The science of weight-loss shakes: How meal replacements work

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A meal replacement diet is, well, exactly what it sounds like.

The idea is to replace your regular meals with low-kilojoule powdered shakes and snack bars to reduce your overall energy
intake and subsequently help you lose weight.
If you can stomach shakes and bars for breakfast (and in some cases, lunch and dinner too), there's evidence meal replacements
may be a feasible weight-loss strategy in the short term.

But they're not a permanent fix, and before you start any weight loss diet, you should check in with your GP.
Here are a few things you should know before swapping out ordinary food for packaged soups and smoothies.


How do they work?

Meal replacements are kilojoule-controlled products designed to facilitate rapid weight loss and at the same time
conserve lean body mass (that's your muscles and organs).
The formulas are largely protein based (from the milk or soy-based powders that are mixed with water or skim milk),
usually contain few carbohydrates, and are supplemented with vitamins and minerals.

"What meal replacements try to do is cover the body's protein requirements in as few kilojoules as possible," says
Clare Collins, professor of nutrition and dietetics at the University of Newcastle.

Meal replacement products can be found in just about every supermarket health-food aisle and pharmacy.
But not all shakes and bars are supplemented appropriately (for a low-kilojoule diet) — some are particularly high in
sugar, and not suitable when trying to lose weight.

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Very low energy diets

Like any diet that encourages an energy deficit, the idea of meal replacements is to take in fewer kilojoules (or calories)
than you use in exercise and daily activities.
To achieve this, meal replacements were originally designed to replace all daily meals, as part of what's known as a "very low energy diet".
The average person needs around 8,700 kilojoules per day to maintain their current body weight.

On a very low energy diet, you're limited to about a quarter of that: 1,800 to 2,500 kilojoules per day.
"If you stick to that, you're going to lose weight fast, provided it doesn't make you feel so bad that you are a lot less
active than you normally would be," Professor Collins says.

When your total kilojoule intake is restricted, the fuel stores in your muscles — called glycogen — start to run low.

Once this runs low, your body has no choice but to turn to its fat stores and burn fat for fuel.
This process is known as ketosis.

Very low energy diets are more likely to be recommended to adults with obesity who need to lose a substantial amount of
weight in a short period of time.

This is usually for health reasons, such as ahead of bariatric surgery, or to improve specific medical issues.
Usually, this involves replacing all meals with meal replacements — except for the addition of a small bowl of salad or
vegetables each day (to boost fibre and other nutrients) and a small amount of oil (to keep your gall bladder
working) — for approximately six weeks.

This type of fasting requires medical supervision with frequent monitoring by a GP and dietician or specialist nurse.


Thanks to Luigi for the above image :)

Partial meal replacements

Meal replacements can also be partially incorporated into your diet — replacing one or two meals per day, or used during particular
fasting periods (for example, on the 5:2 diet, where people severely restrict their energy intake two days a week).

Again, this is generally suitable just in the short term, and needs to be carefully managed as part of a broader approach to healthy
eating, Professor Collins says.

"The more days you have a restricted type of eating pattern or a severe kilojoule restriction, the more important it is that on the other
days, your nutrient intake is maximised by following a very healthy eating pattern."

"You need to get enough fibre, folate, B vitamins, iodine, iron, zinc and other essential nutrients, so that you don't end up with some sort
of nutrient deficiency because your diet is so restricted."

Professor Collins says because of the possibility of nutrient deficiencies, among other risk factors such as gall bladder and liver inflammation,
any diet involving meal replacements requires guidance from a health professional.

"Before you jump in and try meal replacements [on a regular basis], it's a really good idea to go and see your GP for a health check.

"You probably need to have a blood test, check your liver function, get a referral to a dietician, and talk to your GP about monitoring
your health while you are on a weight loss diet," she says.

The National Health and Medical Research Council also recommends that anyone who uses meal replacements as a weight-loss strategy
enlists the help of a healthcare professional to ensure they manage the diet safely and without adverse health effects.

Effective in the short term

Research evidence indicates that, when used under the supervision of a health professional, meal replacements in conjunction with a
low-energy diet, can be an effective way to lose weight in the short term.

"The evidence shows that in the short term, meal replacements lead to greater weight loss compared to regular low-calorie diets,
where you have to monitor kilojoules every day," Professor Collins says.

When it comes to using meal replacements over a 12-month period, research shows they are equally as effective in achieving weight loss
as regular low kilojoule diets without meal replacements.

"The fact that there's no difference means that if [meal replacements] work for you and it's a way for you to help manage your
appetite and kilojoule level … then go ahead," Professor Collins says.

"But, if it does not, other approaches can be as equally as effective."

Research also suggests adults with obesity who use meal replacements (under the guidance of a health professional) tend to lose
more weight than people who receive only general dietary advice, for up to 12 months.

More research is needed to evaluate the ongoing use of meal replacements as part of long-term weight management.

According to the Department of Health, meal replacements are unsuitable for long-term use.

Transition to healthy eating

While meal replacements may be useful for some people in the short- to medium-term, Professor Collins says the
key to maintaining a healthy weight in the long term is to find an eating pattern you can stick to.

"If you can move to a healthier eating pattern than you had before [meal replacements], or to some other type
of dietary approach that's not as severe as a very low-energy diet, you'll be happier with your long-term result," she says.

Meal replacements, like any diet, are no magic bullet to improved health.
To achieve sustained, long-term weight loss, it's important to address lifestyle, social factors and habits that influence
your diet and exercise program.

There's also research that shows the challenge of keeping weight off long-term has a lot to do with hunger hormones
and the way your body stores and uses energy, not just self control.

Professor Collins adds that putting some weight back on immediately after stopping meal replacements is

normal — much of the early weight loss (in any diet) is due to loss of water.

"All that says is to keep the weight off, you need to move to another approach that you can follow on a day-to-day basis," she says.
It's important to note that meal replacements and very low energy diets are not suitable for children, pregnant women,
people with eating disorders, and those taking medications affected by rapid weight loss, including insulin.

They may also exacerbate some mental health conditions.

The science of weight-loss shakes: How meal replacements work - Health - ABC News