Carbohydrate Back Loading or CBL is an approach to diet and exercise conceived and popularised by John Kiefer. It is widely used for its reported benefits on fat loss and muscle gain. Because the particulars of the diet have been largely covered in many other articles in the public domain, this article will not go into great depth about what CBL entails. Rather it will serve to critique some of the specific premises that CBL rests on.
Basic Premises of CBL
In a nutshell, the CBL theory is tied to the tissue-specific (i.e. muscle & fat) circadian fluctuations in insulin sensitivity and the effect of resistance exercise on these fluctuations.
The two overriding principles that CBL largely rests on are:
- In the morning, insulin sensitivity is high in both muscle and fat tissue. Therefore consuming carbs at this time can lead to muscle and fat growth.
- In the evening, insulin sensitivity is low. Therefore carbs at this time are best consumed on the back of resistance exercise, which selectively increases insulin sensitivity in muscle, but fat tissue still remains resistant to carbs. In this sense, PM exercise provides a relatively greater boost in insulin sensitivity, selectively in skeletal muscle.
The crux of Kiefer’s CBL theory is that carbs after a morning workout may be more fattening than those after an evening workout because insulin is more likely to store a proportionally greater amount of glucose as fat in adipose tissue. But in the evening, adipose tissue is supposedly less sensitive to insulin and therefore carbs consumed during this time are less likely to be stored as fat. What’s more, if one performs resistance exercise in the evening, this serves to selectively increase insulin sensitivity in muscle tissue at a time when it is relatively low. The final important piece of information is that CBL specifically advocates high GI carbs in the evening, so as to produce a sharper and shorter insulin spike, which Kiefer asserts is necessary for maximum muscle growth.
Downfalls with CBL
Kiefer’s CBL is plausible providing his theory on the circadian fluctuations in insulin sensitivity are true. The major issue with Kiefer’s theory is that muscle and adipose tisues insulin sensitivity throughout the day is regulated reciprocally. That’s is to say that muscle insulin sensitivity is highest in the morning and lowest at night1, 2 just like Kiefer says, but the opposite is true of adipose tissue. It is less sensitive to insulin in the morning and more sensitive at night2-4! This is a well-established scientific fact, clearly laid out in the cited references below.
Defining Insulin Sensitivity in Muscle & Fat Tissue
The major downfall of Kiefer’s CBL is its definition or understanding of the term insulin ‘sensitivity’. In the medical literature insulin sensitivity is most commonly assessed by measuring blood glucose levels after an oral glucose load. Adipose tissue only has a very small role in clearing this glucose. General estimates are that only 2-3% of the glucose loads ends up in adipose tissue while the vast majority goes to skeletal muscle.
When it comes to insulin sensitivity in adipose tissue, we are primarily concerned with insulin’s effect on fatty acid uptake and breakdown. As it turns out, small fluctuations in insulin have a much more dramatic effect on fatty acid levels in the blood, compared with the effects on blood glucose levels. What’s more, adipose tissue becomes more sensitive to insulin as the day wears on3-5.
What does this polarity in adipose tissue insulin sensitivity throughout the day mean practically? Well studies have shown that fatty acid levels and associated fat burning returns significantly quicker when an identical meal is consumed in the morning versus the evening6.
A Revised CBL Theory
Given the above facts, a revised application of CBL may in fact be:
Carbs in the morning are less fattening (due to lower adipose insulin sensitivity) and more muscle-building (due to higher muscle insulin sensitivity) than carbs in the evening. On the flipside, carbs at night are more fattening because insulin sensitivity is increased in adipose and reduced in muscle.
The Argument For CBL
Even allowing for the above facts, the reality is that Kiefer’s CBL has been successful for a significant number of people. So why might this be the case? It may have to do with the high fat, low carbohydrate diet that is prescribed for the morning and afternoon of CBL. While counterintuitive, low carbohydrate-high fat diets have recently been shown to be most effective at improving blood glucose control, insulin sensitivity and weight loss7, 8. It seems restricting carbohydrates to a level of approximately 50g per day can have dramatic positive effects on insulin sensitivity. Once an individuals’ insulin sensitivity is improved, they are going to store glucose more effectively, regardless of when it is eaten.
The other powerful component of CBL is regular resistance exercise. This too has proven positive effects on insulin sensitivity and blood glucose control in overweight individuals9. It's fair to say that any program that promotes regular carbohydrate restriction combined with regular resistance exercise has the potential to induce positive change in weight and insulin function.
1. Van Cauter E, et al. Roles of circadian rhythmicity and sleep in human glucose regulation. Endocr Rev. 1997;18(5):716-38.
2. Johnston JD. Physiological responses to food intake throughout the day. Nutr Res Rev. Jun 2014; 27(1): 107–118.
3. Sopowski MJ, et al. Postprandial triacylglycerol responses in simulated night and day shift: gender differences.J Biol Rhythms. 2001;16(3):272-6.
4. Burdge GC, et al. Effect of meal sequence on postprandial lipid, glucose and insulin responses in young men. Eur J Clin Nutr. 2003;57:1536–1544.
5. Swislocki ALM, Chen YDI, Golay A, Chang MO, Reaven GM.Insulin suppression of plasma-free fatty acid concentration in normal individuals, and patients with type 2 diabetes. Diabetologia 1987;30:622-6.Keller U, et al. Human ketone body production and utilization studied using tracer techniques: regulation by free fatty acids, insulin, catecholamines, and thyroid hormones. Diabetes Metab Rev. 1989;5(3):285-298.
7. Ballard KD, et al. Dietary carbohydrate restriction improves insulin sensitivity, blood pressure, microvascular function, and cellular adhesion markers in individuals taking statins. Nutr Res. 2013;33(11):905-12.
8. Paoli A, et al. Long term successful weight loss with a combination biphasic ketogenic Mediterranean diet and Mediterranean diet maintenance protocol. Nutrients. 2013;5(12):5205-17.
9. Croymans DM, et al. Resistance training improves indices of muscle insulin sensitivity and β-cell function in overweight/obese, sedentary young men. J Appl Physiol (1985). 2013;115(9):1245-53.