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The Truth About Crash Dieting

Jun 16, 2016 | Austin Gill

The widespread misinformation in the dieting industry is so pervasive it’s almost depressing. Being the mere mortal that I am, I can only dispel one myth at a time - and today the sights are set on crash dieting.

Crash dieting is defined as a drastic reduction in calories (usually below 1,200) for at least 3 days in order to achieve rapid weight loss. There is are a lot of opinions about crash dieting, most of which are substance-less. Here are the top 5 objections to crash dieting - as debunked by science.

1. It's better to lose weight slowly than rapidly.

There is a belief that if you lose weight fast you will put it on even faster. The New England Journal of Medicine debunked this belief. Participants classified as steady dieters became frustrated by slow progress and found it difficult to finish, less than half did actually. Dietician Katrina Purcell, study leader, said: ‘Achieving a weight loss target is more likely and drop-out lower if losing weight is done quickly.’ Makes sense. I can’t imagine why the diet industry would want you to stick to slow long-term weight loss plans. It’s almost as if the sooner you reach your goals the less money they can make from you...

2. Crash dieting sends your body into ‘starvation mode’.

Story goes that drastic reduction in calorie intake sends your body into “starvation mode” causing your metabolism to slow and extra fat to be stored making you incapable of losing weight. Total myth that was disproved in the 40’s by the famous Minnesota experiment where men were put on a 1570 calorie diet for a year, and they consistently lost weight the whole time. Authority nutrition discusses ‘starvation mode’ in greater detail here. Everyone needs to know that fat is a form of energy. When you consume excess food it is stored as fat, when you consume less the fat is converted into energy and used up. Eating less doesn’t make your body store more. That’s just not how it works, because science.

3. The weight you lose is just water and muscle.

This statement is a half truth used as an excuse to not attempt anything difficult. It is true that you will lose water weight when calorie restricting. That is because lowering calorie intake forces your body turns to glycogen stores for more energy. Each gram of glycogen carries with it 3 grams of water, so as those stores are depleted, a lot of water leaves with it. Afterwards your body doesn’t turn to muscle for more energy, it turns to fat - this is explained in point 4.

On the other hand, if you go water fasting for a month (yes, people do that) then you will lose some muscle mass. But that isn’t what is being referred to as a crash diet, and most of us have plenty of fat stores to not have to worry about this anyway 😉

4. Crash dieting can damage your metabolism, causing it to slow down and never return to normal

Slowing your metabolism down is touted as the apocalypse of future weight loss - preposterous. There is a lot we don’t know about the human metabolism, but we do know that our metabolic rate (speed of calorie burn) increases and decreases as necessary - it’s called adaptive thermogenesis. When you lose weight your body becomes more efficient and requires less calories to perform its daily functions.

What you aren’t being told is that in addition to slowing your normal metabolic process, your body activates an alternate process known as ketosis. This is the process of breaking fat stores down into fatty acids that can be used as energy. It’s all very science-y, but if you feel like nerding check out this article covering the basics of ketosis. You aren’t damaging your metabolism - if anything you are improving it by making it more adaptive. If you are still worried here are some ways to speed your metabolism back up.

5. Crash dieting is dangerous

This seems very last resort doesn't it - “oh, well it’s just dangerous”. All this crash dieting talk is actually just a negative language for what is really a fasting diet. Where the crash diet term came from I’ll leave you to postulate. Fasting diets are becoming widely accepted in the medical community. So anyone telling you otherwise isn’t up on their science, or has some personal interest involved. The most recent research behind this was published by USC. The test group did 5 days per month on just 725 calories, for three months. Here are the results.


Fasting Mimicking Diet Results - Credit USC Dr. Valter Longo

Figure 6. Effects of a Human-Adapted FMD Regimen in a Pilot Clinical Trial. Image credit to the University of Southern California, Dr. Valter Longo.
(A) Subjects were randomized to either the fasting mimicking diet (FMD) or a control group. Subjects in the FMD cohort consumed the FMD for 5 consecutive days every month for 3 months and returned to normal diet in between FMDs. Control subjects continued their normal diet. Measurements were performed prior to the diet (Baseline), immediately after the first FMD cycle (FMD), and during the recovery period after the third cycle (FMD-RF). Subjects in the control group were evaluated within the same time frame as the FMD-RF subjects (End).
(B) Glucose (n = 19).
(C) b-hydroxybutyrate (FMD n = 19, Control n = 18).
(D) IGF-1 (FMD n = 19, Control n = 18).
(E) IGFBP-I (FMD n = 19, Control n = 17).
(F) Body weight (n = 19).
(G and H) Trunk fat (FMD n = 18, Control n = 19) (G) and lean body mass (H) evaluated by dual energy X-ray absorptiometry.
(I) C-reactive protein (CRP; FMD n = 19, Control n = 18) levels of all subjects (left) and subjects in the average or high-risk group for heart disease (n = 8; right). (J) Percentage of lin CD184+CD45 mesenchymal stem/progenitor cells (MSPC) in the peripheral blood mono-nucleated cell population (FMD n = 16, Control n = 14).
All data are expressed as the mean ± SEM.


This study brings to light the fact that calorie restriction is about so much more than just weight loss. For all the participants: 1) lean mass improved, meaning fat was lost and muscle was preserved, 2) risk of cancer and disease went down, 3) markers indicating longevity went up.

Lost fat, get healthier, live longer - that’s what happens when you “crash diet”. Now you know.


Stay Golden 🍍

    Austin Gill

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