While prenatal vitamins contain important micronutrients--vitamins and minerals--that you need for a healthy pregnancy and a healthy baby, the nutrients in the vitamin pills don't provide you with energy. As such, they can't affect your body weight or make you fat--whether or not you're pregnant.
Body Fat
Body fat is your body's way of storing extra calories for periods of fasting. If you consume more energy-containing nutrients at a given time than you need to fill your cells' energy needs, the cells will convert the excess into triglycerides, which are fat molecules. Fat cells then store these triglycerides, releasing them into the bloodstream if you become hungry or don't get enough calories from food at a later date.
Energy-Containing Nutrients
Your body can only convert a few types of molecules into body fat. Carbohydrates, proteins and fat--collectively called macronutrients--contain calories. If you eat more of these than you require, you'll store the excess in the form of fat. Your cells also need another class of nutrients, called micronutrients, to remain healthy. Micronutrients include vitamins and minerals. While they're critical to healthy cells, they don't contain calories, and you can't convert them to fat.
Prenatal Vitamins
Prenatal vitamins don't contain any macronutrients. Instead, they're composed of a mixture of vitamins and minerals--micronutrients--appropriate to the needs of a pregnant or breastfeeding woman. If you're pregnant, your doctor has likely advised you to take a prenatal vitamin to ensure that you fill your micronutrient needs. While you're likely to gain weight during your pregnancy, this weight gain is due to hormones and the pregnancy, and isn't related to the vitamins.
Non-Pregnant Women
If you're not pregnant, you may be tempted to take prenatal vitamins for a variety of reasons. For instance, you may have heard that they'll make your hair and nails healthier. These are misconceptions, just as the idea that prenatal vitamins will make you fat is a misconception. In reality, prenatal vitamins can't do anything that regular vitamins--whether you get them from foods or from supplements--can't do. Further, prenatal vitamins aren't appropriate for non-pregnant women, explains dietitian Katherine Zeratsky in an article on MayoClinic.com.
References
- "You: Having A Baby"; Michael Roizen, M.D. and Mehmet Oz, M.D.; 2009
- MayoClinic.com: Prenatal vitamins: OK for women who aren't pregnant?
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