The Risks of Very Low Calorie Diets

By Ali Luke

Some diets promise extraordinarily fast weight loss – achieved by cutting calorie intake to (dangerously) low levels.

If you’re ever tempted to follow a Very Low Calorie diet, make sure you know the facts first.

What is a Very Low Calorie diet?

A Very Low Calorie (VLC) diet is one designed to promote rapid weight-loss at the start of a long-term dieting program. People on the diet consume below 800 calories per day.

Some well-known plans which are cut calories to VLC diet levels are:

When can you go on a VLC diet?

If a patient has a BMI of over 30 (putting them into the “obese” rather than just “overweight” category), their doctor may put them on a VLC diet. However, this is only done when the risks of remaining obese outweigh the health risks posed by the diet.

The article What is a Very Low Calorie Diet? explains:

[VLC diets] are intended for patients whose Body Mass Index (BMI) is greater than 30 with significant comorbidities (illnesses or diseases related to morbid obesity, such as diabetes or high blood pressure). VLC diets are not normally used for patients with a BMI between 27 and 30 unless they have medical conditions related to their weight.

You should never attempt to follow a VLC diet without medical supervision, as mentioned in Very Low Calorie Diets: Good or Bad?

What if you’re dieting for a special occasion?

There might well be times where you’re tempted to cut calories right down in order to lose weight that bit faster – especially if you’ve been dieting for a special occasion.

Try not to regularly drop below 1100 calories per day. If you don’t do any exercise, it won’t damage your health to occasionally eat only 800 – 1000 calories, but this certainly should never be for more than a few days at a time.

Do you really want to reach that special occasion feeling exhausted, ill and stressed because you’ve been starving yourself for days?

Surely the faster I lose weight, the better?

It can be frustrating to only lose a pound each week, but slower weight loss is much more likely to be permanent.

Following a VLC diet can often have a “yo yo” effect on your weight; you lose weight rapidly for a few weeks whilst on the diet, but when you start eating “normally” again, your metabolism has slowed to cope with the lack of food – and you pile the pounds back on.

Eating so little can have a number of side effects such as extreme tiredness, constipation, diarrhoea and nausea. If you are severely overweight when you start rapidly losing weight on a VLC, you also put yourself at more risk of developing gallstones.

What’s best for long-term weight loss?

When you lose weight fast, you often change your eating patterns radically. The three diets mentioned above all involve replacing meals:

  • The Cabbage Soup diet: no prizes for guessing what you eat a lot of here 😉
  • The Cambridge Diet and Lighter Life both involve meal replacements – bars, shakes and soups – which are designed to include the nutrients you need whilst remaining low calorie.

The problem with these plans is that you won’t be re-educating yourself to change your “normal” eating habits. If you go straight back to what you were eating before the diet … you’ll inevitably put on all the weight you lost.

If you want to keep the weight off for good, follow the common sense advice given by nutritionists down the years and increase the amount of exercise you do.

Maybe it’ll take you a few months longer to reach your goal – but isn’t that worth it, if you stay slim and healthy for the rest of your life?

Have you had long-term success using a VLCD?


  1. MissNutrition

    So basically even if you are on a very low cal diet and still strength train the low cals won’t have a large effect on your metabolism. Is that correct?

  2. CindySense

    Our bodies are uniquely made. Both our body and our mind need food to survive. Let alone the metabolic breakdown of VLCDs – How about the breakdown of our minds and our bodies.

    I’ve never heard of anyone who sustained long term weight loss by using VLCD. All successful weight loss has came from eating healthy and execise – changing your lifestyle.

  3. Dr. J

    Thank you for your information! I appreciate what you are trying to say.

    If I may offer an idea. You do not mention any physical activity. Perhaps if you did some light exercise, like walking 30 to 60 minutes a day, it would slowly make a difference.

