Very Low Calorie Diet Lowdown

As reported on the BBC at the end of 2017, obesity rates in the UK have doubled over the past two decades. According to the Organisation for Economic Co-operation and Development, 63% of UK adults are classed as overweight.

Those looking for a quick fix might be tempted to simply cut their calories, and expect to simply return to their old diet once they’ve achieved their target weight loss. I admit to doing this in the past, but always found those pounds creeping back on again. This blog is going to outline the few reasons why crash diets will never work, and why they are actually worse for you in the long term.

A very low calorie diet is when a person consumes under 1000 calories a day. A couple of points on this before I continue:

  • This is going to be less than your body’s basal metabolic rate (the number of calories you burn just to keep your body functioning before any sort of movement is done).
  • Even for those classed as obese, this is not recommended unless there is a medical need (i.e to lose weight for an operation)
  • A diet at this level of calories will not provide the required nutrients needed for health
  • If there is a medical need to follow a very low calorie diet, it should be completed under medical supervision, and for no longer than 12 weeks.

There are many diet products out there that recommend following a very low calorie diet, replacing normal food with their products (i.e meal replacement shakes). If you do this, and are able to stick to it, you will lose weight, but what does that weight loss actually consist of, and what happens when you eventually have to increase your calories?

Out bodies are pretty smart cookies. When you stop supplying it with the energy it needs to carry out all basic functions, it will adapt to ensure whatever energy is available will last longer. It will also seek to protect vital organs (i.e the brain).

Water Loss

The body stores glycogen (stored form of glucose) in both the liver and muscles. The brain utilizes the stores in the liver – and doesn’t have access to any others. The glycogen in the muscles is used to give the muscles the energy they need to contract. These stores are used up quickly in the absence of sufficient calorie intake. This loss will register on the scales as a small weight decrease. However, every gram of glycogen is stored with 3 grams of water, which is also lost when the glycogen stores are used and not replaced.

The amount of glycogen stored in the liver and muscles will vary from person to person, but as a rough guide:

Liver stores: 60-100g

Muscle stores: 200-400g

Taking the lower range of these values, 260g of glycogen + (3 x 260g water) = 1.04KG (or 2.3lbs). Most people would be thrilled to see that level of weight loss on the scale after such a short space of time – but none of it is fat loss, and will return as soon as calories are increased once again in the diet.

Lean Tissue and More Water Loss

What does the body do at this stage without sufficient energy? One way to get the energy it needs is for the body to turn to its amino acid stores. These can be broken down to make glucose. Amino acids are held in skeletal muscle, so at this point, we are losing lean tissue. As with glycogen, muscle protein is bound to water, but this time it’s 4grams for every 1gram of muscle protein. Once again, we will see a drop in weight on the scale, but it won’t be the fat loss we’re after.

This will continue until the brain adapts to using other sources of energy.


Those on either a low carb or a very low calorie diet can enter a state of ketosis – where the body forms ketones from breaking down stored fat, which are used for energy. So all great – the body is now using fat and weight loss will be all from fat? The problem is, the body is in “survival mode” now, and will slow everything down to try and keep itself running. In this state, the basal metabolic rate can drop by as much as 20% (again, this depends on the individual).

Ending the Diet

So what happens when we come off the diet, and begin increasing calories again? As we’ve seen, the weight loss will have been a mixture of water, fat and lean tissue.

Firstly, we will regain the water weight, as the body will begin creating and storing glycogen in the liver and muscles. The metabolic rate will be slower; hence additional calories will over time be stored more easily as fat. We would have to work to rebuild the lean tissue.

Final Thoughts

If you’re thinking about going on a very low calorie diet, question the claims from the company offering the plan. Does it mention the risks, does it recommend discussing with your doctor, does it talk about long-term management?

Losing weight and maintaining long-term health can’t be achieved with a quick fix. To do this you need to review your current lifestyle to determine where permanent changes are needed.

As always, leave me a comment or ping me a mail if any questions!

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