Low-carb diets work.

That is pretty much a scientific fact at this point. At least 23 high quality studies in humans have shown this to be true. In many cases, a low-carb diet causes 2-3 times more weight loss as the standard low-fat diet that we’re still being told to follow (1, 2).

Low-carb diets also appear to have an outstanding safety profile. No serious side effects have been reported. In fact, the studies show that these diets cause major improvements in many important risk factors (3).

Triglycerides go way down and HDL goes way up. Blood pressure and blood sugar levels also tend to decrease significantly (4, 5, 6, 7). A high percentage of the fat lost on a low carb diet comes from the belly area and the liver. This is the dangerous visceral fat that builds up in and around the organs, driving inflammation and disease (8, 9, 10).

These diets are particularly effective for people with metabolic syndrome and/or type 2 diabetes. The evidence is overwhelming.

However, there is a lot of controversy about why these diets work.

People like to debate the mechanism, the stuff that is actually going on in our organs and cells that makes the weight go off. Unfortunately, this is not fully known, and chances are that it is multifactorial – as in, there are many different reasons why these diets are so effective (11).

In this article, I take look at some of the most convincing explanations for the effectiveness of low carb diets.

Carb restriction lowers insulin levels

Insulin is a very important hormone in the body. It is the main hormone that regulates blood sugar levels and energy storage.

One of the functions of insulin is to tell fat cells to produce and store fat, and to hold on to the fat that they already carry. It also tells other cells in the body to pick up glucose (blood sugar) from the bloodstream, and burn that instead of fat. So, insulin stimulates lipogenesis (production of fat) and inhibits lipolysis (the burning of fat). It is actually well established that low-carb diets lead to drastic and almost immediate reductions in insulin levels (12, 13).

According to many experts on low-carb diets, including Gary Taubes and the late Dr. Atkins, lower insulin levels are the main reason for the effectiveness of low-carb diets.

They have claimed that, when carbs are restricted and insulin levels go down, the fat isn't "locked” away in the fat cells anymore and becomes accessible for the body to use as energy, leading to reduced need for eating.

However, I'd like to point out that many respected obesity researchers do not believe this to be true, and do not think the carbohydrate-insulin hypothesis of obesity is supported by the evidence.

Bottom Line: Blood levels of the hormone insulin go way down when carb intake is reduced. High insulin levels contribute to fat storage, and low insulin levels facilitate fat burning.

Water weight drops rapidly in the beginning

In the first 1-2 weeks of low carb eating, people tend to lose weight very quickly.

The main reason for this is reduction in water weight. The mechanism behind it is two-fold:

Insulin: When insulin goes down, the kidneys start shedding excess sodium from the body. This also lowers blood pressure (15).

Glycogen: The body stores carbs in the form of glycogen, which binds water in the muscles and liver. When carb intake goes down, glycogen levels in the body go down, and the water follows along.

This does not happen to nearly the same extent on a higher carb diet, even if calories are reduced significantly.

Even though some people use this as an argument against low-carb diets, reduced water weight should be considered an advantage.

I mean, who wants to carry around excess bloat and water weight all the time? Anyway, despite claims to the contrary, this is far from being the main weight loss advantage of low-carb diets.

The studies clearly show that low-carb diets lead to more fat being lost as well, especially the “dangerous"belly fat found in the abdominal cavity (8, 16).

So, part of the weight loss advantage of low-carb diets is explained by reductions in water weight, but there is still a major fat loss advantage as well.

Bottom Line: When people go low-carb, they lose significant amounts of excess water from their bodies. This explains the rapid weight loss seen in the first week or two.

Low-carb diets are high in protein

In most studies where low carb and low fat diets are compared, the low carb groups end up eating much more protein.

This is because people replace many low-protein foods (grains, sugars) with higher protein foods like meat, fish and eggs. Numerous studies show that protein can reduce appetite, boost metabolism, and help increase muscle mass, which is metabolically active and burns calories around the clock (17, 18, 19, 20).

Many nutrition experts believe that the high protein content of low-carb diets is the main reason for their effectiveness.

