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Weight Loss

Understanding Rapid Weight Loss: When Can It Be Bad?

Published on: May 03, 2019

Understanding Rapid Weight Loss: When Can It Be Bad?

A 900 calorie egg diet has become popular for its claim that it can help you lose 10 kg in 10 days. You may have also come across other weight loss diets and products that claim you can “Drop a dress size in a few days”. But is rapid weight loss safe?

Does it really work? We’re about to walk you through everything you may want to know about fast weight loss, and how to do it the right way.

What Is Rapid Weight Loss?

“I’ve lost 10 kg in 10 days” by doing so and so.

That’s rapid weight loss. Fad diets and trending weight loss supplements play a huge part in this, and rapid weight loss diets are often labeled as “Master Cleanse” or “Detox Diets”.

Here’s what research has to say about rapid weight loss and slow weight loss.

Rapid Weight Loss vs Slow Weight Loss

Here’s what research has to say about rapid weight loss and slow weight loss.

Body Composition

In a 2012 study, 10 obese postmenopausal women underwent either a slow weight loss or a rapid weight loss approach [1]. Results revealed that both groups lost weight, but the rapid group lost more lean body mass. The study concluded that a slower approach seems to be more beneficial for body composition, and metabolic risk factors.

Maintaining Weight Loss

A 2013 study found that rapid weight loss had both short term and long term advantages when compared to gradual weight loss. In this study, 296 obese women were randomly assigned to receive either fast, moderate, or slow levels of weight loss oriented calorie diets [2].

Their findings indicated that those who lost weight fast had greater weight reduction and long term maintenance than participants who lost weight gradually. They also concluded that rapid weight loss did not place participants at higher risk of regaining weight in the future in comparison to those who lost weight more slowly.

In a 2016 study, 57 overweight and obese participants underwent different levels of dietary intervention to lose weight. One group underwent a low-calorie diet (1250 cal/day) for 12 weeks (slow weight loss). And one group underwent a very-low-calorie diet (500 kcal/day) for 5 weeks (rapid weight loss) [3].

They also had a 4-week weight-stable period, and 9 months follow up. Results revealed that both groups lost similar amounts of weight and went on to regain similar amounts 9 months later. This was despite their different weight loss rates.

This study also noted that fat-free mass loss was higher in the rapid weight loss group, and this was associated with weight regain.

Preserving Lean Body Mass

In a 2017 study, 42 overweight and obese participants were divided into two groups. One was a rapid weight loss group where they aimed to achieve at least 5% weight loss in 5 weeks. The second was a slow weight loss group where they aimed to achieve at least a 5% weight loss in 15 weeks.

Results revealed that both rapid and slow weight loss groups had a reduction in fat mass, body mass, lean body mass, total body water, and waist-hip ratio. Both groups also had improvement in their cholesterol profiles [4]. The rapid weight loss group lost more lean body mass than slow weight loss group.

Another notable result from their study is that reduction in LDL (bad) cholesterol, and improvement of insulin resistance and sensitivity were more significant in the rapid WL group than the slow WL group.

The study concluded that both approaches can be effective for weight loss. However, body composition is better in the slow approach group, and improvement in glycemic and lipid profiles was better in the rapid weight loss group.

Rapid weight loss article_graphic

Rapid Weight Loss vs Unexplained Weight Loss

The big difference between rapid and unexplained weight loss is that the former is often achieved by choice, and the latter isn’t. Unexplained weight loss is normal after a stressful life event such as changing jobs, divorce, or bereavement.

And it usually returns back to normal weight after people start feeling relaxed and happy again. In some cases, however, unexplained weight loss can be bad news because it could be due to an underlying health condition, most notably cancer

It’s normal for your weight to fluctuate regularly, but an unexplained weight loss of more than 5% of your weight within 1 month calls for a doctor’s appointment.

If the unexplained weight loss is accompanied by other symptoms such as chronic tiredness, and/or loss of appetite this should prompt urgent referral to medical care.

Let’s have a look at some research-backed reasons why someone might lose weight unintentionally.


