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August 18, 2023

‘Complicated’ answer to common question: How much weight can you lose in a week?

Filed under: Fitness — Tags: — admin @ 9:08 pm

When it comes to weight loss, there are very few people who are keen to lose weight slowly. Rather, as quickly as possible tends to be the goal, even though it may have taken several years to gain the extra kilos.

This desire too explains the fad diet industry in which any regime, however temporary or metabolically damaging, that promises fast weight loss will be momentarily popular.

Should you have a goal of weight loss, it is reasonable to ask how much weight can you realistically lose in a short period of time, and how much of it is actually fat?

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However, the physiology of fat metabolism is complicated and the answer is not black and white, no matter how much we want it to be. 

So let’s take a look of the science of fat loss.

ketogenic diet with nutrition diagram, low carb, high fat healthy weight loss meal plan
Fad diets become popular because they promise fast results. (iStock)

First, we have fluid

When we take a closer look at the science of fat loss, an easy way to look at things is to think of the human body as a machine that runs with different types of fuel. First of all, there is plenty of water around, which is crucial for all the cells to function. This water volume can differ considerably depending on what you have eaten, how hydrated you are and what your kidneys are up to. Fluid levels will generally be the first thing impacted courtesy of any weight loss attempts, which is what you will notice on the scales within a day or two. 

READ MORE: Dietitian reveals how many calories it takes to lose weight

Then, we have carbs

Next, we have our fuel stores. We store carbohydrates in the form of glycogen in the muscles and the liver to fuel the brain and the body. This means the greater the volume of muscle mass you have, the greater your fuel stores. The average person will have enough glycogen stored to fuel the body for a couple of days.

These fuel stores also hold water, which means the greater the amount of carbs you have on board, the greater the volume of water you will hold, and the heavier you will be on the scales. In weight amounts this can translate into several litres or kilos of weight, which too can change dramatically within days of beginning a new weight loss program as the muscle stores of fuel are depleted. 

Women's legs on the scales, close-up of a measuring tape.
Losing weight is never easy and when you do, kilos can go back on as quickly as they left. (iStockphoto)

And then, there is fat

Then, there are your fat stores. Whilst there may be plenty of fat on board, or at least 10-15kg for the average lean adult, it takes time to actively mobilise stored fat and transfer it into the molecules that will be metabolised as part of the burning cycle. As the body is generally always burning a mix of carbs and fat, in order to burn a higher proportion of fat, it means we need to burn significantly more calories each day.

Guides of ½ -1kg a week of fat loss are suggested targets, or 1-2kg for those with larger amounts of body fat.

Whilst you may commit to working out for an hour each day, as you cannot exercise for hours and hours at a time, there is a cut off for how much extra fat the average person can metabolise each day. This is one of the reasons that larger people with lots of weight to lose can lose several kilos each week, as they can exercise for several hours each day. Basically burning more fat takes time, and as such the results may not be seen in measurements or on the scales for weeks after beginning a weight loss program. 

What happens in fat loss?

The first phase of a weight loss regime, in which calories and/ or carbohydrates are restricted and / or energy output increased with more activity will see glycogen stores depleted as the body needs to draw on them more for energy.

The weight of this will differ depending body size, ranging from a kilo up to two or three kilos for a person with a larger frame, more muscle mass and more stored glycogen. As this storage form of fuel also holds water, this drop in weight will also be accompanied with a drop in fluid. This will explain why some people, who are carrying 20-30 extra kilos may lose several kilos on the scale in just a week or two – most will be fluid and some stored glycogen which is being utilised as energy.

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What about fat?

At any given time, the body is also burning a proportion of fat and carbohydrate and the greater amount of energy burnt, the higher amount of total fat this translates into. It is impossible to say exactly how much fat will be metabolised at any one time. But if you consider that a single kilogram of stored fat contains over 9000 calories of energy, or up to four entire days’ worth of energy, it is easier to understand why fat loss takes time.

For this reason, tough guides of ½ -1kg a week of fat loss are suggested targets, while this may reach higher amounts of 1-2kg for those with larger amounts of body fat who are also exercising.

So how much can I lose?

For those with 10 kilograms or less to lose, expecting losses of just 1/2 a kilo each week is reasonable after the initial kilo or so drop courtesy of fluid loss. For those with more to lose, 1/2-1kg a week is a reasonable expectation, after the initial few kilos of fluid loss.

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Female leg stepping on floor scales, close-up. Woman and weighing scales at home. Diet, healthy lifestyle, loss weight, slim concept.
For those with 10 kilograms or less to lose, expecting losses of just 1/2 a kilo each week is reasonable. (iStock)

The downsides of fast weight loss

These numbers are based purely on physiology and so it is understandable things get confusing when you see weight loss programs that report much faster weight loss than ½ – 2kg a week.

Extreme diets that slash carbs and calories and report much more dramatic losses will generally be a) reporting outcomes from those who have large amounts of weight to lose and / or b) using regimes that are causing significant amounts of muscle mass to be broken down and used as energy to deal with the perceived starvation.

Long term, these extreme regimes negatively impact metabolic rate, which is the reason they may work once or twice, but generally see all the weight loss regained, weight which also tends to become more difficult to lose each subsequent time as metabolism has been negatively impacted.

It’s not just about the scales

Another key thing to keep in mind when working on the goal of fat loss is that you can’t expect consistent weight loss results each week. It takes time to mobilise stored fat, which is why you may see changes in measurements some weeks but not changes on the scales. For this reason, whilst scale weight is one outcome, it is not the only positive outcome and changes in body shape and size can also be indicative of fat loss.

Author Susie Burrell is a leading Australian dietitian and nutritionist, founder of Shape Me, co-host of The Nutrition Couch podcast and prominent media spokesperson, with regular appearances in both print and television media commenting on all areas of diet, weight loss and nutrition.

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