I’ve had this article in the back of my mind for a while now but wanted to get the BCAA series out of the way first, not to mention that I’ve been ridiculously busy over the past few months. For those that don’t know anything about the author or the book, John Kiefer holds a bachelor’s degree in Physics and Mathematics as well as a master’s degree in Physics, and is the owner of the Dangerously Hardcorewebsite. In addition to Carb Back-Loading(CBL), Kiefer has written a previous book titled The Carb Nite Solution. From his credentials and by listening to him on various podcasts and YouTube videos, Kiefer comes across as an intelligent and genuinely nice guy, so that’s where the personal judgments stop. As such, in this review, I’ll take a reasonably in depth look at CBL and examine whether the claims behind it withstand scientific scrutiny.
What is Carb Back-Loading?
The premise behind 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. As such, Kiefer 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, Kiefer suggests 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 insulin sensitivity 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). Because of the relative complexity of CBL, it seems to be suitable for the intermediate-advanced weight trainees as opposed to beginners.
The following points briefly summarise how CBL works, in Kiefer’s words:
- 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.
I must admit, all this does sound cutting-edge and very impressive on paper, but is there any research to back up Kiefier’s hypotheses? Before moving onto the more critical aspects of this review, I do actually think that there are some positives that we can take away from CBL and it would be biased of me not to include them.
The strengths of CBL
Firstly, CBL is good for those who like to eat crap but still want to be reasonably lean. By doing so, it demonstrates that the most important factor in manipulating body comp is the total macronutrients consumed by the end of the day. For these reasons, it shows that you don’t have to be obsessive and eat clean all the time in order to get results. This is probably why the ‘if it fits your macros’ (IIFYM) approach to dieting is so popular also, but that is a topic for another day. By focusing on carbohydrate consumption in the evenings, CBL also nicely destroys the myth about not eating carbs after 6pm or some other random time. Although, at the same, it creates worry about eating carbs in the AM! It amuses me somewhat that the very same guys who aimed to front load their carbs by eating the majority in the morning, now do the very opposite in the fear of getting fat/minimising muscle gain. CBL is also good if it fits nicely with your current lifestyle (i.e. you train in the evenings and prefer to eat the majority of your food after training).
Perhaps another positive aspect of CBL is that it attempts to be evidence-based, and in doing so Kiefer cites 48 pages of references to support his points. He also highlights a few common misunderstandings that are prevalent in the fitness community, such as the fact that some types of fibre DO have an energy value (the type that can be fermented in the colon to produce short-chain fatty acids), as well as making the distinction between non-insulin dependent glucose uptake via GLUT4 translocation and insulin sensitivity per se (concepts that people often use interchangeably).
Finally, and perhaps most importantly, the general premise of CBL (i.e. shifting carb intake to later in the day) does have some scientific backing, the details of which are discussed in the following section.
Does research support the idea of CBL?
Before taking a look at some of the more specific claims in CBL, I’ll briefly review the research into whether moving carbs to later in the evening makes a difference in terms of fat loss or muscle gain. Specifically, I will only look at randomised-control trials (RCTs), since this is the only type of research that can establish cause and effect. There are correlational data signifying associations between shifting the intake of calories at different times of the day and adiposity. However, given the observational nature of such studies, which are confounded by a plethora of uncontrolled variables, they cannot establish cause and effect. As such, they are useful for generating hypotheses to be tested under controlled conditions, but drawing conclusions from this type of data is erroneous (perhaps someone should tell science journalists this).
The whole idea of shifting carbohydrate intake to later in the day is largely based on two studies, which are frequently cited throughout CBL. The first study by Keim et al. (1997), 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). As mentioned in part 3 of my BCAA series, BIA isn'tthe most accurate means of assessing body composition.
The more recent trial used to support the evening carb intake of CBL is a 6-month study by Sofer et al (2011), 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. 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 to spare muscle mass and promote satiety and is far below that typically consumed 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 (see here & here) 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. For those with more than a passing interest in the topic, Martin Berkhan of LeanGains does an excellent job of reviewing these studies. 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.
Click here for part 2.