  4. cc

    to Dr. J.
    You need to do some more investigation before you decide whether something is a myth. I’m in my late 50’s and 5 years ago I had my Resting Basal Metabolism measured at a Hospital wellness clinic. (I was 25 lbs overweight and kept telling people I wasn’t a closet potato chip eater.) My Basal metabolism (using a calorimeter to measure metabolic rate) was measured at 950-1100 calories/day. I eat about 1000-1100/day…not out of intent but habit of years of youthful–no breakfast, either skipping lunch or only having a salad and “chicken and peas” for dinner–catching up with me. Normal for my age is 1800 calories. To lose weight they say drop 500 caolories to 1200-1300 and you will drop lbs. To go to 1300 is an INCREASE for me. In addition, the “set point” just keeps increasing. At Christmas, I went out 1/week with friends for 3 weeks and ate full meals each time (and gained 7-9 lbs). 6 months later this weight is still on and the increase is my new set point and will not come off. Low calorie diets do eventually reduce metabolism.

  5. Dave

    Fat loss is rate limited. VLCD’s ignore this.

  6. susan

    I think your metabolism slows down on ANY diet. Didn’t Gina Kolata write about how EVERY diet stops working after six months (in Rethinking Thin)?

  7. Dr. J

    This is not exactly the same thing, but with the upcoming Olympic Games, the very things you mention are used to get the optimal performance from our athletes. I know that each athlete has a diet tailored to their specific metabolic uniqueness. Perhaps some of the data that they accumulate can be applied to healthy weight loss?

  8. Regina Wilshire

    ::: grin :::

    Obviously I don’t either!

    While I do believe that a carb-restricted dietary approach is probably the best to control hunger and provide the densist level of nutrients, one doesn’t have to use that approach to lose weight (obviously)….but no matter what approach one decides to try, I really think it’s critical to pay attention to micronutrients, meeting protein requirements for amino acids and consuming adequate calories to meet basal metabolic rate (BMR). I do think, based on many observations and reviewing hundreds of menus that it’s a perceived famine condition (consuming less than BMR calories) that undermines weight loss in both the short and long-term since our primal brain is wired to survive and famine ain’t a happy place for the body…but if we at least eat around BMR, the body has no specific threat to its survival energy needs for bloodflow, heart beat, body temp regulation, etc.

    That’s my theory, and I’m sticking to it!

  9. Regina Wilshire

    There is interesting animal model data which finds that obese mice die with more than adequate reserves of fat for energy while they’re provided what would translate to a vLCD diet – their muscles are catabolized, leading to their death with more than enough stored energy in their bodies. It’s not simply calories and stored calories.

  10. weight loss program

    A VLCD is impossible to maintain. I have spoken to many that have tried, my father’s doctor recommended he went on a heart diet (vegetable soup diet) for two weeks. He hated it, was always hungry and very quickly went back to old habits.

  11. greentea

    Radically changing eating patterns in the hope if losing weight fast seems to be a losing game. As you’ve said:
    “but when you start eating “normally” again, your metabolism has slowed to cope with the lack of food – and you pile the pounds back on.”

  12. Lose weight for life

    The hardest part about losing weight is maintaining motivation. On a VLCD it is so easy to bing because you are so hungry all the time. Once you bing your motivation gets shattered. They really are not maintainable in the long term and as stated do not encourage healhty eating habbits long term.

    • Rowerie

      I have been on an 800 calorie high protein diet for 9 weeks and have not binged once. My doctor prescribed phentermine and i dont even need it. I exercise 5 days a week, running and heavy weight training and i still do not feel the need to binge. I am 10 pounds from my goal although people are telling me i already look good, and hubby says i am hot. I suppose it is vanity weight but why not go for it… Oh by the way i am 55 years old!

      Dont say this diet is not maintainable – anorexics obviously do it all the time. Change your mindset! You may have to think of certain foods as toxic garbage. Everything in moderation is stupid. Thats what nutritionists always promote. I dont expect to eat pizza, cookies and crap after i reach my goal, i didnt before either, i just ate too big portions. At least this vlcd is teaching me portion control and i know that to maintain my goal weight range i may even have to practice alternate day fasting along with a low carb paleo diet. If my metabolism is permanently slowed, big whoop, at least i will save money eating less grassfed beef, wild caught salmon and organic veg.

