I have to start this post with a biology lesson. You probably already know that there are three macronutrients: carbohydrates, protein, and fat. These are our bodies’ sources of fuel, but they are not all treated equally.
The article I linked to above explains this quite well, and in more detail than I’m going to, but the short version is that carbohydrates are your body’s primary fuel source, largely because they are the easiest to break down. While your body can and will break fats and proteins down into energy, it takes longer than breaking down carbs and therefore, as a less efficient process, is not preferred by your body in most cases.
Fat contains the most energy potential per gram, so it is also commonly used by the body as a source of fuel – either in low intensity exercise where it has the time to break the fat down, or during endurance exercise in an attempt to preserve glycogen (sugar), which comes from carbs and is used by your body to create energy.
Finally, the least preferred source of fuel is protein. It is used in late stages of endurance exercise, when glycogen (sugar) runs really low and the body is forced to break protein into amino acids for fuel. However, this is not ideal because protein is much better used for other purposes – specifically muscle build and repair functions.
So that brings us to how low carb diets work.
If you start a low carb diet, you will notice that you do lose quite a bit of weight rather quickly – at least at first. This is because carbohydrates help your body store water and so your initial losses will be water weight as your body loses some of its capacity to store it.
I put this into practice recently when in the week leading up to my half marathon I did a five day fat-loading period where 65% of my calories came from fat, and therefore only 15% or so from carbs. At the beginning of that period I weighed 104.5 pounds – five days later I was down to 101.2. Then after just two days of more normal eating, where for both I stayed within my calorie limit and for one I ran a half marathon, I was already back up to 103.7. This is because I lost and then immediately gained back several pounds of water weight.
So if you go on a low-carb diet, you will see some immediate losses. But you’ll start gaining weight again as soon as you start increasing your carbs again.
Anyone who has ever tried a low carb diet might say “but I kept losing weight after the first few days!” and that’s probably true. But the fact of the matter is, low-carb diets work by tricking you into eating fewer calories than you would normally. Because you will have to replace at least some of the carbs you’re no longer eating with high-satiety protein, you are likely to feel fuller on fewer calories and ultimately eat less. Additionally, low-carb diets will cut out most calorie heavy sweets – things like cakes, pies, and cookies. It should be pretty obvious that eating fewer desserts will eventually help you to lose weight (or at least stop gaining it) and this has nothing to do with the carbs themselves being bad for you. It has everything to do with food that is mostly sugar with little to no nutritious value being bad for you.
But if low-carb diets work, why not do them?
The biggest concern with low-carb diets is the type of weight that you are losing. I already addressed the fact that the first few pounds you lose will be water, but there’s more to it than that. If you recall from the beginning of this post, I mentioned that carbs are your body’s primary source of fuel. If you significantly reduce your intake, you will find yourself being tired more often and your performance when exercising will suffer because you’ve lost the most efficient fuel source for your body. This can even effect your cognitive function and memory.
The problem related to weight loss, though, is that because your body cannot use carbohydrates to fuel itself anymore, it’s forced to use fat and protein instead. Burning fat is fine – in fact, it’s probably good, but as also mentioned above protein has much better uses. If your body is forced to convert your protein intake to fuel, it can’t use it to build and repair your muscles and that means you will ultimately be losing muscle weight in addition to the fat. (Note: See the link in this paragraph for other problems low-carb diets may cause for your health.)
And that’s what ultimately makes low-carb diets less than ideal. When you are trying to lose weight, the last thing you want to do is lose muscle. Muscle burns significantly more calories than fat, so the more of it you have the higher your resting metabolic rate will be and the more calories your body will burn throughout the course of a normal day. This is why it’s recommended that people who want to lose weight should do strength training; it, in effect, “increases” their metabolism by increasing their muscle mass.
If you decide to take on a low-carb diet despite these warnings, then you should make sure you get at least 30% of your daily calories from protein as this will help to mitigate the muscle loss. However, the absolute best diet is going to be a well-balanced one consisting of all three macronutrients – protein, fat, and carbohydrates – with as much “real food” (i.e. not Twinkies) as possible.