I recently engaged in a discussion (okay, maybe a debate) about low-carbohydrate diets as a tool for weight loss in recreational exercisers (that refers to people who exercise without training for a competition, which sets them apart from athletes, technically). I’ve posted before that diets don’t work, so I’m not a proponent of low-carbohydrate ‘diets’, especially for individuals who train rigorously on a regular basis. There’s no evidence that these diets are more effective than those that contain higher levels of carbohydrates, and there’s plenty of evidence to show that low-carbohydrate diets (<43% total calories) reduce both aerobic and anaerobic performance (1). At the moment, low-carbohydrate diets are the ‘next big thing,’ and while you may see quite a few anecdotal blog posts that make this appear like a controversial issue, it really isn’t within the sport nutrition community. The current evidence-based consensus is that athletes—recreational or elite—require a majority of their calories to come from carbohydrates (1).
When I brought up the topic of evidence-based practice, my counterpart brought up some research that had been done in elite cyclists, citing that their performance had improved on a high-fat diet. I reminded him that his clients, while certainly exceptional, were not elite cyclists, but in fact recreational exercisers engaging in primarily high-intensity anaerobic training. They fall extremely far away from elite cyclists on the spectrum of intensity and duration of training, and therefore exhibit extremely different physiological adaptations which require specific nutritional prescriptions. Evidence-based practice can’t be reduced to the generalized application of the results of a single research study to all of your clients. Even a very well-designed study can’t be used as a one-size-fits-all nutrition prescription!
Likewise, clients—you need to vet your coaches. The regulation of terms like ‘nutritionist’ or ‘nutrition coach’ is extremely loose and variable by state. (You can worry less about the term ‘registered dietitian’ or ‘sport dietitian’ as these are federally-regulated titles that require licensure in the US.) Your nutritionist may have absolutely no background in exercise physiology or sport nutrition, let alone a vetted certification.(I’ll be posting more on this in my next blog which will explain the types of certifications, what to look for, and what to look out for).
So, back to the issue at hand: evidence-based performance nutrition. As Nancy Clark, MS RD CSSD stated, most clients are, “mere mortals,” which is simply to say that most are not elite athletes. (Though, they are exceptional simply for exercising on a regular basis and meeting or exceeding guidelines set forth by the American College of Sports Medicine. Only 20% of Americans meet the guidelines for both aerobic and resistance training!) Between her statement during the seminar last weekend and this recent conversation, I realized that this is the second major lesson I took away from that experience: We must know our clients, inside and out!
The physiology of elite athletes differs from that of recreational athletes in a few key ways, but a major factor in sport nutrition is the fact that elite athletes can use fat for energy at much higher intensities, whereas non-elite exercisers rely primarily on carbohydrates for energy at high intensities. So, in some cases, an elite athlete may respond favorably, or at least not see much of a reduction in performance, on a low-carbohydrate diet. That being said, the current consensus in sport nutrition supports a high-carbohydrate (>43%) diet for all athletes because levels of muscle glycogen—the storage form of carbohydrate in the body—correlates with exercise capacity (1). Resistance training reduces muscle glycogen after just one set, and low glycogen levels lead to a reduced capacity to do work. Work is what burns calories and elicits training adaptations.
The inability to perform with sufficient intensity will result in…well, no results! Low glycogen may even be one mechanism that leads to overtraining syndrome. If you haven’t been recovering between exercise sessions, your progress has stalled, and you’re feeling mentally and physically run-down, it would be an excellent idea to assess your carbohydrate intake; many athletes fail to eat sufficient carbohydrates (1).
Differences exist not only between elite and non-elite athletes/exercisers, but aerobic (endurance) versus anaerobic (resistance-training, HIIT, intermittent) trainees as well. The extent to which a body can use fats for energy is dependent upon exercise intensity. The anaerobic energy systems—those utilized during high-intensity activities like sprinting, HIIT, and circuit-training—can’t utilize fats. (That’s not to say they won’t cause fat loss; combined with a caloric deficit, the excess post-exercise oxygen consumption (EPOC) will burn plenty of calories from fat—but that’s a different blog post!) So, my counterpart’s clients, all of whom perform either resistance training or HIIT-style workouts on a regular basis, would certainly suffer a decrease in their performance, which means reduced work output and therefore wouldn’t be maximizing their potential adaptations. In less scientific terms: their workouts would suffer so they’d burn fewer calories and induce less stimulation required for adaptations like muscle growth and strength. Anaerobic athletes who consume insufficient carbohydrates exhibit reduced power output and intensity (1).
A recent study in CrossFit athletes, many of whom eat a moderately-low carbohydrate diet, showed improved exercise capacity after three days of increased carbohydrate intake (6-8 grams per kilogram of bodyweight as compared to <6 grams per kilogram of bodyweight). Another study in trained cyclists showed that three days of a high-fat diet (68% calories from fat) followed by carbohydrate loading compromised sprint performance even though the single day of carbohydrate loading met the recommendations of 8-10 grams per kilogram of bodyweight. The elite cyclists my counterpart read about weren’t exercising at such high intensities (it’s not physiologically possible as aerobic energy systems are required after about two minutes), and were instead using aerobic pathways that can utilize both carbohydrates and fats, tapping into thousands of calories worth of stored energy in their adipose tissue to spare muscle glycogen for the final sprint. Not to mention, as elite endurance athletes, they had adapted more of the cellular ‘machinery’ required to use fats for energy in comparison to his clients. His ‘evidence’ to support the use of a low-carbohydrate diet was well-intentioned but entirely misappropriated.
The International Society of Sport Nutrition recommends an intake of 5-7 grams of carbohydrate per kilogram of bodyweight for endurance athletes performing moderate-intensity aerobic exercise, and 6-10 grams of carbohydrate per kilogram of bodyweight for anaerobic athletes. The minimum recommended intake is 3 grams per kilogram of bodyweight for individuals who aren’t physically active, though research has shown that intakes of less than 5 grams per kilogram or <43% total calories from carbohydrate will result in low muscle glycogen after successive bouts of exercise (1).
Fat loss comes from a caloric deficit—simply eating fewer calories than you’re burning. There is no need or justification for creating that deficit by reducing your carbohydrate intake. Your ‘diet’ shouldn’t be a short-term solution, but a lifestyle that you can maintain indefinitely while sustaining a rigorous training program to consistently elicit adaptations and improvements.
- Haff, G. C. (2008) Essentials of Sport Nutrition and Supplements: Carbohydrates. Totowa, New Jersey: Humana Press.