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Will Soy Protein Powder Cause Weight Gain?

By Sylvie Tremblay, MSc ; Updated July 18, 2017

When it comes to protein supplements, dairy proteins like whey and casein tend to get the most attention. But they're not the only good option in the protein aisle, and soy offers high-quality protein and nutritional value as well. Soy protein won't single-handedly make you gain weight, and it won't single-handedly make you lose weight, either. You can use soy protein for either weight loss or gain, depending how you use it and how you structure your overall diet.

Calories, Soy Protein Powder and Weight Control

Monitoring your calorie intake is key for weight loss -- it's all about calories in vs. calories out. When you're trying to gain weight, you want a calorie surplus -- to take in more calories than you burn. That allows you to gain mass and store some of those excess calories as fat, which is beneficial when you're underweight. You don't need many extra calories -- 250 to 500 per day is plenty.

To lose weight, you need a calorie deficit, so you'll eat fewer calories than you need to maintain weight. You don't want to cut too much -- reducing your intake by 500 to 1,000 calories daily allows for safe weight loss.

Soy protein is a moderate source of calories -- one commercially available soy protein supplement contains 95 calories per 1-ounce serving of protein powder, while another commercial meal-replacement shake made from soy protein contains 178 calories. Because it's not too high in calories, it can work well in a weight-loss diet -- even a restrictive one that allows just 1,200 or 1,400 calories -- as long as you count it as part of your daily intake. On the other hand, a soy protein shake or two can supply the extra calories you need to gain weight, if that's your goal.

Benefits of Soy Protein for Weight Loss

While soy can help you gain weight, it might be beneficial for weight loss. Several studies have looked into the connection between soy protein and weight loss and, while there's still a limited amount of human research on soy and weight control, it seems to help with fat loss, reports a literature review paper published in the International Journal of Medical Science in 2007. For example, one study highlighted in the review found that including soy-replacement shakes in a weight-loss diet accelerated fat loss, as well as overall weight loss, compared to simply following the weight-loss diet alone. And a study published in the Journal of Nutrition, Health and Aging found that soy protein supplements were just as effective for weight loss as protein supplements without soy.

Make sure you count the calories in your soy protein drink toward your daily calorie intake, though. If you're simply taking soy protein on top of your normal diet, without accounting for the calories you get from soy, you'll actually sabotage your weight-loss efforts.

Soy Protein Benefits for Weight Gain

Protein shakes are popular among bodybuilders and those looking to gain muscle, and soy protein can help you add lean mass. While most plant-derived proteins are incomplete -- they're missing at least one amino acid you need from your diet -- soy is a complete protein, so it supplies all the amino acids you need for tissue and muscle growth.

Soy protein supplements also serve as concentrated sources of protein -- while the protein content can vary from brand to brand, one commercially available soy protein isolate offers 23 grams of protein per ounce of powder. Eating high-protein foods can help you bulk up, as long as you're combining your diet with a proper strength-training program, and you should eat 0.8 gram of protein for every pound of body weight. If you weigh 150 pounds -- and therefore need 120 grams of protein while you bulk -- a single soy isolate shake made with an ounce of powder in a cup of milk would provide about 25 percent of your daily protein goal.

The calories in soy shakes help you gain weight, too. Simply drinking one meal-replacement shake made from soy -- which contains 178 calories -- in addition to your regular diet would allow you to gain one-third of a pound a week, even without making other dietary changes.

Serving Soy for Weight Control

Making your own soy protein shakes at home allows you to control the ingredients and portion size of your meal, so you can tailor it to either weight loss or gain. For example, if you're seeking a low-calorie soy shake for weight loss, you could blend an ounce of soy protein isolate with 1 cup of nonfat milk and a 1/2 cup of frozen strawberries. This shake contains just 227 calories and supplies 32 grams of protein -- satisfy your chocolate craving by blending in a tablespoon of cocoa powder, and you'll add just 11 calories and 1 gram of protein.

If you're trying to gain weight, use soy protein powder to make higher-calorie shakes. For example, blend an ounce of soy protein isolate with a cup of full-fat milk, a cup of frozen strawberries and 2 tablespoons of peanut butter. You'll get a shake that contains 482 calories and 38 grams of protein.

Safety Considerations

Soy protein might not be safe for everyone. Like any protein supplement, soy serves as a concentrated source of protein, so it may not be a good fit for your diet if you need to limit your protein intake. Some groups of people -- such as breast cancer survivors -- should also avoid soy products, until there's more research into their safety or potential health risks.

Talk to your doctor before you take any protein powder, including soy. She can let you know if soy protein is safe for you and, if so, recommend a good protein powder and an appropriate dose for your health goals.

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