How to Lose Weight Fast, According to Science

What do a bride-to-be, a couple booking a long-overdue cruise, and a career powerlifter with an upcoming competition have in common? Oftentimes, they all want (or need) to lose weight — fast.

If you’re even vaguely familiar with health and fitness, you’ve probably heard the standard weight loss dogma more than enough times: Eat your vegetables, be more active, make sure you’re getting enough sleep each night, hit the weights, take this or that “magic” supplement, blah blah blah.

Don’t get it twisted: For the most part, that’s all sound advice (you can probably keep your wallet shut about the diet pills, though). Increasing your physical activity and monitoring your nutrition are sure-fire ways of changing the number you see on the scale … eventually. 

But there are some situations where you need to change your body composition or body weight fast. While health is undeniably more of a marathon than a sprint, there are ways to kick things into high gear quickly. Here’s the science-based guidance behind rapid weight loss.

How Weight Loss Works

Whether you need to squeeze into a wedding dress or make the cut for a weight-class-specific sport, you need to understand the science behind how your body adjusts and manages its own weight.

[Read More: Best Fat Burners for Women]

First things first: The overwhelming scientific consensus regarding weight change is that slow and steady beats fast and aggressive almost every time, especially when it comes to sustainability in weight change. (1)(2)(3) That said, there are certain scenarios that legitimize a temporary, aggressive uptick in weight loss behaviors. 

Understanding Energy Balance

Energy balance, or the ratio of calories consumed to calories expended within a given parameter of time (usually a calendar day), is the guiding principle behind weight loss. Excess calories are stored as fat (or used for other metabolic purposes, such as building muscle), so it follows that adjusting your influx of calories will affect weight change. 

[Read More: Best Fat Burners]

Research indicates that one pound of body fat consists of roughly 3,500 calories. (4) So if you want to burn body fat, you need to expend an additional 3,500 calories worth of energy over time, on top of what you use every day to facilitate general bodily functions like physical activity, digestion, and so on.

The rule of thumb most commonly used is to divide that 3,500-calorie deficit across a seven-day week. Eat 500 fewer calories than you burn all seven days, and you should lose a pound of body fat. But your body’s weight is made up of more than just residual calories stored within your fat cells. 

Digestion, Hydration, and Other Factors

Weight loss and fat loss aren’t necessarily the same thing: Losing fat by creating a caloric deficit will change the number you see on the scale, but your body weight also fluctuates on a daily basis all on its own, and sometimes quite dramatically.

Your eating behaviors can acutely, and of course chronically, affect your body weight. After all, the foods you consume and digest have weight, so you’ll be heavier after eating a large meal. Bowel movements, constipation, and other digestive health factors can impact your weight as well. (5)(6)(7)

[Read More: 8 Best Appetite Suppressants to Burn Fat, Boost Metabolism, and More]

There’s also hydration to consider. Your body stores water in more places than you’d think, and a well-hydrated person will weigh more than someone who is significantly dehydrated. Dropping “water weight” is a common tactic for rapid, temporary weight loss among athletes, particularly those who partake in combat sports like boxing or in strength sports like powerlifting or Olympic lifting. 

How to Lose Weight Fast

If you want to lose weight quickly, you need to change both your eating habits and your physical activity levels. More moderate strategies like mindful or intuitive eating can create change over time, (8) but often won’t start working right away. To create drastic weight loss, you need to take drastic action — safely. 

Step 1: Create a Caloric Deficit

A calorie deficit, also known as a negative energy balance, is all but mandatory if you want to create weight loss that lasts for more than a day or two. In essence, this simply means eating less, but you need to know exactly how much less in the first place.

To get there, you must arrive at a general marker of how much energy you burn every day through your normal habits; that’s your caloric maintenance level. This value can vary tremendously based on how you build your healthy habits, lifestyle, genetic factors like metabolism, and more. (9)(10)

[Read More: Best Supplements for Weight Loss]

Luckily, a bit of math can accomplish a lot of the legwork for you. Plug some simple data into BarBend’s science-based calorie calculator and it’ll tell you roughly where to begin: 

Once you have an idea of how many calories you need to maintain your weight, subtract 500 to 1,000 calories from that to identify your target deficit. A 1,000-calorie deficit (or higher, even) is considered aggressive (11)(12) and may come with side effects like muscle loss, but should lead to faster weight loss if you play it smart.