When we're looking to lose weight, the first thing we often think of is to slash the calories in our diet to as low a level as possible.
We ditch the carbs, fatty foods, snacks and even meat and other protein in a bid to stick to a low-calorie plan.
From now on, we tell ourselves, our diet will comprise only low-calorie salads, steamed vegies, fruit and just a few low-fat dairy products.
We might even skip meals altogether, convincing ourselves that so long as our calories are as low as possible, all will be fine.
Or we'll try some fad diet or a meal-replacement program, thinking that will cure our ills.
We'll achieve the body we want on a diet of barely any calories and we'll live happily ever after.
The problem is that very low-calorie diets (or "VLCDs", as they're known) come with their own problems and even the best of them should be undertaken only under strict medical supervision.
What's more, the likelihood of them having any long-term effects may be small.
How Low is Too Low?
Any diet that restricts your calorie intake to 800 calories a day (or less) comes into the category of being very low-calorie.
It may consist of a 'formula', such as a liquid shake, a snack bar or a soup that you consume regularly as a meal replacement, or it may be one of the fad diets that rely on eating only one type of food. These include the Cabbage Soup Diet or Grapefruit Diet.
Any of these may sound tempting as a way of slashing calories and dropping kilos fast, but it's not wise to embark on one without understanding all the pitfalls first.
The Drawbacks of Very Low-Calorie Diets
Doctors occasionally recommend these calorie-restricted diets for patients who are obese and haven't been able to lose weight with a regular program of healthy eating and exercise.
However, the severe calorie cutback would last for just a limited period of time - four to 12 weeks, for example.
Furthermore, patients are generally put on such a limited-calorie diet only when they face life-threatening consequences if they remain at their current weight.
And a doctor would ensure any patient he or she puts on such a diet returns to the surgery for check-ups each week, because of the health risks and side-effects they may be subjected to by the lack of calories and nutrients.
The risks and side-effects of these diets include:
The reason you may suffer with all or any of these side-effects is partly because of the rapid changes your body goes through, and partly because it's difficult to receive all the nutrients you need from so few calories.
Doctors and dietitians are better qualified than most to ensure that what you're eating meets as many of our bodies' needs as possible, even when it contains few calories.
This is why it's so important not to try one of these diets without consulting a healthcare professional first.
When Can They Work?
It seems that in certain instances, these diets may be useful. For example, a study by the UK's University of Newcastle found they could be an effective way of treating type 2 diabetes.
Eleven people were put on an 800-calorie-a-day diet consisting of liquid shake meal-replacements and starchy vegetables for eight weeks.
At the end of that time, seven of the participants were free of diabetes.
However, 12WBT dietitian Georgie Moore says this can also be achieved by a slower rate of weight loss, and that the person would have to keep the weight off to remain free of diabetes.
These diets have also been shown to help those looking to have surgical intervention to lose weight but who are presently too overweight to have an operation safely.
In a study in Mexico, 20 morbidly obese patients who wanted to undergo gastric bypass surgery were put on 800-calorie meal-replacement diets for six weeks.
The Mexican study showed that the patients lost enough weight and liver volume to make the bypass surgery safe to perform.
Few Long-Term Benefits
For most people, though, what they really want to know is, "Will cutting my calories like this help me lose weight for good?"
It seems that, sadly, the answer is no, even if it might work as a way of losing weight quickly.
A US study, which undertook an overview of a large amount of previously published scientific information, found that very low-calorie diets were usually effective in short-term weight loss when conducted under proper medical supervision for severely obese people - those with a body mass index (BMI) of more than 30.
However, it also concluded that these diets were "not very satisfactory" for long-term weight loss.
In other words, you might shed kilos quickly by restricting your calories to a very low level, but you're unlikely to keep them off for long.
A similar study in 2001 concluded that longer-term strategies were needed for sustained weight loss, suggesting that these diets are not the magic bullet we might hope they are.
The Low-Down on Fad Diets
To find out why some of the popular diets of this kind may not work, let's take a closer look at some of them.
1. Cabbage Soup Diet
Well, for a start, it doesn't sound very appealing, does it? The idea of eating cabbage soup day after day hardly sounds sustainable.
It's intended to last for only seven days, and makes no claims to offer sustained long-term weight loss.
Even if you do manage to stick to it for seven days in a row, then yes, you'd be eating very few calories, but you'd also be taking in very little in the way of nutrients.
And not only would your calories be low, but so would your mood as well!
You'd be eating no protein or carbohydrates, and your intake of calcium, iron, zinc or B vitamins would be minimal.
