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Fat vs carbs.

When you are aiming to lose weight, cutting calories and/or increasing energy expenditure through exercise are the tools at your disposal. But let’s say you are cutting 500 calories per day, which is better to reduce, carbohydrates or fat? If you are eating an equal amount of calories per day, which macronutrient is preferable to cut when losing weight?

The debate over reduced fat or reduced carbohydrates has gone on for decades and shows no sign of ever ending. The pendulum has swung back and forward during these decades. The Atkins diet swung the pendulum towards the very low/reduced carbohydrate diets in the 1970s. In the 80’s, reduced fat diets came into vogue as science indicated that dietary fat was more easily stored as body fat, the Zone diet became popular in the 90’s, but since then things started to fragment, the paleo diet approach, the cyclical ketogenic/reduced carbohydrate diets and intermittent fasting have become ever more popular.

The macro war between those who prefer carbs for fat and vice versa is a lengthy trench war without clear winners or losers. There is some research on this topic, but much of this research is old.

Now a new study sheds further light on the topic. The study is relevant, applying the more precise technology available today, it is a well-controlled study investigating the effect of varying a number of carbohydrates and fat with a fixed calorie intake.

A Fixed Intake of Calories

The study involved obese (BMI 36+) participants put under strict surveillance to check energy consumption and diet. The 19 participants were subjected to a specific diet during a couple of weeks, then another diet for another couple of weeks. The advantage of this approach is that it controls for variables that differ between individuals – eliminating the possibility that a specific diet for some reason is more effective on some people and vice versa – thereby making each participant their own control group.

Before starting a session with a specific diet, the participants were given a standardized diet for five days, to ensure that they started each diet session at the same starting position, with the same conditions.

After these five days, the participants were eating a diet with 30 percent fewer calories than needed to remain weight stable. One dietary session consisting of 30 energy percent from carbohydrates, 49 energy percent from fat and 21 energy percent from protein. The other consisted of 72 energy percent from carbohydrates, 7 energy percent from fat and 21 energy percent from protein.

The participants thereby ate the same amount of calories and the same amount of protein in both diets. But different amounts of fat and carbohydrates. The fact that both diets included the same amount of protein is indeed important, as protein is metabolized very different from fat or carbohydrates.

Higher Fat Oxidation but Less Fat Loss With More Fat

The resulting data indicates that the participants burned more stored fat when they ate more fat and fewer carbohydrates. This is actually expected, as fat oxidation has nothing to do with the actual fat loss. The body is adaptable after all and if you feed it fat as an energy source, it will then primary use fat as energy.

Fat loss, however, is not just about using fat as energy, but it’s also about storing less fat. If the body is too lose fat mass, fat burning needs to be higher than fat accumulation.

When the participants were on the low-fat diet instead, fat burning was higher than fat accumulation and the fat loss was indeed also greater. Fat oxidation was reduced with 50 kcal per day, but since they ate very little fat, the net effect implies that they burned stored fat as energy.

This can be compared to when the participants were on the high-fat diet, with an increased fat oxidation of 425 kcal per day. Even so, the net effect on stored fat was smaller than that corresponding to the low-fat diet as the participants burned less stored fat as energy.

Equal Amount of Calories With Different Results

There was no difference in energy consumption between the participants, the participants ate just as many calories and burned an equal amount of calories. Yet, the participants were able to lose more stored fat mass when they ate the low-fat diet. But in terms of absolute number, the participants who ate fewer carbohydrates than fat, lost more mass in total, 1,9 kilos compared to the low-fat diet, 1,3 kilos.

This is probably due to that carbohydrates binds more water and the fluid levels in the body probably differed. The participants also lost a little more muscle mass when they ate more fat and fewer carbohydrates.


This study shows that most likely there is little difference in how much energy you need to eat each day depending on how much carbohydrates or fat you eat.

As for the small difference in fat loss between the two diets in this study, it probably has little real world application for most people. As stable weight loss spans, a longer time horizon and you need to be comfortable with the diet.

But perhaps the real message here is that easier than counting carbs or fat, it is more important that we are paying attention to where our carbs and fats are coming from. Making dramatic dietary changes is hard and many people won’t achieve a very low level of carb reduction, let alone sustain it. Indeed, a variety of diets can lead to weight loss, but what matters most is consistently eating fewer calories.

Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity