(Note: To make things easy, we will be using the words "carbohydrate", "sugar" or "glucose" interchangeably because the body eventually breaks down all carbohydrates and sugar into glucose. We will also use the terms "blood sugar" and "blood glucose" interchangeably.)

Carbohydrates are currently seen as the enemy of weight loss. Low-carb dieting seems to be the holy grail and a number of studies give evidence it is a great way to lose weight. Low-carb is a great way to lose weight for a lot of people, but not all. Like any strategy or method, it depends on your personal biology, preferences, and specific situation.

Our Bodies Prefer Carbs

First, studies clearly show that carbohydrate is the body's preferred source of fuel. Given a choice between carbs or fats, our bodies will prefer carbs without question. Why is that? Because the body can create energy from carbs more efficiently (it requires less oxygen).

Your brain especially loves carbs because fatty acid particles are too large to cross blood-brain barrier. Your brain requires a constant amount of energy to survive and glucose is its primary/"go-to" source. Your liver has the primary responsibility to keep your blood glucose levels stable and delivery energy to the brain. Depending on who you ask or the studies you look at, the brain can burn between roughly 400 - 550 calories per day (equal to roughly 100 - 140 grams of carbs per day). Interestingly, your body size doesn't matter very much: humans of all sizes have roughly the same size brains and thus similar energy requirements for the central nervous system.

Our muscles also require glucose for high-intensity performance.

The bottom line is that carbs are not inherently "evil" and are certainly not anything like poison. Even prehistoric man got carbs in the diet. Our bodies have clearly adapted to using carbs for energy needs.

Carbs Are Not a Required Nutrient

In spite of our body's preference for carbs and capability of handling them, carbohydrates are not required in our diets at all. If you completely eliminated carbs, you would still be in perfect health and you could function properly (aside from impaired performance of certain athletic activities).

Why? For two reasons:

1) Most tissues in your body can also generate energy directly from fatty acids (fat). In fact, both fat and carbs are always being burned in different ratios all day long.

2) For the few tissues that require glucose (your brain in particular) your body has two backup methods. First, it can create glucose from scratch. Second, it can switch to an alternate fuel source.

With the first method, the body can make glucose from scratch though a process called gluconeogenesis. This process creates glucose from glycerol, lactate, pyruvate and amino acids (protein in the body). The drawback to gluconeogenesis is that if you don't have enough protein in your diet, your body breaks down muscle tissue. This lean tissue loss can be very high in the beginning of low-carb adaptation. Under prolonged true starvation circumstances, loss of lean body tissue would rapidly cause the inability to move, and eventually death.

For that reason, the body has a second backup, an alternative fuel source called ketone bodies or just "ketones" for short. These particles can cross the blood-brain barrier and your brain can use them for energy. Ketones are created in the liver by breaking down fatty acids (which comes from fat). It takes about 2 - 3 days of eating carbs at a low enough level before this occurs at significant levels, a state called ketosis. After another 1 - 2 days or so (around 4 or 5 days total) your brain is running almost entirely on ketones. Over the next week or two your body adapts and uses almost entirely ketones.

Sparing Muscle and Lean Tissue

As mentioned above, during the adaptation period of moving to ketosis, lean tissue loss can be very high. Although the body eventually adapts, it is ideal to construct a diet to avoid this loss -- this is called a "protein sparing" diet.

You can avoid muscle or lean tissue loss by doing one of the following:

1) Consume adequate protein. This ensures that the body uses dietary protein for gluconeogenesis instead of its own muscle or lean tissue. See Protein for recommended protein levels.

2) If protein is low, then at least eat adequate carbs. Research shows that eating as few as 15 grams of carbs per day can limit body protein losses, but raising it to 50 grams per day is ideal. This ensures that the body has enough dietary glucose so gluconeogenesis is not required or minimized.

Ketogenic Diet / Ketosis

As mentioned above, your brain can burn off 100 - 140 grams of carbs per day. If you consume at least that many carbs in your diet, and your muscles don't require any of those carbs (eg, you are sedentary) ... then your body has no need to produce ketone bodies.