Bottom Line: Low carb diets tend to be much higher in protein than low fat diets. Protein can reduce appetite, boost metabolism and help people hold on to muscle mass despite restricting calories.

Low-carb diets have a metabolic advantage

Although this is controversial, many experts do believe that low carb diets have a metabolic advantage.

In other words, that low carb diets increase your energy expenditure, and that people lose more weight than can be explained by reduced calorie intake alone.

There are actually some studies to support this.

A study conducted in 2012 found that a very low carb diet increased energy expenditure compared to a low fat diet, during a period of weight maintenance (21).

The increase was around 250 calories, which is equivalent to an hour of moderate-intensity exercise per day!

However, another study has suggested that it may be the high protein (but not low carb) part of the diet that causes the increase in calories burned (22). That being said, there are other mechanisms that may cause an additional metabolic advantage.

On a very low carb, ketogenic diet, when carb intake is kept extremely low, a lot of protein is being transformed into glucose in the beginning, a process called gluconeogenesis (23). This is an inefficient process, and can lead to hundreds of calories being "wasted."However, this is mostly temporary as ketones should start replacing some of that glucose as brain fuel within a few days (24).

Bottom Line: Low-carb diets appear to have a metabolic advantage, but most of it is caused by the increased protein intake. In the beginning of a very low carb, ketogenic diet, some calories are wasted when glucose is produced.

Low-carb diets are less varied, and lower in "food reward"

Low carb diets automatically exclude some of the world's most fattening junk foods.

This includes sugar, sugary drinks, fruit juices, pizzas, white bread, french fries, pastries and most unhealthy snacks. There is also an obvious reduction in variety when you eliminate most high-carb foods, especially given that wheat, corn and sugar are in almost all processed foods.

It is well known that increased food variety can drive increased calorie intake (25). Many of these foods are also highly rewarding, and the reward value of foods can impact how many calories we end up eating (26).

So, reduced food variety and reduced intake of highly rewarding junk foods should both contribute to a reduced calorie intake.

Bottom Line: Low carb diets exclude many foods that are highly rewarding and extremely fattening. These diets also have less food variety, which may lead to reduced calorie intake.

Low-carb diets significantly reduce your appetite

Probably the single biggest explanation for the weight loss effects of low carb diets, is their powerful effects on appetite.

It is well established that when people go low carb, their appetite goes down and they start eating fewer calories automatically (27). In fact, studies that compare low carb and low fat diets usually restrict calories in the low-fat groups, while the low-carb groups are allowed to eat until fullness (28).

Despite that, the low carb groups still usually lose more weight.

There are many possible explanations for this appetite reducing effect, some of which we have already covered.

The increased protein intake is a major factor, but there is also evidence that ketosis can have a powerful effect (29). Many people who go on a ketogenic diet feel that they only need to eat 1 or 2 meals per day. They simply don't get hungry more often.

There is also some evidence that low carb diets can have beneficial effects on appetite regulating hormones like leptin and ghrelin (30).

Bottom Line: Low carb diets lead to an automatic reduction in calorie intake, so that people eat fewer calories without having to think about it.

The long-term effects on weight loss are not very impressive

Even though low carb diets are very effective in the short-term, the long-term results are not that great.

Most studies that last for 1-2 years show that the difference between the low-carb and low-fat groups mostly disappears. There are many possible explanations for this, but the most plausible one is that people tend to abandon the diet over time, and start gaining the weight back.

This is not specific to low carb diets, and is a well known problem in most long-term weight loss studies. Most "diets" are incredibly hard to stick to.

The take-home message

Some people refuse to accept that low carb diets can work, and that people can eat as much as they want, because that must violate the calories in, calories out model. However, when you understand the mechanisms behind low carb diets, you can see that the CICO model is not being violated, and the laws of thermodynamics still hold.

The truth is, low carb diets work on both sides of the calorie equation.

They boost your metabolism (increasing calories out) and lower your appetite (reducing calories in), leading to automatic calorie restriction.

Calories still count, it's just that low carb diets automate the process and help prevent the biggest side effect of conscious calorie restriction, which is hunger.