Unintentional weight loss is a major indicator of malnutrition. In a 2012 study, researchers examined the weight loss changes in 104 preoperative surgical patients cancer. They found out that patients with the highest percentage of unintentional weight loss had a higher risk of malnutrition [5].

They also noted that unexplained weight loss was the only major predictor associated with malnutrition.


Hyperthyroidism is the result of an overactive thyroid gland producing too much of the thyroid hormone. One of the major symptoms of this disorder is unintentional weight loss [6].


Cancer patients often experience unintentional weight loss. In a 2014 study, researchers examined the clinical, hematologic, biochemical parameters of 290 inpatients. Their results revealed that 72 out of 290 (25%) of the patients who had unintentional weight loss had cancer [7].

Unexplained weight loss could also be caused by:

  • Losing teeth
  • Alcohol or drug misuse
  • Side effects of some medications
  • Inflammatory conditions
  • Undiagnosed diabetes
  • Bacterial or viral infections
  • Dementia

Is Rapid Weight Loss Safe?

According to medical experts, losing 1-2 lb per week is considered safe, and losing weight faster than this could increase the risk of malnutrition, fatigue, and gallstones [8].

However, as you may have noted from the studies we discussed earlier, rapid weight loss wasn’t that severe in most cases. In one study, it even resulted in greater weight loss and maintenance results than gradual weight loss.

But one thing that kept recurring in all of those studies is that rapid weight loss puts you at a higher risk of losing more lean body mass than gradual weight loss. Body composition was better in those who lost weight more slowly than those who lost weight fast.

The real health hazards of rapid weight loss depend on the type of method you choose. For example, a prolonged liquid diet or water only diet could increase the chances of malnutrition, fatigue, and other health conditions.

Some weight loss supplements also have deadly side effects that have only been mentioned in research papers.

Other side effects of losing weight too fast include:

  • Irritability
  • Muscle cramps
  • Dizziness
  • Constipation
  • Diarrhea
  • Dehydration

Does Rapid Weight Loss Work?

As far as research is concerned, rapid weight loss works, and calorie restriction is the most commonly used method to achieve fast weight loss. For safety concerns, we’ll stick to rapid weight loss results from research studies rather than what’s trending on social media.

If you scroll back up to the rapid weight loss vs slow weight loss section, you’ll see that 2016, and 2017 studies we discussed did their rapid weight loss experiment for a minimum duration of 5 weeks.

It wasn’t a “Lose 10 lbs in 10 days” or “Drop a dress size in 2 days” approach. The studies considered 5 weeks as a minimum duration to lose 5% of body weight fast. So that’s what we’d consider a safe duration to aim for fast weight loss.

Intermittent fasting can also be effective for rapid weight loss. For instance, in a 2018 study, 51 obese men were assigned to either continuous or intermittent energy-restricted diet for 16 weeks. Results revealed that those in the intermittent fasting group had greater weight loss and fat loss [9].

clock-near-empty-white-plate-with-measuring-tape-and-wooden-spoon-and-fork-on-blue background-intermittent-fasting-concept

How to Do Rapid Weight Loss the Right Way

We’d always recommend following the gradual weight loss approach for overall health benefits, however, here’s what you can do if you need to lose weight fast.

1. Think about your current condition.

Do you have any health conditions that could get in the way or be impacted by rapid weight loss? Are you on any medications or supplements that could have a negative impact?

If you answered yes, consult with your doctor first to see if it’s safe for you to lose weight fast. Your doctor may also give you the best advice on what approach to take.

2. Safety first.

Stay away from bogus weight loss products that don’t have any valid evidence to support their claims. As stated earlier, a lot of them can do more harm than any good.

3. Reduce your carb intake.

Low carb diets have been proven to be successful for fairly quick weight loss.

4. Consider the ketogenic diet.

The keto diet has been found to be effective for relatively fast weight loss. Most people begin to see results within a month of starting the diet. In a 2017 study, overweight participants with type 2 diabetes were assigned to either a very low carb ketogenic diet and lifestyle online program, or a conventional, low-fat diabetes diet program for 32 weeks [10].

Their results revealed that those who were in the ketogenic diet group had significant improvement in glycemic control and lost more weight than those who were in the conventional diet group.

