We’ve all been there. You told yourself you were going to get in shape in time for a wedding, holiday or another important event. Now it’s a week away and the scales haven’t changed. Or perhaps they have, but in the opposite direction you intended.

So how much weight can you actually lose in a week?

Healthy weight loss is generally a slow and gradual process, but you’ve probably heard stories of people dropping significant amounts of weight in a short space of time – or seen some of our inspiring weight-loss transformations.

Sometimes that’s achieved sensibly. Other times, people resort to drastic measures in an attempt to shift weight quickly. But is there a safe way to make meaningful progress when you’re short on time?

The answer depends on several factors, including your starting weight, training, nutrition and previous dieting history. And, importantly, there’s a significant difference between how much weight you can lose in a week and how much you should lose.

Here’s what the experts say.

Everyone wants to know how much weight they can lose quickly – and, inevitably, they want that number to be as big as possible. Unfortunately, the answer isn’t straightforward.

How quickly you lose weight is influenced in part by your starting size. Generally, someone carrying more bodyweight may see a larger initial drop when they first begin trying to lose weight, says David Creel, PhD, RD, a psychologist and registered dietitian at Cleveland Clinic.

‘Unfortunately, our brain doesn’t have a scale saying, “when you get to a healthy weight, we’ll stop all this compensation,”’ Dr Creel says. ‘It doesn’t work that way.’

As we lose weight, our metabolism also starts to adjust, meaning our bodies require fewer calories than they did at a higher bodyweight.

‘It’s kind of like taking a backpack off,’ Creel adds. ‘Breathing requires less calories, walking down the street requires less calories, everything requires less energy.’

There’s a difference between how much weight you can lose in a week and how much you should lose – and focusing too heavily on the former can lead to some potentially dangerous approaches.

‘Wrestlers, boxers, people who have to make weight for a sport will often dehydrate themselves,’ says Creel. ‘You can see people who maybe lose 20 pounds in a week intentionally – but it’s very risky.’

And that’s the problem with extreme diets. However tempting rapid weight loss might sound, a dramatic change on the scales over a matter of days doesn’t necessarily represent an equivalent amount of body fat lost.

Very low-calorie diets can cause dehydration, meaning a significant proportion of the weight you initially lose may simply be water, says Creel.

Another issue with extreme dieting is the potential loss of muscle, particularly when weight is being lost rapidly. Maintaining muscle matters not only for strength and performance, but also because muscle tissue contributes to your daily energy expenditure.

‘If someone goes on a pretty restrictive diet, they might lose 20 percent to 25 percent of their weight as muscle,’ says Creel.

Konstantinos Spaniolas, MD, associate director of the Bariatric and Metabolic Weight Loss Center at Stony Brook University, suggests around one per cent of your bodyweight per week as an upper target. He cautions that this still represents relatively rapid weight loss.

For example, someone weighing 136kg (300lb) could potentially aim to lose around 1.4kg (3lb) per week. But trying to lose the same amount from a relatively lean 73kg (160lb) frame would be considerably more aggressive and could increase the likelihood of losing muscle.

The more time you give yourself, the better. Generally, Creel says people should aim to lose around 1-2lb (roughly 0.5-0.9kg) per week. Even then, progress won’t necessarily happen at the same rate every week.

‘A lot of people lose weight kind of like stair steps, not a straight line,’ says Creel. ‘They might drop four pounds in a week, and then their weight doesn’t change for two weeks, and then they drop two or three pounds.’

If you want to lose fat while preserving muscle – or lose fat and gain muscle simultaneously – strength training should be part of the equation.

One Columbia University study demonstrated how useful resistance training can be for preserving lean mass during weight loss.

Researchers asked participants to reduce their calorie intake and then perform either strength training or cardio three times per week. After eight weeks, everyone had lost more than nine per cent of their bodyweight. But in the aerobic group, 20 per cent of the weight lost came from lean tissue, compared with eight per cent in the resistance-training group.

You’ll also want to make sure you’re eating enough protein, which provides the essential amino acids your body needs to maintain and build muscle.

Spaniolas recommends eating around 0.8-1g of protein per pound of bodyweight when trying to retain muscle.

Your starting weight isn’t the only factor that influences how quickly you lose weight.

Your Dieting History

Scientists are still working to understand all of the mechanisms involved, but research suggests that people who have previously lost weight may expend fewer calories than people of the same size who haven’t dieted.

Not only will a smaller body generally require less energy, says Yoni Freedhoff, MD, author of Why Diets Fail and How to Make Yours Work, but hormonal and metabolic adaptations can also occur during weight loss.

That doesn’t mean you’re doomed if you’ve previously lost and regained weight. It simply means weight loss isn’t always a linear process, and the rate of progress can change over time.

Lack of Sleep

Not getting enough sleep can also affect hormones involved in hunger and appetite, including leptin and ghrelin.

One small study published in Annals of Internal Medicine had volunteers following a reduced-calorie diet sleep either 5.5 or 8.5 hours per night. After two weeks, both groups lost a little more than 6.5lb (3kg), but those who slept more lost around twice as much of that weight from fat.

Your Current Diet

Unsurprisingly, what you’re eating also plays a major role in how much weight you lose.

Spaniolas says accurately estimating how many calories your body requires can be complicated. Once you’ve estimated your maintenance needs, he suggests beginning with a moderate calorie deficit rather than making a dramatic reduction from the outset.

Even then, maintaining that deficit may become more difficult if you’re already relatively lean and have lower overall energy requirements.

Alcohol

Alcohol can quickly increase your daily calorie intake if you aren’t accounting for it.

One study found that men consumed an additional 433 calories on days when they drank a moderate amount of alcohol. If fat loss is your goal, calories from drinks still contribute to your overall energy intake.

If you’re looking to lose a significant amount of weight or make substantial changes to your body composition, you might consider using a body-composition scale alongside your regular weigh-ins. A tape measure, progress photographs and the mirror can also provide useful information.

Just remember that progress isn’t solely about the number on the scales.

You might want to lose as much weight as possible in a single week, but a more useful question is what changes you can make to your lifestyle that will improve your health – and that you can actually sustain – over the long term.

As Creel puts it: ‘There are other things to focus on other than one-week weight loss.’

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