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Pre-Workout Snacks Increase Calorie and Fat Burn

Scientists, more and more, are taking the guess work out of what to eat before working out. Kyle J. Hackney, M.Ed, Department of Exercise Science, Syracuse University, Syracuse, NY, and his colleagues investigated the effect of eating protein or carbohydrate 20 minutes before heavy resistance training.

Strength training stimulates post-workout calorie burn (metabolism) two ways. First, the body has to work overtime to replenish fuel, and secondly, the body also has to put extra effort into rebuilding muscle tissue. Both carbs and fat are burned in the process. 

Hackney and his team predicted that the protein snack would increase metabolism more than the carb snack, theorizing that extra protein would encourage added muscle formation. That appeared to be true at 24 hours, but not at 48 hours.

The study measured calorie burn two ways: Resting energy expenditure (REE) and respiratory exchange ratio (RER). (No attempt was made to differentiate between the metabolic effects of the snacks and the workout.)

REE is what you'd assume. It represents the minimum amount of energy required to maintain cellular processes at rest. It has been estimated to account for 60% to 70% of total energy expenditure. (Total energy expenditure would include active states.) Energy expended for muscle rebuilding is estimated to account for up to 20% of REE in well-trained athletes. An increase in post-workout REE would influence energy balance and body composition.

RER is less straightforward. A decrease in RER means an increase in fat burn.

Respiratory exchange ratio (RER) refers to the amount of oxygen inhaled in relation to the amount of carbon dioxide exhaled. The ratio indicates whether carbohydrate or fat is being used to supply the body with energy. A low ratio means fat is being burned, and a high ratio means carbs are the predominant source of energy. (Fat can only be burned in the presence of oxygen; carbs are burned when the oxygen supply is depleted.) 

Respiratory exchange ratio (RER) is normally about 0.8 at rest.

An RER of 0.70 indicates that fat is the predominant fuel source, 0.85 suggests a mix of fat and carbohydrates, and a value of 1.00 or above says that carbohydrate is the predominant fuel. (These figures come from Wikipedia.)

The Study

The study found that both protein and carb pre-workout snacks increased resting energy expenditure (REE) and decreased respiratory exchange ratio (RER). Here’s how they did it, what they found, and what it means.

Eight resistance trained subjects (five men and three women) participated in a double-blind two-trial crossover designed study; double-blind means neither researcher or subject knew which snack was consumed. REE and RER were measured (7:00 a.m.) on four consecutive days. On the second day (after measurements), subjects consumed 376 calories of either protein (18 g of whey protein, 2 g of carbohydrate, 1.5 g of fat) or carbohydrate (1 g protein, 19 g of carbohydrate, 1 g of fat) 20 minutes before a single bout of heavy resistance training (whole body, nine exercises, 4 sets, 70%-75% of 1-rep max). REE and RER were measured again at 24 and 48 hours (days 3 and 4) after the workout.

During trial 2, at least 30 days later, the same protocol was followed, except subjects consumed the other snack before the workout. If they had the protein snack before, they had the carb snack the second time around.

Day 1 was used for REE and RER baseline (no snack or workout). Total energy intake was recorded during the first 4-day trial, and subjects were instructed to replicate the diet in the second 4-day trial. Subjects kept a dietary journal. So total calorie intake was essentially the same for both trials, protein and carb. 


Compared to baseline, REE was significantly elevated in both the protein trial (PRO) and the carbohydrate trial (CHO) at 24 and 48 hours after exercise; at 24 hours, it was 99.7 for PRO and 94.6 for CHO, and at 48 hours, 95.0 for PRO and 97.7 for CHO. In short, resting energy expenditure (REE) was higher after the protein snack at 24 hours, but higher after the carb snack at 48 hours. (The figures used represent calories per kilogram—of muscle mass?)

The researchers speculated that the greater energy increase after the protein snack was due to “preferentially increasing amino acid availability in skeletal muscles that were damaged during the heavy resistance training session.” In short, they believe the added protein allowed more muscle synthesis to take place, which upped  energy demand.

“In this regard, our investigation has shown that protein before heavy resistance training (HRT) increased REE 8.5% at 24 hours after the exercise session compared with a 3.5% increase in the CHO [trial]. Therefore, PRO before HRT resulted in an additional 5% increase in REE…” (Again, the results were reversed at 48 hours.)

In addition, respiratory exchange ratio (RER) was significantly decreased 24 hours after exercise for both CHO (0.74 compared to 0.78 at baseline) and PRO (0.74 compared to 0.79 at baseline). In short, resting fat burn increased by about the same amount after the two snacks.

Which Snack?

“Timing PRO before heavy resistance training may be a simple and effective strategy to increase energy expenditure by elevating REE the day after resistance training,” the researchers concluded. “Increasing REE could facilitate reductions in body fat mass and improve body composition if nutritional intake is stable,” they added. (As noted earlier, calorie intake was essentially the same in both trials.) 

That doesn’t mean that eating carbs before workouts is a bad idea. Clearly, it’s a good idea.

Both snacks significantly elevated energy expenditure 24 and 48 hours after training. The study also found that both snacks decreased respiratory exchange ratio 24 hours after training, indicating that fat burn increased.

What’s new is that protein should also be included in your pre-workout snack. That’s “the main finding” in the study. A pre-workout snack should include carbs—and protein.

I usually have a Tiger’s Milk Nutrition Bar (145 calories, 18 g carb, 7 g protein, 5 g fat) and a glass of skimmed milk (90 calories, 12 g carb, 9 g protein, 0 g fat) 20 or 30 minutes before workouts. The carbs stabilize my blood sugar after a morning of work, putting me in the mood to train. (I usually train at about noon.) The carbs also provide fuel to keep me going until the end of the workout. The new study says that carbs also increase my metabolism, burning carbs and fat. 

The protein revs up my metabolism even more—burning carbs and fat—and provides the building blocks for my muscles.

What’s in your pre-workout snack?

The new study was reported in the May, 2010, issue of the journal Medicine and Science in Sports and Exercise.

(BTW, training on empty kills motivation and cannibalizes muscle. It’s a bad idea. http://www.cbass.com/TrainingOnEmpty.htm )

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