A new analysis suggests that low-carb diets cause you to burn more calories and lose weight. But the details tell a different story.
According to Google Search data, interest in the ketogenic diet appears to have waned significantly lately. But the popular and scientific fascination with low-carb diets persists.
The ketogenic diet, of course, is just the latest reincarnation of other diets that severely restrict carbohydrates, including the Atkins, South Beach, and Dukan diets. The goal of these very-low-carb diets is to induce ketosis, where the body’s cells are forced to burn fat for energy instead of sugar.
Just because the body is using fat as its fuel source, that doesn’t necessarily mean it’s burning body fat.
Although sugar (or carbohydrate) is the body’s preferred source of energy, we have very flexible metabolisms. If sugar isn’t available and fat is, the body can switch over to burn fat instead. Sort of like a car that can run on both gas and electric power. If the battery runs out of juice, the car can burn gasoline until you have a chance to recharge your batteries.
Just because the body is using fat as its fuel source, that doesn’t necessarily mean it’s burning body fat. First, it will be burning the fat that you eat, and ketogenic diets tend to be very high in fat. You'll only start burning body fat for energy when your body runs out of dietary fuel. So if you take in fewer calories than you burn, your body will turn to its fat stores to make up the difference. But this is true of any diet, not just low-carb diets.
So why the continuing fascination with very-low-carb diets? Well, one advantage of being in ketosis for a prolonged period of time is that it suppresses your appetite. That can make it easier to eat less. But in order to reap this advantage, you have to be pretty strict in your avoidance of carbohydrates.
What is the carbohydrate-insulin hypothesis?
Very-low-carb diets also tend to reduce blood sugar levels and insulin production. One of the normal (and necessary) biological functions of insulin is to promote fat storage. The carbohydrate-insulin hypothesis supposes that a diet high in carbs would result in higher insulin levels, which would, in turn, cause you to store fat and become obese. Restricting carbohydrates, therefore, should suppress insulin, trigger fat burning, and reverse obesity.
Study after study, including some designed and funded by proponents of the carbohydrate-insulin theory, have failed to prove the theory.
The carbohydrate-insulin theory also argues that low-carb diets lead to weight loss because they increase your metabolism, or the number of calories you burn. Theoretically, you would lose more weight on a low-carb diet than you would on a high-carb diet that provided the exact same number of calories.
The problem is that study after study, including some designed and funded by proponents of the carbohydrate-insulin theory, have failed to prove the theory. One strictly controlled study that evaluated a very-low-carb diet against a high-carb diet providing the exact same number of calories found no significant difference in the number of calories burned and a slightly lower rate of fat loss in the low-carb group.
How do low carb diets affect metabolism?
This brings us to the latest study, a meta-analysis, in which the results of 29 different studies were assessed to see whether low carb diets increase resting energy expenditure, which is a measure of your basic metabolic rate.
Lower carb diets did, in fact, speed up the metabolism, but ...
The lead author is David Ludwig, one of the primary champions of the carbohydrate-insulin hypothesis. Clearly, he has not given up on his theory. And the finding did support Ludwig's hypothesis: Lower carb diets did, in fact, speed up the metabolism. But before we get too excited about this, there are a few details that bear mentioning.
- Most of these 29 studies lasted just a few days. Some weren't even an entire day long but compared the effects of a single meal.
- Many of the diets in the "low-carb" arms were not very low in carbs.
- In many of the studies, the difference in carbohydrates between the two arms was quite small.
Despite the headlines, this analysis doesn't add much to our understanding of the effects of low-carb diets on metabolism because most of the studies included weren't really low-carb diets or were too short to be meaningful.
But the researchers now conclude that for every 10% decrease in carbohydrate, you can hope to burn an extra 14 calories a day. Keep it up for a couple of months and that might increase to 50 calories per day.
So, would reducing carbs cause you to burn more calories? Perhaps. Enough to turn the tide on obesity? Probably not.
Is a low-carb diet for you?
Low-carb diets can increase inflammation markers, cholesterol, and other blood fat levels, so keep an eye on those risk factors, especially if you are concerned about heart health.
If you are content eating a low-carb diet on an ongoing basis (not just as a short term strategy to lose weight), and you also like the way it affects your energy levels, mood, and appetite, then a low carb diet might be for you. Because they restrict so many foods and food groups, low-carb diets tend to be lower in fiber, magnesium, vitamin C, and other nutrients. So if this is your strategy, make sure you've got those bases covered. Low-carb diets can also increase inflammation markers, cholesterol, and other blood fat levels, so keep an eye on those risk factors, especially if you are concerned about heart health.
But most people eventually get tired of limiting their diet to meat, fat, non-starchy vegetables, and artificial sweeteners. They'd like to be able to enjoy the occasional sandwich, plate of pasta, bowl of cereal, fruit salad, or ice cream cone. And when they leave the ketogenic lifestyle, any weight they’ve lost tends to come back on.
While this latest analysis isn't exactly a home run for the carbohydrate-insulin theory, there may be a silver lining for the rest of us.
I have long advocated for a modified approach to the low-carb diet. The goal is not ketosis or anything close to it. And I'm not suggesting that you need to avoid whole foods that contain carbohydrates, such as fruit, dairy products, potatoes, legumes, or whole grains
Rather, the goal is to limit your intake of refined carbohydrates: things made with white flour and added sugars. These foods add very little to your diet nutritionally and are notoriously easy to overeat. And when we limit our intake of them, it tends to make more room for foods that are more nutritious and fill you up for fewer calories. That may not be quite as sexy as the carbohydrate-insulin theory or metabolism-boosting diets, but it is a proven approach to long-term health.