As suggested by @MarkRuffaloCrumble ...
...here's a brief/simple explanation of what happens when we eat carbohydrate:
- Carbohydrate is broken down into glucose (sugar) before being absorbed into your blood. The glucose then enters your body's cells with the help of the hormone insulin.
- That glucose is used by your body for your immediate energy needs, fuelling your activities, whether that's going for a run or simply breathing.
- Any glucose not used for immediate energy requirements is converted to glycogen, which is stored in the liver and muscles.
- If more glucose is consumed than can be used immediately, or stored as glycogen, it's converted to fat for long-term storage of energy.
When we eat a high carbohydrate diet, as currently recommended by the NHS, we're creating a constant cycle of insulin production.
As an example: we start the day with a high carb breakfast - cereal and/or toast, fruit juice and/or some fruit. That means that your body has to product a load of insulin to deal with all the glucose created in your body. This load of insulin quickly reduces your blood sugar levels - but the result is that you feel hungry (sometimes even weak and irritable). This will typically be around 11am - so you grab something else to snack on, which at that time of day is likely to be a biscuit or fruit, or another sugary/carby snack. This then provokes another spike of insulin.
For lunch, you're probably eating a sandwich, maybe with a packet of crisps - again, a high carb meal. So the same thing happens, meaning that at about 3pm you'll start to feel ravenous, and reach for a piece of cake, or another biscuit, or some more fruit.
This constant cycle means not only are you producing more and more insulin, but the insulin is causing the excess carbohydrate to be stored as fat and prevents body fat from being used for energy. As Dr Charles Clark says "you can never break down body fat if insulin levels are elevated" (from The New High Protein Diet).
What we need to do to burn body fat, then, is to reduce the amount of insulin our body is producing. And the way to do this is to reduce the amount of carbohydrate we eat. And if we lower insulin levels, the body turns to burning fat for fuel, instead of carbohydrate.
To quote Dr Clark again:
"The production of insulin is essentially controlled by carbohydrate intake, not fats or proteins, so if you cut down drastically on refined carbohydrates, your insulin levels reduce naturally, and you start to burn body fat."