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One Major Side Effect of Drinking Protein Shakes, Says Science

Your quest to slim down may end up causing you to get fatter.
FACT CHECKED BY Olivia Tarantino

Protein shakes are wildly popular for muscle building and body slimming, thanks to research that suggests the potential benefits of getting more protein in your diet. This macronutrient has been linked to such benefits as:

  • Preventing muscle loss and building lean muscle.
  • Reducing appetite by keeping you feeling full longer.
  • Boosting metabolism.
  • Dropping pounds. (The National Academy of Sports Medicine calls protein "one of the key 'levers' in a diet that increases the likelihood of someone's ability to lose weight.")

It's safe to say that many people have become obsessed with protein, and food and beverage marketers have taken note. Simply using the word "protein" in packaging seems to anoint a food with a halo of health that drives purchases "by symbolically linking [protein] to wellness and lifestyle values," according to a paper in Food Culture & Society that examined how protein snacks are marketed as good food choices through their packaging.

But getting extra protein can backfire, especially when it's delivered in the wrong kind of protein shake. (See: 7 Ways Eating Too Much Protein Can Harm Your Health)

Some protein powders, premade shakes, and smoothie shop protein drinks can lead to weight gain and the tagalong health downsides of extra pounds, nutrition experts say. Weight gain is only one major potential side effect of drinking protein shakes that you'll want to avoid if you're using them to muscle up or slim down. And it can be true no matter the source of protein—eggs, milk (casein or whey protein), or plants (peas, hemp, rice, or soybeans).

Related: Check out our recommendations for the best and worst store-bought protein shakes.

How do protein shakes pack on pounds?

The answer in two words: calories, sugar. Many protein beverages are very high in calories and contain lots of added sugars.

Check the nutrition facts and ingredient lists. A serving of Gatorade Recover Chocolate Protein Shake packs 280 calories and 19 grams of added sugars—that's more added sugar than you'll find in a serving of Breyer's Chocolate ice cream!

"Liquid calories are sometimes not as well acknowledged by the brain as calories from solid food," says physician and medical writer Leann Poston, MD, of Invigor Medical. "So, you may not feel full after drinking a protein shake and follow it with a full meal."

A Purdue University study published in Nutrition Reviews suggests that drinking protein shakes as snacks may end up causing you to swallow more daily calories and gain weight. The analysis of research papers found that when people consumed protein supplements with meals, they tended to adjust the number of calories they ate at mealtime to account for the protein supplement calories. But when they had protein in between meals, they didn't reduce the number of calories eaten at mealtime and gained weight.

"People who consume protein supplements in between meals may be less successful in managing their body weight," said study author Wayne Campbell, PhD, a professor of nutritional sciences at Purdue.

Too much protein can add weight

Remember, while protein contains fewer calories per gram than fat does (4 calories versus 9), those calories still add up. And when you consume more protein than your body needs, those protein calories are stored as fat. A study in Clinical Nutrition demonstrated this: Researchers analyzed dietary protein using a food-frequency questionnaire and found a significant association with risk of weight gain when protein replaced carbohydrates in people's diets but not when protein replaced the higher calorie fat.

"Most of the time, you should be able to get all of the protein you need from your normal diet, eating meat and dairy, as well as plant-based sources like beans, nuts, and whole grains," says Heather Hanks, MS, a nutritionist in Plymouth, Mich., with Instapot Life. "Drinking too many protein shakes may contribute to blood sugar fluctuations and weight gain."

Carbohydrate, after all, isn't the only macronutrient that stimulates the release of insulin from the pancreas. Protein does, too. More insulin is released when both macros are consumed together than when either is eaten alone.

Other potential negative side effects

In addition to weight gain, consuming too many protein shakes could cause other negative side effects.

  • Nutrient deficiency. If you use protein powder shakes as replacements for several meals a day, "there could a risk of not getting enough of other important nutrients," says registered dietitian nutritionist Trista Best, RD at Balance One Supplements.
  • Constipation. Many protein powders lack fiber, which can cause constipation, Best says. And long-term use of protein supplements may cause a gut microbiota imbalance, according to a 2018 study in Nutrients.
  • Impaired kidney function. Before drinking high-protein shakes, "check with your doctor if you have kidney disease to ensure added protein is safe for your kidneys and what your limits should be," Poston says. Protein metabolism can be taxing on the kidneys, according to a 2020 study.

Making your own protein shakes at home may help you avoid some of these potential side effects. Spark up your blender, and try these 13 Best Protein Shake Recipes for Weight Loss.

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Jeff Csatari
Jeff Csatari is responsible for editing Galvanized Media books and magazines and for advising journalism students through the Zinczenko New Media Center at Moravian College in Bethlehem, PA. Read more