Carbohydrate Timing and Portioning
Carbohydrate support immediate energy needs and glycogen synthesis up to its maximum rate. (Once you exceed immediate energy needs and glycogen synthesis rates, remaining carbs are converted to fat tissue.) Although laboratory glycogen synthesis studies have reported rates up to around 0.8 g per pound per hour (g/lb/hr), these are unlikely outside of laboratory conditions. Intakes over 0.4 g/lb/hr have been shown to exceed intestinal absorption rates. In reality, the upper limit for glycogen replenishment in individuals is probably around 0.3 g/lb/h. This means that a 150-lb. individual should not exceed approximately 50 g of carbohydrate per hour, even when doing their hardest training. If this person eats every 4 hours, 200 g of carbs per meal should be their absolute maximum. Not the most relevant limit, as that is much more carbohydrate than most people need or would eat per meal, but pertinent for timing extremes. For example, in some intermittent fasting diets, people might eat all their daily carbohydrates in one meal.
Maximum glycogen synthesis rate becomes relevant here because if you weigh 150 lb. and your daily carb allotment is 500 g of carbs and you only eat one meal, as many as 300 g of those carbs may go toward fat storage.
Because carbs are significantly anti-catabolic, their consumption in most meals is recommended in order to prevent muscle loss (figure 4.8). This is especially true on a hypocaloric diet; if smaller meals are composed of only protein, most of that will be burned for energy with little left to go toward muscle retention. Carbs are ideal as a fuel source to spare protein, and in fact your body will tend to use them preferentially for that purpose if they are co-ingested with protein.
The benefit of increasing carbohydrate intake for anti-catabolism is non-linear, however, and probably loses potency above values of 0.1 g/lb/hr. This means that our 150-lb. athlete does not likely need more than 60 g of carbs per meal for this purpose when eating every 4 hours. In contrast to protein intake, lower carbs in one meal make the musculature more sensitive to absorbing higher amounts of carbs in the next meal, so under-eating carbs in one meal can be compensated with overeating carbs in the next to a greater extent. Maximum carbohydrate intake recommendations are therefore in the range of 0.1 to 0.2 g/lb/hr, which translates to around 3.0 to 5.0 g per pound of bodyweight per day. This is the maximum amount of carbohydrates that most individuals would be able to absorb under heavy training conditions. General daily recommendations for normal training and for non-training days will be substantially lower.
Details for appropriately assigning carbs will be discussed in depth in chapter 10.
Figure 4.8 A. Eating only your needed protein at a given meal can prevent catabolism (reaches the threshold to prevent FBR; fractional breakdown rate of muscle tissue), but leaves no protein to support anabolism (reach the threshold to support FSR; fractional synthetic rate of muscle tissue). Adding carbs allows the meal to reach the needed protein and total calories to reach the FSR threshold. B. Different ratios of macronutrients in addition to protein can be consumed to reach the FSR threshold.
STRUCTURING MEALS AROUND PHYSICAL ACTIVITY
Carbohydrate also plays a very important role in meal timing around activity–an important component of nutrient timing. Your body has different nutritional needs depending on recent, present, or upcoming physical activity. We can outline six general periods, each with unique nutritional needs:
1.Pre-training window
2.Intra-training window
3.Post-training window
4.High-activity, non-training periods
5.Low-activity periods
6.Bedtime
Pre-Training Window
The pre-training window refers to the 30 minutes to 4 hours before training begins. During this window, carbohydrate is needed to top off muscle glycogen stores and help regulate blood glucose levels in preparation for the high-energy demands of training. A state where muscle glycogen stores are full has two interesting benefits. The better known of these is to energetically support high-intensity muscular activity. A lesser known benefit is that full muscle glycogen stores directly signal the muscle to become more anabolic, which enhances muscle retention or growth (depending on calorie intake).
Pre-training window meal restrictions include limiting meal size and avoiding slow-digesting foods when the meal is eaten closer to training. Most blood will leave the GI tract during high physical exertion in order to circulate between the working muscles, heart, and lungs. Without blood to pull nutrients out of the intestines, undigested food in the GI tract can lead to discomfort, nausea, or even vomiting. If an individual is too full to perform well after a large meal, training will be impaired, ultimately defeating the purpose of the pre-training meal. Likewise, having insufficient energy for training from fasting all morning can have equivalently detrimental effects.
The size of the pre-training meal and digestion time of its contents should be scaled to the time between this meal and training (figure 4.9). Larger, slower digesting meals can be an option if the pre-training meal comes three to four hours before training. In contrast, a very small amount of quickly digesting protein and carbs (with minimal fat and fiber) should be consumed if one is eating around 30 minutes before training. In the latter case, some intra-workout carbohydrate ingestion may be advisable to prevent sudden changes in blood sugar due to the fast absorbing carbs and their rapid use during training. Maintaining stable blood sugar is preferable for best performances.
Figure 4.9 A. Smaller meals can be eaten closer to training time and still provide sufficient nutrient availability to support training. B. Larger meals should be eaten if the pre-training meal will occur earlier so that there is still sufficient nutrient availability by the time training occurs.
Intra-Training Window
Most workouts, especially those that last less than an hour, rely almost exclusively on stored glycogen and pre-workout nutrients. Intra-workout nutrition becomes more useful for longer workouts (more than 60 minutes). Fast-digesting carbohydrates can be combined with small doses of fast-digesting protein to supply the working muscles with an anti-catabolic mix. In addition, fast-digesting carbs can help maintain blood glucose levels through the workout, which is both anti-catabolic and supports performance. The top end recommendation for intra-training nutrition is about 5 to 10% of daily protein per hour up to the maximum carbohydrate depending on the workout. This protein amount is reasonable for most, but the carb amount should be decreased from the maximum for all but super-high-volume workouts like endurance