Most of you have probably heard the old saying, “You need to eat every 3 hours or your body will enter stress mode and your metabolism will grind to a halt!” You’ve probably heard this from friends, trainer, and maybe even Doctors. When I hear people state this myth it drives me nuts. For one, it shows a complete lack of understanding of the stress response. Secondly, it shows a complete misunderstanding of how the human body works in general. So let’s see where we can shoot this thing down.
To start off, the Thermic Effect of Food (TEF), the mechanism by which you are trying to increase caloric expenditure by eating 5 times a day, accounts for about 10% of calories. That’s right, TEF is related to the calorie load, not the number of times you eat. So, if you eat 2000 calories a day, the TEF will be around 200 calories whether you eat 3 times or 5 times. Now, if you lower calories to 1800, your body will eventually adjust metabolism to that rate and you will stop losing weight. However, if you increase consumption to 2200 calories, your body will actually increase metabolism. The point here is that there is no stress mode, the body just adapts to the number of calories you put in to it by increasing and decreasing efficiency as well as increasing fidgeting and non-exercise related energy consumption. This is fairly common knowledge outside of the Muscle and Fitness crowd.
Now, does your metabolism grind to a halt when you fail to eat every 3 hours? Given that studies of fasting have actually shown INCREASES in resting energy expenditure during fasts of 84 hours (1), I’m going to go out on a limb and say that skipping your mid morning snack should leave your metabolism pretty well intact. Oddly enough, given that the Autonomic Nervous System (ANS) partitions resources based on eating and physical activity, eating 5 times a day may be a bad idea for energy levels.
Your ANS controls most of the automatic processes in your body. You know, things like heart rate, blood pressure, digestion, breathing rate, etc. Most people have heard of the fight or flight, or stress, response. The fight or flight response is controlled by your ANS. You don’t need to know the specifics of it, but your ANS is a continuum from rest and digest to fight or flight and is handled by 2 branches of the ANS, the Sympathetic and Parasympathetic Nervous Systems.
When you encounter a stressful situation, your ANS increases blood flow to your muscles, floods your bloodstream with stress hormones, and increases heart rate, blood pressure, and breathing rate so that you can either fight what is coming at you, or flee and live to fight another day. This is handled by an increase in Sympathetic Nervous System activity. When you are resting or eating, your ANS directs blood flow to the organs of digestion while also lowering heart rate, blood pressure, and breathing rate. This is accomplished by an increase in Parasympathetic Nervous System activity.
This is why irritable bowel syndrome is so tightly linked to stress. If you are highly stressed, blood is shunted away from your organs of digestion and towards your muscles, preventing you from digesting your food properly. In the same way, if you are constantly digesting the food you’ve eaten, blood is shunted away from your muscles and toward your organs of digestion. This is why most people tend to feel sluggish after they eat. Optimally your body should switch between the two easily so that you have energy and you can digest your food properly. As such, it’s probably a good idea to give your organs of digestion a rest every now and again by eating 3 times a day or less. In fact, the above referenced study showed the increase in caloric expenditure during fasting was due to an increase in norepinepherine, one of the stress hormones secreted in response to fight or flight.
According to most accounts, we evolved by eating a very small meal in the morning followed by a large meal at night with no lunch in between. To say eating 5 times a day is necessary for fat loss shows a thorough misunderstanding of rudimentary human physiology.
References
1. Zauner, C. et al. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000 Jun; 71(6): 1511-5.
