When we set out to get into shape and start to eat healthily and exercise, the question that's on all of our minds is: how long will it take to lose weight and see results?
Naturally, when we hop on the scales after a week of working hard, we want to see a fat loss of at least two or three kilos. When it doesn't go to plan, this is often when we turn to fad diets, restricting food or excessively exercising.
But is quick weight loss really that safe, or even sustainable?
What's the best way to lose weight?
"The one thing I tell all the athletes and clients I work with is: if it sounds too good to be true, it is. That is definitely the case when it comes to quick fixes and magic bullets with weight loss," Spendlove told HuffPost Australia.
"It might not sound as appealing as a magic bullet, but the most sustainable way to lose weight is through improvements to your daily routine and lifestyle, in particular better eating habits and more movement."
"Eating fresh, minimally processed foods and controlled portion sizes is the most effective way to lose weight. Eighty percent is what we eat, 20 percent is exercise," Tuck told HuffPost Australia.
"To most effectively lose body fat we need to find a good balance with what we are eating and what training we are doing," Spendlove added.
If we are restricting foods, exercising too much or doing 'detoxes', this can actually throw our bodies out of whack, which makes losing weight even harder.
"One of the biggest mistakes I see people make is they over-restrict on training days or around intense training sessions, but then end up overeating on low output [rest] days," Spendlove said.
"Most people go wrong by undereating on the days they have trained, when they can afford to eat more, and their body will actually utilise the nutrients better. Getting the energy intake and output balance in check is key."
Is losing weight quickly dangerous?
Although fast weight loss is a tempting idea, shedding kilos in a short amount of time is likely to result in muscle or water loss, not fat loss. Unlike healthy, sustainable weight loss, it means you're also more likely to put the weight back on once you resume normal eating patterns.
"Fast weight loss can be unsustainable. This is because when the body is in extreme energy deficit, it is programmed to lose muscle before fat. Fat loss is a much slower process, and to be successful it requires long-term sustainable improvements with diet, as well as exercise," Spendlove explained.
Fast weight loss can also result in the slowing down of your metabolism, which is not what we want.
"[Rapid weight loss] can lead to weight gain when someone starts to eat normally again. It can take the body into starvation mode, thereby lowering the metabolism, making it harder to lose weight long term and easier to gain," Tuck said.
Other side effects of quick weight loss can include imbalances of electrolytes, malnutrition, dehydration, fatigue, irritability, headaches, constipation, dizziness, irregular menstruation and loss of muscle.
What's a safe amount of weight to lose in a week?
While this answer depends on the person, their weight and current activity levels, there is a guideline for the safe amount of weight to lose in a week.
"I generally would recommend 0.5–1 kilogram per week across the board," Spendlove said. "Losing more than that is often not sustainable."
In the first couple of weeks, weight loss tends to be higher as excess fluid is often lost.
"Some people will therefore safely lose several kilos in the first few weeks, depending on body size," Tuck explained.
It's also important to keep in mind that if you don't see a change in weight but you feel leaner, this could mean you have gained muscle mass -- a great thing.
"Around one kilo per week is a safe amount to lose for most people. However, these factors vary and if muscle mass is being built, this may give a false reading as muscle weighs more than fat -- so the scales may suggest less weight is being lost than it actually is as fat is being lost but replaced by muscle," Tuck said.
"Muscle mass is the metabolic powerhouse, so the more muscle mass an individual has, the faster their metabolism will be," Spendlove added.
How many calories do you need to burn to lose one kilogram?
A general rule of thumb is you need to be in an energy deficit of around 7,000 calories (29,400 kilojoules) to lose one kilogram of fat.
A calorie deficit is a state in which you burn more calories than you consume. This means, to lose one kilo the difference between your basal metabolic rate (the energy you burn just by existing) and daily intake of energy (food) needs to be 7,000 calories over the course of the week.
"There are 7,700 calories in one kilogram, so if you wanted to lose weight at a rate of one kilogram per week, you would need to reduce your overall calorie intake by 7,700 calories, or 1,100 calories per day," Tuck said.
If reducing 1,100 calories each day feels unachievable, to lose 0.5 kilograms per week you need to be in a deficit of 3,500 calories, which equates to about 500 calories per day.
Need a better picture? Here is how long it takes to walk off popular junk food meals.
Is it easier to lose weight if you're overweight?
According to Spendlove, technically yes. This is for a few reasons, including:
- The more someone weighs, the more energy they burn in activity;
- The more someone weighs, the higher their basal metabolic rate. This means the number of calories burnt even at rest are higher;
- If the person is making poor food choices, such as consuming excess energy from snacking or energy dense foods, even a few small changes which reduce energy intake can have a significant impact for weight loss.
"If you don't have much weight to lose, your body is slower at losing weight as it wants to ensure body weight does not go too low so you have some emergency energy reserves in times of need," Tuck explained.
"If you have more fat and fluid than the body could possibly need then it doesn't have to worry that there will be no emergency fat stores to use in times of survival."