“I lost 5kg in a week!”

“Fit into your high school jeans in time for reunion!”

“LOSE 10 kilos IN 20 DAYS!”

Most grown women know that if a weight loss promise sounds too good to be true, the only thing that will be significantly lighter in short order will be your wallet after you shell out for the “breakthrough!” “life-changing!” supplement, book, app, eating plan, meal delivery service or whatever.

Which is not to say that you can’t slim down, but how much you lose and how fast depends on a handful of factors, says nutritionist Lauren Slayton.

First and foremost: “If you have more to lose, or are male, you lose faster,” she says.

Oh, the injustice! Not only do they earn more money for the same work, but men have more muscle mass; because a kilo of muscle burns more than a kilo of fat, a typical 75kg guy burns more kilojoules than a woman of the same weight. That’s another way of saying that guys' basal metabolic rates (the rate at which a body burns kilojoules when it's just sitting there being alive) tend to be higher.

Okay, I’m a woman with a few kilos to lose. How much can I drop in a month?

Well, it also depends on your weight and your history: If you’ve been 65 kilos since your 20s and you’re up 5, getting back down is reasonable, says Slayton. But if the last time you dipped below 65kgs was when you had glandular fever in the 8th grade, and your goal is 60kgs, that’s likely not gonna happen - even if that’s what’s considered a “healthy weight” for your height. “Your history is more telling than any BMI chart,” Slayton says.

We can handle the hard truth, because we’re adults, right? No matter what you weigh, your body wants to defend that weight, says weight management expert Dr Caroline Apovian.

Dr Apovian works with obese people aiming to lose 5 percent of their body weight in three months, not 5kg in a week, she says. But even if you’re just looking to undo the fallout from too many fun nights out, and you cut back majorly on kilojoules, your body will similarly think you’re starving. Research shows that various bodily systems then kick in to slow your kilojoule burn, and certain hormones start cranking to up your appetite, among other things.

Then how come people on [fill-in-the-blank trending diet] lose big numbers, especially at first?

How much you lose depends, too, on what you’ve been eating up until then. If you’re a fast food fan or scarf a lot of processed carbs, and then you cut them all out, you’ll see a big sexy needle swing on the scale - say, 2kg in the first week - which is mostly the water your body was retaining.

The good news? If you don’t go back to eating junk, that water weight should stay off, says Slayton, but you shouldn’t expect that big a drop in month two.

Just answer my dang question! How much can I lose in a month?

Okay, okay! “We’re more tortoise-y with long-term weight loss and suggest that 0.5-1kg a week can be lost, though you don’t lose each week. 2.5kg in a month is completely reasonable,” says Slayton.

But she’s loathe to make promises because of all the factors that go into this issue. “Goals are great but two clients can eat the same way for two weeks and lose weight differently,” says Slayton. “I always say let’s see what kind of a loser you are…which sounds wrong.”

But my high school reunion is next month! What if I want to lose faster?

You could see a smaller number on the scale if you go to extremes, says Dr Apovian, but it’s not a good idea. “People who try to lose weight in a week or a month are going to gain it back,” she says. Fact.

Besides, you don’t only lose fat when you drop kilos quickly - you lose muscle, the very muscle that you need to burn kilojoules; that means your metabolism will slow and the weight will pile back on and then some - even if you keep restricting your intake, she says.

A better question to ask:

For non-obese people who want to drop a few kilos for cosmetic reasons, “I wouldn’t focus on the scale,” says Dr Apovian. The better question - one that’ll get you where you want to be, she says - is “how can I lose fat and increase muscle mass?” If you build strength, she says, you’ll have a much better chance of keeping the kilos off, because you’ll be boosting your metabolic rate.

That’s not to say you should toss your scale off a highway divider. “You can get on the scale so you can look at how you’re doing, but I wouldn’t say to lose more than a kilo a week,” she says. “You want to do resistance exercises - that’s what I would tell anyone who wants to lose any amount.”

The bottom line: Losing up to 3kgs a month is reasonable, but even then, it might not be every month. Building muscle mass will help you keep it off.

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