Thus if you eat less than 100 grams of carbs per day your body will at some point enter into and remain in ketosis. The lower you go below 100g, the more ketones will be generated. Most ketogenic diets tend to set a maximum carb intake of around 30 grams per day but there isn't really any scientific support for that value -- that figure simply guarantees you'll be in ketosis because it's hard to mess up aiming for a low target.

If you plan to test for ketosis, keep in mind it is defined as the amount or concentration of ketones in the blood (serum concentration reaching 0.5 mM). Some people rely on Ketostix as evidence of ketosis -- yet the fact is you can be in ketosis without ketones in the urine -- particularly as you get closer to that 100 gram mark.

Carbohydrate Needs For Exercise

Exercise can change how many carbs you decide to consume. High-intensity exercise burns carbs which means you can be in ketosis (even losing weight) while consuming more carbs than the regular 100 gram limit.

For exercising at low intensity (such as walking), you don't really need any extra carb intake. You can use the 100 gram level to enter (or avoid) ketosis.

High-intensity exercise requires extra carb intake -- specifically, it burns the glycogen stored in muscle. This pathway cannot run on anything else: not fatty acids, not ketones. Thus a certain level of carbohydrate intake is needed for certain kinds of uses of our muscles. This includes resistance exercise and sprinting in particular, although long endurance exercise can deplete glycogen as well. When muscles reach a certain state of glycogen depletion, they are forced to run entirely on fatty acids and that cannot supply energy quickly enough.

In fact, too much exercise while in a ketogenic state or low-carb diet can really cause problems. It can impair thyroid function and metabolic rate, affect natural production of neurotransmitters like serotonin and dopamine (which affects mood and sleep).

This can lead to: Feeling like your metabolism is shot, make you grumpy, depressed or just plain put you in a foul mood. You may have insomnia. You may not be losing weight and holding on to fat in spite of exercise and super low calories.

On the other hand -- again, everyone is different. If you feel great when exercising on low-carb, don't change. Otherwise you'll need to boost your carb intake -- or reduce your exercise significantly (or entirely, depending on how low-carb and low-calorie you are).

If we assume that maintaining glycogen levels for exercise performance is the goal, here are some carb-intake suggestions:

Weight lifting: It's estimated that each work set lasting 30-45 seconds requires 2.5 grams of carbs to replenish glycogen used. So if you did 12 sets, you'd need to eat 30 grams of carbs to make up for it.

Endurance exercise: Glycogen usage varies greatly, depending on exercise duration and intensity. A very fit, trained athlete can deplete glycogen in 1-2 hours when working near lactate threshold. At lower intensity but longer (say, a 2 - 6 hour session) both liver and muscle glycogen can be fully depleted. Muscle glycogen stores for such athletes might be roughly 300-400 grams, requiring 1200 to 1600 calories of carbs to replenish. Studies show that most competition endurance athletes consume around 2.2 grams of carbs per pound of lean body mass -- which means an average person should be consuming much less.

Weight loss: For fat loss, it may actually be beneficial to deplete glycogen entirely or maintain a lower level. In that case, simply consume fewer carbs than the above suggestions.

Max Carb Intake?

In theory (and according to studies) you could eat your entire maintenance level calories as carbs without fat gain. If a typical maintenance level is 14 - 16 calories per pound (eg, a person who moderately exercises), that works out to be 3.5 to 4 grams of carbs per pound. Your glycogen stores would increase but unless you went into a calorie surplus over multiple days you wouldn't gain fat.

Athletes sometimes practice "carb loading". Some take this to an extreme by first depleting muscle glycogen with a combination of heavy training and a low carb diet, then consuming a lot of carbs at once. In those conditions, the body can process and store (as glycogen) as much as 7 grams of carbs per pound over a 24 hour period. For a 150 pound person that would be 1050 grams or 4200 calories of pure carbohydrate - that's a lot of carbs!

Is Low-Carb (Ketosis) Ideal for Everyone?

No, not everyone functions well in ketosis. We all have slightly different metabolisms.

