How low-carb diets work

Pastry with a bite outInterested in losing weight by restricting the amount of carbohydrates you eat? Mumsnetter BIWI explains how your body will react and how you will feel physically.

How the low-carb diet works

Carbohydrates are a great source of energy for your body. But the amount and type of carbohydrates typically eaten in the Western diet exceed what our body needs to convert into energy, and the excess ends up being stored as fat.

When you eat carbohydrates, your body converts them into glucose. Your pancreas releases a hormone called insulin to deal with this glucose. The insulin does three things:

  1. It moves the glucose in your blood stream into your body's cells for immediate energy (think of that sudden boost you get when you eat a biscuit or a piece of chocolate).
  2. It transports what is left into the liver, where excess energy is converted into glycogen, which is stored in the liver and the muscles, accessible for future energy needs.
  3. Once the glycogen stores are full, what is left over is converted to fat.

When you eat protein or fat, your body does not produce the same insulin response.

When you consume a lot of carbohydrates, especially carbs that are easy for your body to access, such as sugar or white bread, the levels of glucose in your blood rise rapidly. Your body has to produce a lot of insulin to deal with this and level out your blood sugar.

The more insulin you produce, the more fat you're likely to lay down, as not all of the glucose will be required for immediate energy. 

When you eat a lot of carbohydrates in one go, eg a typical breakfast of a bowl of cereal, a piece of toast and a glass of fruit juice, your blood sugar levels spike quickly. Lots of insulin is produced to deal with this excess glucose, which is then followed by a swift fall in glucose levels. This leaves you with low blood sugar, which can make you feel shaky, irritable and hungry.

A typical breakfast of cereal and toast will generally mean you're hungry again around 11am, and reaching for the biscuit tin. Lunch of a sandwich, packet of crisps and a chocolate bar will produce the same effect around mid-afternoon, when you might have a cake or another biscuit. Then an evening meal of pasta or rice or something with potatoes will have the same effect, resulting in late-evening snacking.

When you eat like this, your blood sugar levels are constantly peaking and troughing. The peaks mean you produce insulin, which lays down fat in your body; the troughs encourage you to eat more, meaning you produce more insulin, meaning you store more fat. 

What is ketosis?

If you've heard about low-carbing, then you've probably also heard the word 'ketosis'. When you restrict the carbs you consume, you correspondingly eat more fat and more protein. Fat and protein have a minimal effect on the amount of insulin your body produces and, in turn, the amount of fat it stores.

But your body also has less glucose available to use for energy, so it switches from burning carbs for fuel to burning stored body fat. Dr John Briffa (in Escape the Diet Trap) describes this process, ketosis, as follows: "This metabolic state is induced when dietary carbohydrate is restricted and the body turns to fat for its primary fuel. Fat can be broken down into ketones, which provide ready fuel for the body, including the brain." 

If you notice that your breath has a sweet-ish, 'pear drops' type smell, this is a sign your body is in ketosis ie your body is burning fat rather than carbs. Water and (if you must) sugar-free chewing gum will help, and your breath should return to normal within a few days.

A positive side-effect of low carbing when you are in ketosis is that you will not feel hungry. Your blood sugar levels will be stable, so you won't find yourself with the munchies at 11am or 11pm. You may even find yourself forgetting to eat. This lack of hunger makes it a lot easier to stick to a low-carb diet - you no longer have to rely on willpower and gritted teeth to keep going.

Reported benefits of a low-carb diet include:
  • Better sleep
  • No acne
  • No thrush
  • Fewer irritable bowel symptoms
  • More energy - no more afternoon 'slumps'
  • Lower blood pressure and cholesterol readings

Other low-carb diet side effects

As well as ketosis, there has been plenty of media coverage about other side-effects of low-carb diets.

It's true to say that in the first few days of low carbing you may not feel great. This is the time when you are experiencing carb withdrawal, or carb flu as it is often known, and you may feel tired, headachey and as if you are coming down with a cold or flu.

As long as you drink plenty of water and keep your electrolyte levels up - especially sodium and potassium - you should get through this stage pretty quickly. Many people report feeling much more energetic and 'sharp' once they are past this stage. 


Disclaimer: The information on our diet and fitness pages is only intended as an informal guide and should not be treated as a substitute for medical advice. Mumsnet would urge you to consult your GP before you begin any diet if you're concerned about your weight, have existing health conditions and/or are taking medication.

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Last updated: about 3 years ago