  13. Andy

    So how about high protein diet to lose weight? I read from a book entitled “LA Shape Diet” that high protein diet can help to curb hunger control and lose weight. Are there any supporting evidence for this?

  14. doris

    Note: Cxmbridge Diet is typically sold via direct selling in my country, sometimes found in pharmacies. No supervision from doctor.

  15. Mike OD - IF Life

    Cycling parameters will lead to great results. As you said…the body likes changes. Like exercise, nutrition must also be varied…of course the key is understanding how foods effect our weight loss gains…which ones help and which ones don’t. Cycling high and low calories leads to a negative energy balance without compromising the metabolism. Cycling macro nutrients like carbs and proteins will also help increase sensitivity and utilization….as you want carbs to be used for muscle glycogen and not spilling over into fat cells. That being insulin resistance is the key factor in weight gain…and the best 2 ways to help reverse that is periods of low insulin (low carbs) and resistance exercise (that helps increase muscle sensitivity. Like I tell people, insulin being released all the time is like listening to loud music all the time…your hearing starts to go (aka you build up a resistance to it)…so the best way to get your hearing back? Turn off the music and let your ears have time to heal. When you do turn the music back on…your sensitivity is increased..and you get more from it.

  16. doris

    I had taken Cxmbridge Diet 8 yrs ago (for the first 2 to 3 months and swtiched to other fad diets) and lost > 40KG in 5.5 months time. But I regained all the weight plus a few more pounds in less than 3 years. Side effects: lost hair, no spirit, and most importantly the distorted and depressed mind, and I believed I had overeating disorder.

    Big lesson, long recovery, and now still working on it.

  17. Tom

    I sort of agree with you here. I guess with most nutrition plans it depends on each individual, but I agree that for me I would not want to do something that I wasn’t able to maintain for the long-term. I’m not a huge fan of continuously changing my diet, but that’s not to say that this isn’t effective.

  18. Spectra

    A lot of people swear by the calorie cycling method to lose weight very quickly. I think there is some merit to it…when I was losing weight, I sometimes just had days where I would eat 1800 calories a day and other days where I would eat 1500 calories a day or 1200 calories a day….the body thrives on change. It gets used to things quickly and adapts.

  19. Jeff

    800 calories a day? 1100 calories a day? I can’t imagine actually sustaining that for more than a day or two. I’m sure the only ones who do must be extremely motivated (by their disorders!)….

  20. Dr. J

    That’s interesting information, Regina! Hopefully, we will eventually nail it down. I never said, but I’m not advocating VLCD.

  21. Mike OD - IF Life

    Fat cells have different percentages of receptors, those being alpha and beta. Those receptors will dictate whether the fat cell will open up and release fat to be burned or not. Very Low calorie diets for too long will convert fat cells to have more A2 receptors (bad)…aka those problem areas that are stubborn to get rid of. So crash dieting will give you more stubborn fat cell receptors while also losing muscle (if you protein is not high enough…which most VLCD are not). So now people are in worse shape then when they started off at in regards to metabolism from muscle and their ability to burn fat is blunted (A2 receptors). Not good. You can always cycle low days and high days…like said above many professional BB will cycle in low weeks to get the final cut before a contest, but it’s temporary….and most need B12 shots just to walk around as they have zero energy.

  22. Heather

    I wonder if there’s truth in that.

    I tend to believe in it myself… but every time I went on a strict diet like that my bloodwork changed and I started showing up hypothyroid (with symptoms). When I’m eating enough, I have perfectly normal numbers.

  23. Spectra

    My mom went on some sort of VLC diet back when I was a kid. It consisted of 3 of these “milkshakes” that she drank per day…no solid food at all. I think it was maybe around 1000 calories per day (and that may be overestimating). She lost weight really quickly, but when she got down to close to her “goal” weight, she stopped the diet and gained all the weight right back again. The reason for that was that the diet never taught her what healthy eating was; it just starved her for a couple months.

    Incidentally, I don’t think the whole metabolic slowdown thing is as true as people say. If you have a very large reserve of fat and you don’t give your body enough fuel to use outright, your body pulls fat out of storage to use as fuel. If you exercise a lot in conjunction with a pretty low cal diet, you’ll lose fat rather quickly without slowing down your metabolism too much (think Biggest Loser: those people are eating 1300-1500 calories a day and are working out like madmen and they lose weight REALLY fast). You don’t start using muscle for fuel until you don’t have much fat left; then your body tries to preserve essential fat and you DO burn muscle for fuel. So I think the whole starvation mode doesn’t come into play until your body really IS close to a “starvation” status.

  24. Spectra

    It’s not like eating a VLC diet makes your metabolism stop or anything, especially if you have a very large reserve of fat. Your body does burn off fat if you aren’t giving it enough fuel to use outright. The danger comes in when people don’t have a large reserve of body fat. In that case, your body tries to preserve the fat it does have for purposes such as hormone production/regulation and body heat regulation, etc. and you start burning muscle mass. That’s why they only recommend VLC diets for seriously obese people.

  25. Regina Wilshire

    Based on my own personal experience with stalling at 1600-1800 calories but losing predictably if I eat 2200-2300 a day, there is something going on with metabolic response to perceived “famine” conditions I think.

    While there is a lot of data supporting that a reduction in basal and resting energy expenditure is real, there are also studies that counter that data (most weakly, but it’s still contrary data).

    I’m not convinced it’s all calories driving what happens on an individual level and am more ‘certain’ it’s calories + essential nutrients together than play into the end of the day changes that appear to happen to quite a large number of individuals.

  26. Supplements Canada

    I am not a fan of any nutritional program that you can’t realistically keep over the long term.

  27. Dr. J

    Hey to you!!

    It’s a matter of quality and quantity with each individual, but the numbers are probably not large. I mean Dumbo’s got some severe ear problem there! Despite his questionable looks, however, he’s quite the flyer 🙂

  28. MizFit

    lots of great info here as always—both in post and in comments

    not sure what I think yet.

    being both a (wanna be bodybuilder) and a woman who possess ears (*waves to drj*)

    I guess Id always thought it was HIGHLY personal whether it did or DID NOT slow ones metabolism…unique to each individuals ‘make up’


  29. Dr. J

    As an interesting aside, cold constricts and heat dilates, yet our ears will turn red in the winter cold. Why, because after the initial constriction, vessels dilate to compensate. There may be a short term metabolic lapse, but I doubt it lasts very long.

  30. Heather

    I don’t think it’s a myth because I was able to start losing weight after increasing my Calories (on the recommendation of a nutritionist.)

    I was eating 1100-1600 a day (usually ~1300)– but I was exercising 15-20 hours a week with fairly intense activity. Stopped losing weight at that amount. Went to a nutritionist and increased to 2100-2300 (so like 1000 Calories a day) and started losing easily and pretty quickly. Without feeling like crap all the time or changing my activity.

    I do think people jump to metabolic slowdown a lot of times when they probably just need to measure more carefully and be more realistic, or be patient– which makes it seem like a myth just because of so much wrong information.

    I know some people who have to eat ridiculously low amounts to keep their weight under control despite a lot of activity, and I think it probably relates to metabolic slow down features… I mean, it only makes sense that the body has measures in place to survive. We have a lot of survival mechanisms, and we come from a history where food wasn’t as abundant and constant as it is now.

  31. Mike H.

    Here’s a study that took training variables into account. Hat tip to Alwyn Cosgrove for this nugget.

    Bryner RW, Ullrich IH, Sauers J, Donley D, Hornsby G, Kolar M, Yeater R.

    Effects of resistance vs. aerobic training combined with an 800 calorie liquid diet on lean body mass and resting metabolic rate.

    J Am Coll Nutr. 1999 Apr;18(2):115-21.

    The aerobic group performed four hours of aerobics per week. The resistance training group performed 2-4 sets of 8-15 reps, 10 exercises, three times per week.

