Diet for weight loss with limited fat or carbohydrates

What is the best way to lose weight? Low-carb or low-fat diet? This is one of the most heated debates in the field of nutrition and dietetics. In this article, PhD student in dietetics and nutrition Richie Kirwan is here to clear up this question once and for all.

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How does the body burn fat?

If you want to understand how to melt fat, you must first understand how the body burns it. Let’s turn to science for a bit to find out.

One of the main functions of fat or adipose tissue in the body is to store energy in the form of triglycerides. They are molecules that are composed of three fatty acids and are linked to a glycerol molecule.

Triglycerides, or fats, are a great way to store energy because they can contain much more energy than other macronutrients. For example, one gram of triglycerides has nine calories, and one gram of protein or carbohydrates contains about four calories, meaning they are less energy efficient.

With this in mind, the body stores carbohydrates in the form of glycogen in the muscles and liver and uses them to maintain blood glucose levels when we have not eaten or are performing high-intensity activity.

When we haven’t eaten for a while, our blood glucose levels are normal, and we’re not exercising intensely, the body gets energy from fat. An enzyme called lipase breaks down triglycerides into glycerol and fatty acids. They are transported out of the fat cells and into the blood via protein carriers called albumins. These fatty acids travel through the body and supply cells with energy. Therefore, if we burn more fat through fat cells than we take in through food, we will lose weight over time.

But that’s just fat. When we eat a balanced diet and have protein, carbohydrates and fat in one meal, something completely different happens. Protein and carbohydrates are absorbed directly by the body and enter the bloodstream directly from the stomach in the form of amino acids and sugars. Increased levels of sugars and amino acids cause the body to produce more insulin to normalize blood sugar levels.

Insulin lowers blood sugar levels by slowing the action of lipase. In other words, when this happens the body cannot break down more fat for a certain period of time.

Insulin also stimulates the replenishment of stores of glucose and free fatty acids to help remove them from the blood. Glucose is stored in the form of glycogen, but if its stores are already full, then it can be converted into fat contained in the liver or in other parts of the body.

It is clear from this explanation that if you eat carbohydrates you will raise your insulin levels and the body will start storing fat. Yes, this is true, but it is also an overly simplistic way of looking at such complex processes in the body and begs the question of energy balance and how to achieve it.

Energy balance

Whether you consume carbohydrates or fats, your body needs the same amount of energy as usual to function. If you don’t eat enough food to meet your daily energy needs, you will be in a state of negative energy balance and burn fat. On the other hand, however, if you consume more energy, that is, food, than your body needs, you will achieve a positive energy balance. Excess energy will be stored in the body in the form of fat and over time will lead to weight gain.

Low fat or low carb

Now let’s look at some studies to find out what science has to say on the matter.

First on our list is a study that was conducted with the participation of 19 overweight people. During the study, participants were required to live at the study center, and their activity levels and the amount of food they consumed were tracked there.

The method used required participants to follow a high-carbohydrate, low-fat diet for two weeks, followed by a four-week rest period. Participants then followed a high-fat, low-carbohydrate diet. During each diet, participants consumed an amount of calories that placed them in a 30% caloric deficit.

Let’s find out what the results were. On the low-carb diet, insulin dropped by more than 20%. In addition, fat burning increased significantly, which is not unexpected considering that the participants were getting the energy the body needed from already stored fat. Remember that the fat they burned was a combination of the fat that was stored in the body and that taken in through food.

The scientists calculated that when consuming fewer carbohydrates, people burned an average of 53 grams of body fat per day, but when consuming less fat, they burned 89 grams of body fat per day. So, in the end, eating less fat burned more body fat. This was a study conducted over just two weeks, but it suggests that when it comes to shedding body fat, a caloric deficit is far more important than keeping insulin levels low.

But what do longer-term studies say? Fortunately, there are many of them who have researched this topic.

One study compared weight loss by following a healthy low-fat diet versus a healthy low-carbohydrate diet over 12 weeks. 600 participants followed a certain diet but otherwise led a normal lifestyle.

Participants were asked to reduce their carbohydrate or fat intake to the lowest possible levels depending on the group they were assigned to. In one, people followed a low-fat diet with an average ratio of 48% carbohydrates and 29% fat. The other group followed a low-carbohydrate diet with an average ratio of 30% carbohydrates and 45% fat.

The results indicated that in both groups people lost weight. In the group that ate less fat, each person lost an average of 5.3 kilograms, and in the other group (eating fewer carbohydrates) people lost an average of 6 kilograms. For over a year, the difference of 700 grams is not particularly important. Insulin levels also had no effect on the weight loss process.

One caveat about the study is that it did not specifically target body fat. Therefore, the difference in the weight of the participants in the two different groups may be due to water loss, which is common with lower carbohydrate diets.

Other comparisons between lower-fat and lower-carbohydrate diets have also shown similar results. In a recent study, a meta-analysis of data from 53 studies conducted with more than 63,000 participants over 12 months showed that low-carb diets resulted in weight loss of 1.15 kg more than low-fat diets. Here again we have to ask how much of this difference was due to water loss?

The results indicate that when it comes to clearing fat, low-carb diets are likely to be more effective than low-fat ones. In the end, it all comes down to achieving a successful and sustainable calorie deficit.


The choice is yours. Less fat or less carbs or just a well balanced diet that will put you in a calorie deficit. Richie can’t tell you what to do, but he can share with you what the science says and you can make an informed decision about what’s best for you and your lifestyle.

As always, we must note that for a diet to be successful, you must be able to stick to it for a certain (and long enough) period of time. Therefore, if you cannot live without pasta, you better not pretend that you will manage a low-carb diet for a long time. Choose what works best for you and your personal preferences.


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