Study Spotlight: Are you better off cutting carbs or fat from your diet for fat loss?

As a researcher I try to be as objective as possible but I must admit that I have my own biased opinions on few topics and this is certainly an example of one of them.  However, I must give credit where credit is due and this is a very well-controlled study and therefore impossible to ignore the results.

A lot of people question whether it's more effective to cut carbohydrates or fat from your daily intake if your goal is fat loss. There are several theories supporting both sides of the argument and evidence to support both. However tightly controlled metabolic-ward studies in human models are far and few between.  Recently an article was published that sought to to examine whether fat reduction or carb reduction would have a greater impact on fat loss and to confirm results from a mathematical prediction model.  It should be noted, that most of the time when researchers or people in a real-world scenarios reduce a certain macronutrient they often compensate by consuming more of another. For example, if people follow a low-carb diet they often consume more protein and fat in their diet to account for a desired total calorie intake per their goal, which is not the case in the current study.

That's what makes this study so interesting.

What did they do?
The researchers put individuals on a 6-day reduced fat diet and reduced carbohydrate diet equating to a 30% reduction in total calories each day.  Each subject completed both periods of dietary restriction using a cross-over design and changes in metabolism, body composition and hormones were assessed.

What did they find?
Calorie for calorie, the researchers concluded that reducing fat intake resulted in a greater loss in body fat with rates of fat oxidation remaining the same when compared to carbohydrate reduction.  However the researchers did find that carbohydrate restriction increased rates of fat oxidation, reduced insulin levels and reduced carbohydrate oxidation suggesting an improvement in metabolic fuel selection aka metabolic efficiency (Which in the long-term could potentially be more advantagous).  Body composition results via DEXA revealed that both diets resulted in reduced body mass and overall fat mass with no differences observed between diets.

Keep in mind, this is when the remaining macronutrients and total calories are kept in check.

Again, I hate to be biased but this is a situation where I can't help but to speculate on a lot of "WHAT IF's...."  For example, the researchers mentioned that the remaining macronutrient amounts were kept in check, but what would happen if protein was increased as is often the case in a low-carbohydrate date. Also, the subjects did exercise each day but it was limited to 1 hour on a treadmill. What if they would have completed HIIT training or resistance training instead, aiming to increase EPOC, FFM and ultimately REE during the controlled testing period? AND, what if they combined a higher protein intake WITH a resistance training protocol? Again, these are questions beyond the scope of the initial study and situations where we can only speculate.

We have to give credit where credit is due and this is a beautiful study that answered some fundamental physiological questions regarding energy balance, selective macronutrient restriction and resulting changes in body composition and metabolism.  I myself have to remain objective on the issue and respect the data and what they are telling us.

Take Home Messages:
From this study we can conclude that both carbohydrate and fat restriction, totaling a 30% drop in total calories are effective strategies for fat loss.  A reduced fat intake may be slightly more beneficial for greater daily fat loss while a reduced carbohydrate intake may help reduce insulin levels and improve fuel utilization.

Find the study here.