    V02 max increased equally in both groups. Both groups lost weight. The resistance training group lost significantly more fat and didn’t lose any LBM, even at only 800 calories per day. (The reason the calories were so low was to really take any dietary variables completely out of the equation and compare the effects of the exercise regime on LBM and metabolism.)

  32. Claire P.

    I don’t think the metabolic slowdown is a myth at all. If I eat much less than 1200 cals (my BMR) I completely lose my appetite and actually start gaining weight. I can lose at a good pace if I keep calories around 1200-1400. I actually am able to predict whether I’ll lose weight the next day based on how frequently I eat and how my hunger is. Hunger tells me that my metabolism is working. VLCD’s just don’t work for me.

  33. Barry

    Are you kidding me? Ask any body builder who has done contest preparation.

    The metabolism slows as your body fat level drops, and you have to either cut calories further or increase the amount of cardio that you’re doing in order to keep fat loss continuing.

  34. Barry

    You forgot to mention the only scientifically based, safe and effective very low calorie diet on the planet: The Rapid Fat Loss diet by Lyle McDonald. It’s what’s known as a protein sparing modified fast and you will consume between 600 and 800 calories per day. However, you only diet for 4 weeks before returning to maintenance level calories.

    The diet is essentially 600 to 800 calories in lean protein – chicken, very lean red meat, etc. Plus, you can eat as many vegetables as you like.

  35. angiers

    I think another point should be is you’re not likely to stick to such a severe diet for any extended amout of time. When I was younger and more naive I would try the very low calorie diets and after 2-3 days I would be so ravenous I would binge totally negating all the starving I did the previous days.

  36. Liam

    The number one principle of nutrition for weight loss is… sustainability. People need to find a way of eating that they can stick to for life, not just for a few months. These VLCDs are completely unsustaible, so inevitably weight is regained when people return to their old eating habits.

  37. Regina Wilshire

    Here’s one:

    Changes in body composition and resting energy expenditure after rapid weight loss: is there an energy-metabolism adaptation in obese patients?

    Int J Obes Relat Metab Disord. 1995 Feb;19(2):119-25

    The aim of this study was to assess changes in resting energy expenditure (REE) related to changes in fat free mass (FFM) in nine morbid obese (BMI 43 +/- 5.1 kg/m2) hospitalised females on VLCD.

    REE was measured by 30 min indirect calorimetry before and after 28 days of hospitalisation. Changes in FFM were assessed by bioelectrical impedance analysis (BIA), hydrostatic weighing (HW) and nitrogen balance (N).

    REE decreased 11.5% from 7.8 +/- 1.0 to 6.9 +/- 0.8 MJ/d.

    Total weight loss was 8.4 +/- 1.9 kg or 7.4% with an estimated FFM loss of 3.4 +/- 1.8 (BIA), 2.9 +/- 1.9 (HW) and 1.8 +/- 1.0 (N).

    As the fall in REE was larger than the loss of FFM, it is concluded that morbid obese patients develop an energy saving adaptation during rapid weight loss.

  38. Dr. J

    I can’t offer any real studies, because I think the metabolism slowing is not too far from an urban legend.

    My opinion is that humans evolved through a feast or famine existence and our “metabolism” has adapted to those origins.

  39. Charlie Hills

    Yes, there’s nothing like eating 800 calories a day to put the ‘die’ back in ‘diet’. I can see the reasons (both good and bad) for trying a VLC diet. Personally, my left leg alone requires 800 calories a day. It’s hard to imagine someone doing this voluntarily.

  40. Quito

    Following a VLC diet can often have a “yo yo” effect on your weight; you lose weight rapidly for a few weeks whilst on the diet, but when you start eating “normally” again, your metabolism has slowed to cope with the lack of food – and you pile the pounds back on.

    Does anyone have any pointers to studies that establishes how much metabolism slows when dieting? I know that this is common wisdom, but bariatric surgery works primarily by cutting back on calories (there is also reduced absorption of calories). Why doesn’t the resulting slowing of metabolism doom bariatric surgery?