Tempted to give low carb diets a go? If you’re serious about your sport, you should know that low carb high fat (LCHF) diets – fashionable as they are – might actually hinder your performance. Here’s why.
First, the theory behind LCHF diets: By restricting carbs you force your muscles to burn more calories from fat and less from carbs during exercise, thus ‘sparing’ valuable glycogen stores and allowing you to exercise longer before experiencing fatigue.
Indeed, studies have indicated that the body adapts to LCHF diets by producing more fat-burning enzymes (such as hormone sensitive lipase) although the jury is still out as to whether this translates into superior performance in events. Many people also experience a (desirable) drop in body weight and body fat, which is regarded as advantageous in most sports.
So what’s not to like about LCHF diets? Well, if you’re more interested in high intensity performance as opposed to long distance low-moderate intensity training, then LCHF diets will not help your quest for gold. The reason? Your muscles cannot generate enough ATP (“energy”) per minute from just fat to keep pace with demand. You need (at least some) carbs to fuel this kind of activity. Without it, you simply won’t be able to generate maximal power and perform at your best. What’s more, high intensity exercise actually suppresses fat breakdown (lipolysis), making exercise feel much harder. In other words, high-intensity training on a LCHF diet feels an awful lot harder than training on a higher carb intake.
This was illustrated in a recent study. Eight competitive mountain bikers were placed on either a low carb high fat diet (15% carbs, 70% fat, 15% protein) or a balanced diet (50% carb, 30% fat, 20% protein) for four weeks in random order. Those following the low-carb diet became leaner than those on the balanced diet (11% body fat vs 14.9%). But the fat loss that the athletes experienced on the low-carb diet was associated with a significant drop in performance. On the balanced diet, the subjects managed 257 watts at lactate threshold intensity and 362 watts during a 15-minute maximal effort. On the low-carb diet, their power output dropped to 246 watts and 350 watts. According to the study authors, the cause of the decline on the low-carb diet was impairment of the muscles’ ability to burn carbs, which is critical to performance at higher intensities.
So, while a low carb diet may be useful if you’re trying to drop weight or if you’re focusing mainly on high volumes of low to moderate intensity training, it is not useful for training at high intensities (usually defined as anything over lactate threshold). The exact amount you should consume is individual and depends on how long and hard you train but as a guide, aim for 5 – 7 g/ kg body weight (that’s 350 – 490g for a 70kg athlete) if you train at high intensities for between 1 and 2 hours daily. If you train longer than 3 hours daily, you’ll need more like 7 – 10g carbs/ kg of bodyweight (that’s 490 – 700g for a 70kg athlete).
For more, see Food For Fitness (4th edition)