Fasting and weight loss - does it work? Will you burn fat while fasting, or will it make your metabolism crash... along with plaguing you with mood swings?
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The Simple Version:
When you fast, uou're not using food for fuel - so you have to use fat. Short term, weight loss and fasting go hand-in-hand.
The Longer Version:
When you fast, you aren't using the food in your stomach for fuel. So your blood glucose levels gradually fall as you use up that source of energy, and your insulin levels fall as well - since the primary trigger for insulin release is consumption of carbohydrates and, to a lesser extent, protein.
This allows your fat to be mobilized so you can start using it for fuel, and it has been known for some time that fat is your primary fuel source when you're not eating. In the words of the 1907 German physiologist Adolf Magnus Levy, "The fat streams from the deposits back again into the blood…".
Fasting and weight loss happens naturally every 24 hours when you go to sleep, since you're in a fasted state then. Your insulin drops, you use fat for fuel, and you aren't hungry again 'till you wake up.
Interestingly, if you eat a diet high in fat and protein and low in carbohydrates, your body gets more used to metabolizing fats and proteins as fuel. Since your body gets used to using them for fuel, you don't get a sugar 'crash' if you haven't eaten for 3 hours.
I noticed this lack of sugar crash when I tried out a low carb diet. The low carb diet, however, also makes exercise weird at first - I felt totally drained for the first few days of the diet, before my body got used to using fat and protein for energy regularly. Eventually, though, the low carb diet complemented my intermittent fasting nicely.
OK, this part of fasting and weight loss gets a little science-y, but stay with me. I promise that it'll make sense!
Your muscle and fat cells have the enzyme lipoprotein lipase (LPL) in their outer cell walls. LPL's job is to pull fats out of your bloodstream and use them for fuel.
During exercise, the LPL activity in the muscles increases. This gives your muscles a much greater ability to use your fat for fuel.
Additionally, after exercise LPL activity in fat cells is increased. This is done so that you can re-stock any fats that were released into the blood.
If you do exercise after eating carbohydrates, you burn the glycogen (sugar) stored in your muscles and then start burning the carbohydrate's energy. That's why carbohydrate loading is used to increase athletic performance.
But fasting to lose weight with exercise is different. With fasted exercise, and not feeding yourself right after the exercise, you more directly metabolize fat for fuel - basically, you burn more fat! Cool!
From personal experience, I advise that you to not jump headlong into fasted exercise. Especially if you're not used to it, and your body isn't used to metabolizing fat for fuel, you will feel pretty crappy the first few times you try it. Just a friendly warning!
From short term fasting, no.
Fasting and weight loss go together because when you're fasting, fat is your primary source of fuel. Fat makes up about 85% of the energy that you use, while protein from your muscles makes up only 15% at most for longer fasts.
In fact, some of the effects of fasting can actually help you build muscle. Short-term fasting causes the release of growth hormone and increases your body’s sensitivity to insulin, which helps you get bigger muscles and burn fat.
If you fast for long periods of time, like three days or longer, losing muscle may be an issue. But if you’re skipping a meal or fasting for one day, you won't lose muscle.
You fast for about 8 hours every night when you're asleep and don't worry about losing muscle. 16 or 24 hours really isn't that different.
Fasting for a day or two will not cause your metabolism to crash, or you to stop burning calories. Neither will skipping a meal.
Fasting for more than 3 days will start to lower your metabolism. But up until then, you'll be pretty fine, if a little hungry ;-)
Think about it from an evolutionary perspective: food was not always plentiful. It was not necessarily possible to eat 3 meals every day, or even a six small meals a day diet like some people recommend.
Fasting for long periods of time causes bad stuff to happen, but skipping a meal or not eating for just a day, or doing intermittent fasting? That's not going to hurt you, and there are actually a lot of benefits you can get from fasting.
Fasting and weight loss work together naturally for short periods of time. Try it, and see if it works for you.
Just remember: the first time you try a fast, it'll probably be uncomfortable. Though it does work, fasting and weight loss isn't a walk in the park.
Oh, and be sure to sign up for the e-zine Starting Strong to get monthly strength training, exercise, and diet tips e-mailed to you - and access to the free e-book Train Smart, Eat Smart: Exercise Nutrition Hacks!
Fasting and Weight Loss: The Technique That'll Make All The Difference
1. Taubes, Gary. 2008. Good Calories, Bad Calories: Fats, Carbs, And The Controversial Science of Diet And Health. New York: Anchor Books. Pp. 385.
2. Ibid. Pp. 432.
3. Ibid. Pp. 341.
4. Ibid. Pp. 385.
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