Carb Back-Loading: What Does the Evidence Say?

By Joseph Agu

Read Time: 8 minutes

TL;DR: Consuming most of your carbs later in the day isn’t some magical tool and the insulin fairy still isn’t the key to getting lean. Overall consumption of calories and macros is the most important thing to control

What is Carb Back-Loading?

Carb Back-Loading (CBL) –basically revolves around taking advantage of the supposed fluctuations in insulin sensitivity (IS) within the muscle and fat tissue throughout the day, as well as the non-insulin mediated uptake of glucose within the exercised muscles.

For example, insulin sensitivity in both muscle and fat tissue is generally higher in the morning relative to the evening [1]. As such, it has suggested that eating carbs in the morning/earlier in the day (when overall IS may be higher) relative to the evening, will result in greater glucose uptake by the muscle (a good thing), but also in the fat tissue (a potentially bad thing).

Therefore, it is is suggested that a way to get around this problem would be to train in the evenings as well as consuming almost all of your daily carbohydrate post-workout (PWO), whilst eating as little carbs as possible throughout the day. That way, you would supposedly take advantage of the reduced IS in fat tissue in the evenings, but also have the benefits of increased insulin sensitivity (more specifically, non-insulin mediated uptake of glucose) in the muscles PWO due to the evening training.

In addition, by avoiding carbs as much as possible during the day (when overall IS is high), fat gain via de novo lipogenesis (the creation of new fat tissue via carbohydrate), would apparently be minimised. Overall, this would hypothetically result in the potential for successful body recomposition (i.e. gain muscle whilst losing fat).

The following points briefly summarise how CBL works:

Shift calories to later in the day, eating lighter in the morning and early afternoon, and feast at night. This may include skipping breakfast. Keep carbs at an absolute minimum throughout the day until training. Train in the afternoon, at around 5pm or so. Start ingesting carbs after your training session, up to 30 minutes later. Continue eating carbs throughout the night.

Does research support the idea of CBL?

There are correlational data signifying associations between shifting the intake of calories at different times of the day and adiposity (generally favouring eating earlier in the day). However, given the limitations of observational research, I’ll solely focus on randomised control trials here.

The whole idea of shifting carbohydrate intake to later in the day is largely based on two studies, which are frequently cited throughout the book on CBL.

The first study by Keim et al [2] compared the effects of eating 70% of the day’s calories in the morning (AM) vs. the evening (PM) on body mass and body composition during a six-week hypocaloric diet (60% CHO, 18% PRO & 22% FAT) in a group of 10 women. It was found that the ingestion of the larger AM meals resulted in greater weight loss compared with the larger PM intake, but this extra weight loss consisted of lean body mass (LBM). Therefore, the consumption of larger PM meals resulted in greater preservation of lean body mass (LBM) and resulted in a greater reduction of fat mass (see table below).

This study possessed several design strengths, the most notable of which was that it was conducted in a metabolic ward, meaning that food intake was strictly controlled. Furthermore, the 10 women underwent a structured exercise programme consisting of cardio and resistance training, making the results somewhat more applicable for those implementing CBL. However, two notable limitations of this study include the relatively small sample size (10) and method of assessing body composition (total body electrical conductivity, which is similar to BIA). BIA isn’t the most accurate means of assessing body composition! [3]

The more recent trial used to support the evening carb intake of CBL is a 6-month study by Sofer et al [4], in which the authors compared the effects of carbs eaten mostly at dinner vs. eaten throughout the day, in diets consisting of 1300-1500 kcal (40-50% CHO, 20% PRO & 30-35% FAT) in a group of 78 Israeli police officers.

It was found that reductions in weight, body fat and waist circumference were greater in the evening-carb experimental condition vs. the control condition (Table below). In addition, glucose control, inflammation, blood lipids and satiety were improved to a greater degree in the evening-carb group.

Moreover, leptin levels decreased to a lesser degree in the experimental condition and may partially explain the better maintenance in satiety within this group, as well as the greater observed weight loss. It is possible that the greater reductions in satiety in the control group led to a greater caloric intake in comparison to the evening-carb group, and thus explaining the more favorable body composition results seen in the experimental group.

Although this study looks extremely promising for CBL with respect to all the anthropometric, hormonal and biomarker data, the methodological limitations of the investigation are worth briefly discussing. Whilst a specified diet was prescribed, dietary intake was self-reported (unlike the shorter trial above).

It is therefore possible that the participants’ reported intakes were inaccurate, especially when considering the hectic work patterns on police officers. Similarly, caloric intake wasn’t set according to the individual. Whilst still on the subject of food intake, an intake of 20% protein is the equivalent to roughly 65-75g per day.

As the participants in this trial had an average body mass of 98.3kg, this would equate to a daily protein intake of 0.66-0.76g/kg, which is below the RDA of 0.8g/kg. This intake is below that required [5] to spare muscle mass and promote satiety and is far below that typically consumed [6] by weight trainees looking to improve body composition. Therefore, the study’s relevance to such populations is questionable (not to mention the lack of a structured exercise programme).

Finally, when we look at the differences in weight loss between groups, the experimental group lost 2.54kg more than the control group over the 6-month trial, and was the only anthropometric measure to reach statistical significance. To put things into perspective, this equates to a greater weight loss of 14g per day, or 100g per week; this is hardly anything to write home about and surely isn’t worth the hassle if it doesn’t easily fit into your routine.

There are other controlled studies [7,8] similar to the two aforementioned, but no changes in body composition or weight loss were observed, probably due to short study durations (15 & 18 days, respectively) and other inherent limitations. Due to their neutral findings, these studies tend not to be mentioned by CBL advocates. Though, by the weight of the limited controlled evidence, it does seem that shifting caloric (and carbohydrate) intake to later in the day would provide a SLIGHT benefit with respect to body composition, hormonal changes and makers of health and disease.

Summary and Practical Recommendations

The general concept of CBL is supported by science. However, this science is limited to two studies with their fair share of limitations, rendering the topic inconclusive in the absence of compelling evidence. Moreover, if we consider shifting only carbohydrate as opposed to overall caloric intake, the data supporting the concept of CBL diminished to a single study. Nevertheless, if we consider the overall impact of calorie/carbohydrate placement on body composition from these two studies, though significant, in reality such differences are trivial.

In my opinion, the total macronutrients consumed by the end of the day will have the largest impact in terms of body composition changes; the author of the book on Carb Backloading even admits this (at least in regards to meal frequency), “the breakdown and distribution of calories and macro nutrients throughout the day matters far more than the number of meals”. At the end of the day, CBL will get some people results, but it will do so because of the caloric deficit and sufficient protein, not because of the intricate protocols.

As such, with respect to carb placement for body composition and performance, total intake is the primary consideration. A secondary consideration would be the positioning of these carbs in relation to training (around-workout nutrition) in order to optimise training performance. Once these factors are in place and consistently achieved, then, and only then, should someone have the option to experiment with hypothetical protocols.

References