i used to work at power world as an instructor - back in 1996... something we were instructed to do was to advice clients to have a pre-exercise snack about an hour or two before their workouts... recently i had a small argument with a bunch of doctors who said there is no bio-chemical basis for that claim... and that one can lose more fat stored in their body if they exercised hungry...

friends of mine buying fruits - possible pre-exercise snacks

quoting the manual

the personal trainer manual (which we used as a bible)¹ says:

• there are three methods in which muscle cells obtain their energy: fatty acids, glucose and a little bit (about 10%) from protein
• muscle cells can be in their aerobic state up to about 60% of its maximum effort - this is called the anaerobic threshold
• exercise intensity is ratio between the muscle cells present effort to its maximum effort
• depending on the exercise intensity muscle cells might be in aerobic or anaerobic states
• beyond this threshold, the muscle cell relies on getting its energy without oxygen
• protein is used as a fuel the muscle cell for its metabolic activities
• your body responds to higher intensity exercise - body's aerobic capacity (maximum oxygen consumption, VO₂ max) increases with exercise intensity over time (in terms of months)
• muscles are very active cells... even during sleep, muscles amount to about 25% of a person's metabolic rate
• loss of muscle cells or decrease in tone would lead to a lower basal metabolic rate (bmr)

high 7+ MET activity

according to the manual¹, the recommended intensity range is 60-85% for cardiovascular fitness... people with very low fitness levels (like our former speaker) can benefit from intensities as low as 40-50%... above 85% the body goes in to the anaerobic mode and although there is no fat burning effect there are cardiovascular benefits at this level of intensity...

Table 7.7: Training Method Selection¹

a MET is a multiple of resting oxygen per minute... 1 MET equals a person's oxygen uptake at rest... which is about 3.5 ml of oxygen per kg body weight per min...

low 2.0 MET activity

so for a beginner (2-3 METs) the recommended aerobic activity would be: ballroom dancing and walking... hmmm... as you progress from each fitness level - you are able to burn more calories during exercise and your bmr also goes up and thus you can burn more fat!

our thinking was...

• the purposes of weight loss exercise are to increase your cardiovascular fitness, increase basal metabolic rate (bmr), increase anaerobic threshold, VO₂ max, reduce muscle loss...
• an increase in your bmr contributes to consuming more energy
• an increase in your anaerobic threshold contributes to using more fat

if you were expending 1800 calories per day in the first month and lets say you consumed 1800 calories for argument sake... after a progressive cardiovascular routine your metabolic rate would have increased up to 2000 calories per day (on a rest day)... so thats a 200 increase... and you didnt even workout... this is the benefit of progressive exercising and that what we were trying our clients to get at...

back to the issue with eating before the workout

the manual¹ has cited on several occasions that protein is used in special cases like when you dont consume enough energy... according to the doctors who i had this argument with - they claim that this is true when you've consumed below 1000 calories...

the doctors claimed that if you're in a fasting state and provided that you are exercising in a "safe aerobic" zone... the muscles would obtain its energy from stored fat because fatty acids are produced by breaking up fat stores in your body when you're in fasting state...

but what happens if you increase the intensity? some amount of energy will be obtained from glycogen... but you're in a fasting state... so fine there is still some amount of glucose in the blood to supply that energy... which will starve other organs - like the brain... so you cannot safely increase the intensity above 60% if you're in a fasting state...

why increase the intensity? because we want to progress to a higher fitness level because it burns more fat (see above)... the most you'd burn during your 30 min or 45 min routine would a few hundred calories... the actual benefit is during rest of the day... when your body is not exercising... compromising any muscle loss is disadvantageous because it only brings your bmr down... you'd rather expend a few calories off the fat you ate before your exercise routing and allow your body to be in fasting state during rest of the day because the elevated bmr would deplete fat stores in your body

quoting...

Energy for physical activity comes from glycogen stores in the muscles. Because glycogen stays in your muscles basically until you need it, you don't need to fuel up right before working out, says Katherine A. Beals, an associate professor in the Division of Nutrition and Department of Family Practice and Preventive Medicine at the University of Utah.²

"If your brain isn't fed, it says, 'This isn't fun,' and your motivation goes down," Clark says. "People think, 'If I don't eat [before working out] I'll burn fat and lose weight,' but it doesn't work that way. Losing weight is determined by your calories-in/calories-out equation at the end of the day." As we say often, to safely lose weight you want to run a daily deficit of 200 to 300 calories.²

conclusion

• the reason why you exercise is because you have a fitness goal - it could be weight loss, muscle building, general fitness or whatever... you want to be slimmer, stronger, muscular or fitter six months or one year from now...
• working out hungry would mean that you would be burning more fat that is stored in your body during your workout but it works against you when you're trying to progress with your fitness level...
• if you're working out after a pre-exercise meal (after 1-2 hours, allowing time for food to digest) stomach your alertness is high and thus the motivation to increase the exercise intensity... increasing exercise intensity progressively elevates your fitness level and thus your metabolic rate...
• over time (months) you will be burning more calories than you used to burn before... the pre-exercise meal enables you to achieve your fitness goal faster because your are more alert and focused during your workout... you are able to push yourself a bit more than the previous week...

references

1. Personal Trainer Manual (2nd Edition) published by the American Council on Exercise (ACE)
2. Washington Post: Feeding an Exercise Habit