You'd probably end up feeling bloated and gassy, which is not that pleasant either for you or those around you!
If you tried this diet over a longer period, you'd be likely to keel over, rendering any weight loss you might achieve useless!
2. Grapefruit Diet
It sounds marginally more appealing than the Cabbage Soup Diet (who doesn't love a grapefruit for their breakfast once in a while?), but it has few long-term effects.
The Grapefruit Diet restricts carbohydrate intake (including cutting out most fruits, vegetables, grains and cereals) and encourages a high intake of meat, eggs and other protein.
High-fat foods such as butter and salad dressings may also be allowed.
Some versions last for 12 days, while others last for 12 weeks.
Popular in the US since the 1930s, it has a few variations, but each one includes eating half a grapefruit with every meal.
This is based upon the idea that grapefruit contains a special fat-burning enzyme.
However, this idea is a myth, with no scientific data to show that it's true.
People who've tried the diet may have upped their vitamin C intake, but aside from that, the grapefruit has little positive effect.
If anything, it could be harming you, especially if you're female. A 2007 study from the University of Southern California found that eating large quantities of grapefruit could raise your risk of breast cancer by up to 30 per cent.
Meanwhile, researchers at Canada's Lawson Health Research Institute have found grapefruit inhibits an enzyme in the stomach and can prevent a number of medications working properly (including anti-depressants and drugs for high blood pressure or ones that lower cholesterol).
3. Meal-Replacement Products
These go under various brand names and are often marketed with the term 'VLCD' on their packaging. They may include shakes, bars, desserts and soups.
Meal-replacement products are aimed at severely obese people (those with a BMI of more than 30) or those whose BMI may be under 30 but who have other conditions such as type 2 diabetes or high blood pressure.
Eating them as part of a very low-calorie program brings about a state of 'ketosis', in which the body switches from using carbohydrates as its main source of fuel to using fats instead.
While this type of diet may be safe for a certain period of time, it's important to talk to your doctor or dietitian before going down the very low-calorie route, for all the reasons listed above.
And, as mentioned earlier, the long-term benefits of such diets are dubious at best, unless a more sustainable healthy-eating and exercise program is put in place.
Some people should avoid meal-replacement diets altogether. They include:
Children and infants
Adolescents under 18
Adults over 65
People with certain conditions, such as cardiovascular disease, renal disease or type 1 diabetes, should be wary of trying this kind of diet and need to be extra-diligent about taking their doctor's advice before proceeding.
A Better Way
After reading all of that, you may be wondering which way to turn to find the answers to your health and weight problems.
If you've tried severely restricting your calories in the past, it's worth looking at whether that's been effective in helping you reach your goals and then to maintain them.
Because you're reading this, the chances are you've discovered that such drastic calorie-cutting hasn't worked.
In that case, what is the alternative? Several of the studies cited in this article recommend that for long-term benefits, the best solution is a combination of healthy eating and daily exercise.
The idea is that the calories you consume each day should be fewer than the calories you burn - to make it easier to remember, just think "calories in versus calories out". The first number should be lower than the second.
The Right Amount
It's true that watching your calorie intake is key to reaching your health goals, but there are ways to do so that will work well in the long term.
For a woman to lose weight, the ideal amount to eat a day is 1200 calories, and for a man it's 1800 calories.
This should be made up as much as possible of whole, unprocessed foods, with a good combination of lean protein, complex carbohydrates, fruit and vegetables, grains and cereals and low-fat dairy products. See our article on healthy foods for more detail.
In addition, you'll need to incorporate a healthy amount of exercise into your life. Aim for at least four or five workouts a week, in which you burn anything from 400 calories to 1000 calories a session (depending on your weight - the more you weigh, the more calories you'll burn).
This might seem outlandish if you've not exercised for a while, but bear in mind that in March 2013 Australia's National Health and Medical Research Council recommended we perform at least 60 to 90 minutes of moderate-intensity exercise a day.
Exercise: A Friend With Benefits
Exercise not only boosts weight loss but has numerous health benefits as well.
These range from reducing your risk of developing heart disease, type 2 diabetes, osteoporosis and cancer to alleviating depression and anxiety and making you stronger, fitter and better equipped to deal with daily life.
Once you've reached your weight-loss goal, you'll be able to change your calorie intake, but you'll still want to make sure the changes you've put in place are ones you can keep [italics] in place.
The aim is to build a healthy nutrition and exercise plan that ensures your body meets all its needs for nutrients and which gets you moving each day.
And that plan and calorie intake should be something you can sustain over the long term, so that you lose weight, keep it off and stay fit and healthy for life!