The biggest things to avoid in protein powder are heavy metals, artificial fillers, added sugars disguised under unfamiliar names, proprietary blends that hide ingredient amounts, and cheap amino acids used to fake higher protein content. Not all of these will appear on the label, which is what makes choosing a protein powder tricky. Here’s what to watch for and why it matters.
Heavy Metals
Heavy metals are the most concerning contaminant in protein powder because you can’t taste them, they aren’t listed on the label, and they accumulate in your body over time. Arsenic, lead, cadmium, and mercury have all been detected in protein powders at varying levels. A risk assessment published in Toxicology Reports found that plant-based protein powders tend to carry a higher heavy metal burden than animal-based options. Weight gainer products tested worst of all, containing significantly higher arsenic levels than standard whey protein.
To put numbers on it: a single daily serving of a weight gainer product delivered between 3.7 and 5.6 micrograms of arsenic, compared to 0.2 to 2.3 micrograms for whey protein. At three servings per day, the gap widens dramatically. Weight gainers delivered up to 16.9 micrograms of arsenic and up to 13.5 micrograms of lead, while whey products topped out at 7.0 and 5.4 micrograms respectively.
The Clean Label Project’s independent testing confirmed the pattern: the five products with the lowest heavy metal content were all pure or 100% whey protein supplements. If you use a plant-based protein, look for brands that publish third-party heavy metal testing results. If they don’t test, or won’t share results, that’s a red flag.
Amino Spiking
Amino spiking is one of the more deceptive practices in the supplement industry. Some brands add cheap, free-form amino acids like glycine, taurine, creatine, or beta-alanine to their powder. These substances contain nitrogen, and since standard protein testing measures nitrogen content, they inflate the protein number on the label without providing the complete protein your muscles actually need.
The result is that you think you’re getting 25 grams of protein per scoop, but a meaningful portion of that number comes from filler amino acids that cost the manufacturer a fraction of the price of real whey or casein. To spot amino spiking, check the ingredient list for individual amino acids listed separately from the main protein source. If taurine, glycine, glutamine, or creatine appears on the label and the protein count seems unusually high for the price, you’re likely looking at a spiked product.
Proprietary Blends
A “proprietary blend” on a supplement label means the manufacturer lists the ingredients inside the blend but not the individual amounts. Only the total weight of the blend is required by law. This makes it impossible to know whether you’re getting a meaningful dose of any single ingredient or just a dusting of it for label appeal.
This matters most in protein powders that combine multiple protein sources (whey, casein, egg, soy) or add performance ingredients. The blend might list whey isolate first but contain mostly cheaper whey concentrate. You have no way to verify. Choose products that list exact amounts per ingredient on the Supplement Facts panel rather than hiding behind a proprietary blend.
Added Sugars Under Different Names
Sugar shows up on protein powder labels under dozens of names. Some are obvious, like cane sugar or honey. Others are easy to miss: dextrose, maltose, corn syrup solids, evaporated cane juice, fruit juice concentrate, malt syrup, trehalose, and turbinado. A useful shortcut is to look for any word ending in “-ose,” which is almost always some form of sugar.
Maltodextrin deserves special attention. It’s technically classified as a carbohydrate, not a sugar, so it won’t appear in the “sugars” line on the nutrition label. But its glycemic index is higher than table sugar, meaning it spikes blood glucose faster. For anyone managing insulin resistance or diabetes, that rapid spike increases insulin demand and can worsen metabolic control over time. Maltodextrin is especially common in weight gainer and “recovery” protein formulas.
Artificial Sweeteners
Most protein powders use non-nutritive sweeteners to keep calorie counts low while tasting palatable. The most common are sucralose, acesulfame potassium, and aspartame. Research on their long-term effects is still evolving, but the evidence on gut health raises legitimate questions.
A review in Advances in Nutrition found that sucralose decreased the total number of several beneficial bacterial groups in the gut, including bifidobacteria and lactobacilli. In mice, sucralose altered the gut microbiome at 14 different taxonomic levels, and those changes were associated with inflammation. Saccharin showed a different but equally concerning pattern: it drove glucose intolerance through changes to gut bacteria, an effect so directly linked to the microbiome that it could be transferred to other mice through fecal transplant. Aspartame consumption raised fasting blood glucose regardless of diet composition.
If you prefer to avoid artificial sweeteners, look for protein powders sweetened with stevia or monk fruit. Sugar alcohols like xylitol and erythritol are another alternative. Some sugar alcohols, particularly isomalt and maltitol, may actually increase bifidobacteria counts and have mild prebiotic effects, though moderate doses can still cause bloating or digestive discomfort in sensitive individuals.
Thickeners and Emulsifiers
Carrageenan, polysorbate 80, and carboxymethylcellulose are common thickeners and stabilizers in protein powders. Their safety record is mixed. Carboxymethylcellulose is flagged in European clinical nutrition guidelines as potentially increasing the risk of inflammatory bowel disease. Polysorbate 80 has been shown to reduce colon weight and length in animal studies and can reduce gut bacterial diversity while promoting pro-inflammatory bacterial strains.
Carrageenan is the most debated. It may increase intestinal permeability, the condition sometimes called “leaky gut,” and preliminary evidence suggests it could worsen glucose intolerance and insulin resistance. The research isn’t definitive enough to call it dangerous for everyone, but if you have existing digestive issues or an inflammatory bowel condition, avoiding it is a reasonable precaution. On ingredient labels, look for carrageenan, polysorbate 80, and cellulose gum (another name for carboxymethylcellulose).
Lactose in Whey Concentrate
If dairy gives you trouble, the type of whey protein matters. Whey concentrate contains up to 3.5 grams of lactose per serving, while whey isolate contains 1 gram or less. Isolate goes through additional filtering that removes most of the lactose and fat. For people with mild lactose intolerance, either form is likely tolerable, but if you consistently experience bloating, gas, or cramping from protein shakes, switching from concentrate to isolate often solves the problem. Isolate typically costs more, but the digestive comfort is worth it for sensitive individuals.
How to Verify Quality
The supplement industry in the United States is not required to prove safety or accuracy of labels before selling products. That puts the responsibility on you to verify what you’re buying. The most reliable shortcut is third-party certification.
NSF Certified for Sport is the gold standard. Products carrying this label have undergone formulation and label review, production facility inspections, and testing for over 290 banned substances including stimulants, steroids, narcotics, and masking agents. Testing happens on a lot-by-lot basis, meaning every production batch is checked. Informed Sport is another respected certification with similar banned-substance testing. Both programs were designed for professional athletes subject to drug testing, but their standards benefit anyone who wants to know their protein powder actually contains what it claims.
When evaluating a protein powder, flip the container around and read the full ingredient list, not just the front label claims. A short ingredient list with recognizable components, no proprietary blends, transparent per-ingredient dosing, and a third-party testing seal covers most of the risks outlined above.