There's a common misconception in the general public that weight gain is a result of digestive disorders and, more specifically, of digestive enzyme deficiencies. This is not the case; if anything, such a disorder would have the opposite effect. Having lactose intolerance can't cause you to gain weight, but it can cause a variety of other symptoms.
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Lactose intolerance is the inability to digest lactose, which is milk sugar. You need different enzymes to digest the various sugars -- table sugar, milk sugar, and malt sugar, for instance -- in your food, and if you're missing the lactose-digesting enzyme, you can't digest the sugar. It's relatively uncommon to have lactose intolerance when you're young, but it becomes increasingly common with advancing age. You can also become lactose intolerant if your intestines have been affected by illness or if you've sustained an injury to the digestive tract.
Common perceptions to the contrary, digestive enzyme deficiencies don't cause weight gain. Weight gain is the result of taking in more energy-providing nutrient molecules each day -- proteins, carbohydrates and fats -- than you need to sustain your cellular function. You can convert proteins and carbohydrates to fats, and you store fats in your adipose tissue, explains Dr. Lauralee Sherwood in her book "Human Physiology." Regardless of what triggers your weight gain, it's always ultimately the result of caloric imbalance.
Lactose Intolerance and Weight Gain
You can't gain weight as a result of lactose intolerance because the intolerance actually means you're getting less nutrition from your food -- at least when the food contains dairy -- not more. If you don't produce sufficient lactase to break down the lactose you take in, the lactose simply passes on through to your large intestine. There, bacteria break it down and use it for energy. This can cause gas and cramping, explains MayoClinic.com.
If you're lactose intolerant, you may wish to take supplemental lactase enzyme to help prevent the uncomfortable gastrointestinal symptoms you get when you consume dairy. These won't help you lose weight -- if anything, they'll mean you're getting more calories from your dairy-containing food, so you'll want to make sure you're eating an appropriate amount of food -- but they will help you digest lactose so that it doesn't cause problems in your lower gut.