each morning after the toilet and before tea, and compare weekly averages. From the third week, a steady fall of half a kilo to a kilo a week means the deficit is roughly where the arithmetic says. A fall above a kilo a week, week after week, usually means the cut is steeper than it needs to be, and some of the loss is muscle.
If the average does not move for three weeks, the deficit is smaller than you think. The most common reason is food that never makes it into the log: cooking oil, a handful of namkeen, the biscuits with evening tea. Measure the oil by the spoon for a week before deciding your metabolism is the problem.
Sources: [3]