There is a short section on this in the FAQ, but the subject really does merit a larger article.
Trying to keep both your fat and carb intakes low in the hope of losing weight more quickly? That really is not a good idea, and you do it only at your own risk.
Your body needs energy to perform all the little daily tasks it’s called upon to do. It takes energy to walk, to digest food, to sit in an erect position, to move, to breathe — even to think. It even requires energy to sleep, and for your body to repair itself of all the little damages it incurs during daily life.
Fortunately, your body is a very efficient power plant. It can use all of three macronutrient fuels as well as body fat and muscle to generate the energy it needs. Only if it runs out of all of these fuels will it be totally unable to produce energy and cease to operate (i.e. starvation death).
As starvation progresses, the body warns you via symptoms such as hunger, aches and pains, bowel irregularities, problems with the texture of your skin and hair, extreme fatigue and syncope(blackouts)
I’m sure that you will agree with me when I say, that one should never let the body get to the point of warning you (beyond being a bit hungry) that it’s out of fuel. Here’s why:
In terms of what you eat, the macronutrients carbohydrates, fats, and protein are not all seen by the body as “fuel”. Carbohydrates are used exclusively as the body’s “go-to” fuel, it can both use and store carbohydrate easily. However, both Fat and Protein would rather by used by the body to heal and repair itself.
Only if there are few carbs (sugars and starches) being eaten, will the body then start to use both consumed and body fats and finally, and only as fuel of last resort, the body will protest loudly and then start to use protein as a fuel.
Our bodies conditioned from birth to use carbohydrates first for fuel, Human breast milk, on average, contains around 7% lactose (carbohydrate), as well as 4.5% fat, 1.1% protein and 0.2% of “lots of other good-for-baby micronutrients”. Baby is using the essentials fatty acids and amino acids from the fat and protein as building blocks for the body and the energy it is receiving from carbs to drive growth.
As we continue to grow, our bodies will continue to want us to seek out fast-burn fuel, and in our current dietary environment, most people get rather more than enough carbohydrates to fuel their bodies’ daily activities. The body puts the leftovers in storage to use in the future as it’s needed. It does this by converting consumed carbohydrates into glucose. Glucose is then either turned into immediate use fuel (ATP), into a short-term storage fuel (glycogen) which is used for fuelling muscles during “fight or flight” responses or triglycerides (fat) for long term storage. Crucially, Glycogen can be turned back into Glucose for use, but triglycerides cannot.
The switch over from using carbs for energy to using fats for energy is only semi-automatic. In the absence of carbs the body will use fat, but only sparingly. It does this for you overnight when we are not eating, but as the body wants to have fat stores laid in against the time when it runs out of fuel it considers its fat as “emergency rations” and will try and encourage you to find a carbohydrate source.
Also note that in a famine, where carb or fat fuel of any sort of are not coming in, your body would also rather break muscles down for to make glucose than use up its precious fat stores.
So, during the 3-5 days the switch over takes, your body ramps down its energy requirements, producing only the amount of energy from body muscle and fat that’s necessary to sustain life. You will feel hunger, as well as probably fatigue, muscle aches, joint pain, etc. and you may become extremely constipated. This happens to most people when they begin a low-carb diet, and the “feeling ill” is off-putting if you do not know the biological reasons why. These types of people will be saying “Oh, I tried that, I felt dreadful! I felt it couldn’t be doing me good, so I stopped.”
However, feeding your body a good supply of dietary fat whilst restricting carbs tells it that right now (because of course, it has no sense of time!) calories in the form of fat are abundant, there is no famine and it doesn’t need to horde its own stores.
This then allows the body to feel OK about to using its fat automatically as its preferred fuel because it has both dietary fat and stored fat to draw upon. It has no reason to stay in “conservation mode” and so the body starts to produce lots and lots of energy in the form of Ketone bodies, which will continue for as long as you eat enough fat to keep your body out of conservation mode. Also, and crucially, Ketones either convert to ATP for immediate use or get excreted. They cannot be converted back to fat for storage. (Which is why the body doesn’t want to let them go!)
As a sidebar, why the body doesn’t like using protein as fuel: Although the body can break protein down into glucose, the body would much rather break it down into other molecules. This is because protein you eat is primarily used by the body to rebuild itself. After all, your organs and muscles are all made from protein. So, dietary protein is constantly used by the body to keep your organs and muscles in good repair.
So, either not eating enough protein or asking the body to use what you are eating as a fuel means that the body cannibalises itself, breaking down muscles to repair organs. If there is severe caloric and protein depletion, it will then act to break down organs as well in a last ditch effort to simply sustain life.
In short, trying to eat a reduced-fat diet along with a reduced-carb diet is almost a sure recipe for failure. It may appear to be effective, at least for a while. You may lose some weight, but it will be mostly muscle, and despite cutting your carbohydrates down to almost zero your body will see what’s coming in as a severe state of famine, put you deeply into conservation mode and act accordingly to preserve itself by storing every spare calorie it can as fat.
No two ways about it, you will end up falling ill if you restrict both fat and carbs, as the body taking energy from protein leads to very unbalanced body chemistry. So, please don’t, just enjoy fat instead, and know it’s doing you good as you eat it.