If you're naturally thin, chances are you have a high metabolism. And while your metabolism makes it easy to avoid packing on excess pounds, it can actually get in the way of your results if you're trying to gain weight. Unfortunately, there's no supplement -- including herbal supplements -- proven to slow down your metabolism, or even impact your metabolism in any significant way. So you shouldn't count on any supplement to change your metabolism enough to make you lose weight or worry that your supplement of choice is secretly making you gain. However, some supplements -- like protein powders and weight-gainer powders -- provide lots of calories, which can help you gain weight, even if these supplements don't actually affect your metabolism.
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At its most basic level, your "metabolism" refers to the billions of chemical reactions that take place in your body as a function of your day-to-day life. Each one of those reactions requires a small amount of energy, and that collective energy makes up the calories you burn each day.
Part of your metabolism relies on your genetics; you might burn significantly more calories each day than someone with a slow metabolism, even if you're the same weight and height. Your lean muscle tissue also affects your metabolism because the more muscle you have, the more calories you burn. An active lifestyle increases your energy burn, too, and certain dietary choices, such as eating lots of protein, can also increase the calories you burn each day.
If you're muscular, have a genetically fast metabolism and live an active lifestyle, you likely have a relatively fast metabolism. That means, if you're looking to gain weight, you'll need to eat more than someone less muscular, less active or with a genetically slow metabolism.
Supplements and a Slow Metabolism
Most metabolism-related supplements, such as herb- or vitamin-containing pills, are marketed to speed up your metabolism, not slow it down. And, as of March 2016, no supplements are designed to slow down your metabolism. You're probably not missing out, though; very few studies show that simply taking a supplement can change your metabolism enough to significantly affect your weight, reports a review published in the International Journal of Sport Nutrition and Exercise Metabolism in 2012. The authors note that, while a few supplements can have a very slight effect on helping lower body weight, by less than 5 pounds, most of them have no appreciable effect.
The bottom line; you shouldn't look to supplements to significantly change your metabolism, whether you're trying to speed it up or slow it down.
Overcoming a Slow Metabolism
Supplements that contain caloric nutrients -- protein, carbs and fat -- can still help you gain weight if you have a slow metabolism, though. You can mix weight gainers and protein powders into water or milk to make drinkable supplements, which will help you put on pounds. One commercial weight gainer supplement, marketed to bodybuilders, contains 750 calories, or about one-fifth of the 3,500 calories you need to gain a pound. If you can't get enough calories for weight gain through diet alone, these high-calorie supplements might help.
However, you can get the same results from simply eating more food; for example, adding high-calorie foods, like nut butters, to your meal plan, cooking with more oil to boost your calorie count, and getting more protein from foods like chicken breast, salmon and eggs.
Considerations With a Fast Metabolism
While it's possible your genetics and body type make it harder for you to gain weight -- if you're naturally lean and lanky, for example, you won't develop a stocky body type through lifestyle changes -- you should talk to a doctor if you're having trouble gaining weight. Some health issues, like an overactive thyroid, can artificially speed up your metabolism, prompting weight loss even if you haven't made any changes to your diet and lifestyle. Undiagnosed diabetes can also affect how your body processes carbs, triggering unexplained weight loss. A medical professional can rule out an underlying health issue and suggest a supplement to help you gain weight.