The weight gain sometimes associated with drinking beer isn't necessarily caused by a decrease in metabolism. However, drinking beer can temporarily affect your metabolism and increase hunger levels, which can lead to the dreaded beer belly. How much beer you drink, and how you drink it, determine its overall effects on your metabolism and weight.
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Alcohol and Metabolism
You burn less fat and are more likely to store any calories as body fat when you drink alcohol. That's because alcohol metabolism uses some of the same pathways as fat metabolism, so drinking beer may temporarily slow fat metabolism. And because your body can't save the calories from alcohol to use later, it needs to use them first, which also delays fat breakdown.
Your body can only process about one drink per hour -- the equivalent of 12 ounces of beer, a 1-ounce shot of hard liquor, or a 5-ounce glass of wine -- so drinking more than this increases the amount of time before your body can start to metabolize the fat from the food you eat.
Beer and Calorie Consumption During Meals
Drinking beer or wine may make it more likely you'll eat more at meals than drinking soda, notes a study published in the International Journal of Obesity in 2002. (ref 9 abstract) Your body doesn't compensate for the calories you get from alcohol, which means that these calories aren't likely to decrease the number of calories you eat from other foods, according to a review article published in Physiology & Behavior in 2010. (ref 10) However, this effect doesn't come from any change to your metabolism -- it just means you're taking in more calories.
Alcohol and Weight Gain
Beer doesn't necessarily cause weight gain, but how much and how often you drink affect the likelihood alcohol will cause you to put on weight. People who regularly drink small amounts of alcohol are less likely to be overweight than those who drink large amounts of alcohol infrequently, according to a review article published in the American Journal of Epidemiology in 2005. However, if you're already overweight, drinking beer or any kind of alcohol might increase your risk of further weight gain, notes an article published in Critical Reviews in Clinical Laboratory Sciences in 2005.
You need to take into account the calories in your beer, as well as those of anything extra you eat while you're drinking. Regular beer has about 145 calories per 12-ounce serving, while stout beers have about 190 calories each; lagers have approximately 120 calories each. Eat too many calories and you'll gain weight, regardless of where the calories come from.
Moderate Beer Consumption
Moderate alcohol consumption means no more than one drink per day for women and no more than two per day for men. This doesn't mean you can "save up" your drinks from one day and have extra at a later date. Drinking more than this makes it more likely you'll suffer adverse effects and less likely to experience any potential health benefits. You'll also be more likely to overeat if you drink more than this, as it can adversely affect your blood sugar levels and make you especially hungry.