SEE ALSO: Why keto is the best diet to lose weight

5. Incorporate pure MCT oil.

MCT oil is much better for weight loss than all the trendy weight loss supplements out there. It also has many other health benefits such as improved cognitive and physical performance.

In a 2008 study, 49 overweight participants were assigned to have either 18-24 g/d of MCT oil or olive oil as part of a weight loss program for 16 weeks. Results revealed that those who consumed MCT oil had greater weight loss [11].

6. Practice intermittent fasting.

Research has shown that intermittent fasting can speed up weight loss, and it also has other health benefits such as reducing stress, and inflammation.

In a 2018 study, diet-induced obese mice were assigned to either ‘obese’ baseline control, no intervention, intermittent fasting, high-intensity intermittent exercise, or combination of intermittent fasting and high-intensity exercise [12].

Their findings showed that intermittent fasting with or without high-intensity intermittent exercise resulted in significantly less weight gain, fat mass accumulation, and reduced cholesterol compared to the other groups.


7. Practice resistance training.

You may have noted in the studies we discussed that rapid weight loss can make you lose a lot of lean body mass. Thus it’s better if you can incorporate resistance training into your weight loss program to reduce that risk and further enhance weight loss.


Research tells us that losing more than 1 to 2 pounds per week may lead to poor side effects and health issues such as muscle cramps, diarrhea, constipation, malnutrition, and more. If you want to lose weight, make sure to do it in a way that’s safe and healthy. Strategies such as reducing your carb consumption, fasting intermittently, and doing resistance training help.


  • According to medical professionals, losing 1-2 lbs per week is considered to be safe. Anything more than that is considered as rapid weight loss.
  • However, it’s important to remember that you may lose a lot of water weight in the early days, and that should not be confused with fat loss. As shown in the studies we’ve discussed, rapid weight loss is not entirely bad and has been proven to be effective in some cases without gaining the weight back.
  • Energy-restricted healthy diets such as a low carb ketogenic diet is much safer for weight loss than experimenting with bogus supplements. If you want to speed things up more, try incorporating pure MCT oil and intermittent fasting into your weight loss plan.
  • Be sure to take advice from your doctor first if you have any health conditions, and if you’re taking any medications or supplements.


  1. Sénéchal M et al. Effects of rapid or slow weight loss on body composition and metabolic risk factors in obese postmenopausal women. A pilot study. 2012 January -
  2. Nackers LM, Ross KM, Perri. MG. The Association Between Rate of Initial Weight Loss and Long-Term Success in Obesity Treatment: Does Slow and Steady Win the Race? 2013 September -
  3. Vink RG et al. The effect of rate of weight loss on long-term weight regain in adults with overweight and obesity. 2016 February -
  4. Ashtary-Larky D et al. Rapid Weight Loss vs. Slow Weight Loss: Which is More Effective on Body Composition and Metabolic Risk Factors? 2017 May -
  5. Loh KW et al. Unintentional weight loss is the most important indicator of malnutrition among surgical cancer patients. 2012 October -
  6. De Leo S, Lee SY, Braverman LE. Hyperthyroidism. 2016 March -
  7. Baicus C et al. Cancer and Involuntary Weight Loss: Failure to Validate a Prediction Score. 2014 April -
  8. Should you lose weight fast? 2016 August -
  9. Byrne NM et al. Intermittent energy restriction improves weight loss efficiency in obese men: the MATADOR study. 2017 September -
  10. Saslow LR, PhD et al. An Online Intervention Comparing a Very Low-Carbohydrate Ketogenic Diet and Lifestyle Recommendations Versus a Plate Method Diet in Overweight Individuals With Type 2 Diabetes: A Randomized Controlled Trial. 2017 February -
  11. St-Onge MP, Bosarge A. Weight-loss diet that includes consumption of medium-chain triacylglycerol oil leads to a greater rate of weight and fat mass loss than does olive oil. 2008 March -
  12. Wilson RA et al. Intermittent Fasting with or without Exercise Prevents Weight Gain and Improves Lipids in Diet-Induced Obese Mice. 2018 March -

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