It typically has to do with insulin sensitivity: People with good insulin sensitivity who function well on carbs (feel good, no energy crashes, etc) tend to have trouble on low-carb diets; they just don't feel well or function well. People with insulin resistance tend to do very well reducing carbs to some extent, and often, they also do every well going to ketogenic levels.

Go by how you feel. It can take some time to adapt. Some people go through what is sometimes referred to as the "low carb flu". Taking an electrolyte supplement has been a big help to many people (see Supplements). If you still feel awful after 2 - 3 weeks then don't force it just because it seems like everyone is doing it. Some people's brains simply run better on carbs; others prefer ketones.

Main Problem With Carbs

The main issue with carbs is the fact that we tend to eat them as refined or processed carbs (white flour, cakes, sweets, etc) which are extremely easy to overeat compared to other foods which have natural "brakes". Refined carbs are also combined with fats -- a bad combination. That's a significant problem because refined carbs and fats have a very high energy density: they pack a lot of calories into a small amount of food. So it's easy to eat at lot.

In one study, 6th graders were told they could eat as much as they wanted of either cheese wedges or potato chips. Even though the cheese had about 50% more calories than the same amount of chips, the children in the potato chip group consumed over THREE TIMES more calories than those in the cheese group. Furthermore, they found that overweight kids fared even worse.

Not only that but they are not as satiating -- meaning you get hungrier sooner. Refined carbs are rapidly digested and absorbed. Once your body is done storing the sugars, it shouts "give me more". If you resist that urge then your body will in fact start burning off those stored sugars -- but it would rather you eat more quick-energy carbs instead.

See Avoid Trigger Foods for more.

Naturally-occurring carbs in whole foods such as starchy vegetables or beans are much less of a problem in an everyday diet. True whole grains (brown rice, 100% whole wheat) can also have their place in a balanced diet.

Of course, it's a different story for people who are either diabetic or insulin resistant. And, there are some advantages of low carb for weight loss but it might not be what you think.

Why Low Carb Works

Many people mistakenly believe that weight loss is caused by lack of carbs or entering ketosis. However you will only lose weight if you are eating less calories than your body burns. A low-carb, or low-carb/high-fat diet cannot violate the Energy Balance principle. If you consume more energy than your body can burn, that excess energy must be stored in the body.

A low-carb diet does provide certain advantages that make it more successful than other dieting approaches - but it's not because it magically alters the human metabolism and defies the energy balance principle. Simply put, a calorie deficit is still required.

The reason low-carb works:

Restriction or elimination of many foods that we typically overeat, particularly refined carbs and sweets of various kinds that have a high energy density.
Those foods are hard to replace with anything else, so caloric intake is automatically kept low
Replacement foods tend to be high in protein, a very filling food; fat is also higher which helps keep hunger in check between meals

We can compare the current low-carb craze to the low-fat craze that was proposed decades ago by virtually everyone. Everyone thought removing or reducing fat would be magic. It worked for many people simply because it eliminated so many types of food. But eating low fat makes for a bland diet. Most couldn't stick with it and/or compensated by consuming more carbohydrates -- compounded by the fact that companies began heavily marketing "low-fat" food items containing a lot of refined (junk) carbohydrates. Most people analyzed food choices as "it's okay to eat as much as we want of this... because it's low fat". In short, caloric intake didn't come down - it actually increased due to high-density carb intake that replaced fat.

With low-carb dieting, when you eliminate or severely reduce carbs, your remaining food choices are typically high protein with some fat. In spite of fat making food taste good, we can't or wouldn't want to eat a pure fat diet (1800 calories of lard, anyone?). Even an 80% fat diet is surprisingly hard to do because of limited foods: if you can't eat carbs, then fat typically can only come bundled with protein. And protein just so happens to be a fantastic diet food because it's so filling. As well, the most tasty high-density food is typically carb AND fat-based food. Since we've eliminated carbs... there goes the fat content as well.

So although low-carb dieting may seem like metabolic magic, in reality it's just a more effective way of moderating calories.