I 3rd-party tested the UK’s top protein powders

Seven popular products. Nitrogen, amino acids and heavy metals. Then a second lab for the results I wanted to check. Here’s what was in the powders, how close it was to the label, and what the protein actually cost.

A big protein number is only the start

If you’re paying for protein powder, you want to know how much protein you’re getting. That sounds simple. It gets less simple when you look at how the number is measured.

A conventional protein test measures nitrogen, then multiplies it by a conversion factor. In this project the lab used Dumas combustion and a factor of 6.25. That gives a useful protein estimate, but it does not identify where every bit of nitrogen came from.

Protein contains nitrogen. Other ingredients can contain nitrogen too. And different protein sources do not all have exactly the same nitrogen content. So a nitrogen number on its own cannot answer every question about what is in the tub.

That’s why I also tested the amino acids: the building blocks that make up protein. Comparing the two methods, and looking at the amino-acid profile, tells us more than either headline number alone.

Food-labelling rules define protein using nitrogen × 6.25. The amino-acid estimates below answer an additional composition question; a difference from the label is not, by itself, a finding of illegal labelling. Protein definition · FAO guidance on amino-acid residue analysis.

What I bought and how I tested it

I bought the products at retail and sent them to the labs with the manufacturer’s seals intact. I also bought backups, so there would be unopened product available if a result needed checking. The first round covers one flavour and one batch of each product. These are findings about those samples.

RSSL coordinated the first round and tested the heavy metals. Normec i2 FAST measured nitrogen-derived protein. Eurofins carried out amino-acid analysis, including a screen for free amino acids. Alta Bioscience then analysed four products for a second look at the amino-acid results.

The lab reports distinguish accredited analyses from results outside accreditation scope. You can inspect those markings in the report download; a lab’s accreditation does not mean every test on its report has the same status.

Making the amino-acid numbers comparable

The first lab reports amino acids after hydrolysis, which breaks protein apart. The project calculation converts those figures to the weight of the residues in a protein chain. Alta’s results are already on that residue basis.

Tryptophan was not quantified by either lab, so the totals include an estimate: 1.3% of protein for the dairy products, 1.1% for Serious Protein’s blend and 1.0% for Free Soul’s blend. I’ve called the resulting number an adjusted amino-acid estimate throughout. It is a calculation from lab results, not a direct measurement of every amino acid or of what your body absorbs.

How close were they to the label?

Optimum Nutrition was basically bang on: 79 g per 100 g on the label, 79.23 g by nitrogen and 77.64 g from the adjusted amino-acid estimate. MyProtein and Protein Works also met or slightly exceeded their label figures on the nitrogen test.

Applied Nutrition was 4.5% below its photographed 71.5 g label on nitrogen and 10.7% below on the amino-acid estimate. Bulk was 10.0% below on nitrogen and 14.8% below on the amino-acid estimate. Those were results worth looking at again.

The largest differences were Free Soul and Serious Protein. Free Soul’s amino-acid estimate was 46.52 g against a 66 g label, a 29.5% gap. Serious Protein’s was 59.28 g against 80.86 g, a 26.7% gap.

Measurement uncertainty matters. Applying the project’s 14% illustration, the upper ends for Optimum, MyProtein, Protein Works and Applied reach the label. Bulk’s first result stays a little below it; Free Soul and Serious Protein stay substantially below it. I would not turn small differences between these powders into a confident ranking of label accuracy.

What happened at the second lab?

I sent Applied Nutrition, Bulk, Serious Protein and Free Soul to Alta Bioscience. All four adjusted estimates came back higher than the first round, by around 5–7.5% relative to the first result. The broader pattern remained.

Applied Nutrition moved from 63.83 g to 68.18 g, bringing the amino-acid estimate very close to its 68.31 g nitrogen result. Bulk moved from 60.49 g to 63.53 g. Serious Protein moved from 59.28 g to 63.72 g, and Free Soul from 46.52 g to 48.73 g.

Free Soul was still 26.2% below its per-100 g label figure on that second estimate. Serious Protein was still 21.2% below. For Bulk and Serious Protein, this is evidence from two batches; it is not two labs testing the exact same sample.

Does a nitrogen gap mean spiking?

No. A gap is a reason to investigate, not proof of deliberate adulteration.

Serious Protein lists soy protein isolate, glutamine peptides from wheat, and milk protein concentrate. The wheat ingredient is declared. It has a different amino-acid profile from whey and contributes nitrogen too. Free Soul lists pea and hemp proteins alongside cocoa, plant extracts, vitamins and other ingredients, some of which can contribute non-protein nitrogen.

The testing did not establish deliberate spiking in any of these products. The free-amino-acid screen does not show the kind of large free-amino-acid addition people usually mean by amino spiking. Hydroxyproline, a marker used when investigating collagen addition, was below the reporting limit in all seven. Neither result proves the absence of every possible adulterant.

What Free Soul said

I asked Free Soul how they calculate protein and whether nitrogen from the additional ingredients can contribute. They declined to share their manufacturing processes or testing methodology, describing that as proprietary information. They also said:

“our products are fully compliant with UK and EU labelling standards and the nutritional information on our packaging is accurate”

Free Soul support · email response, 18 September 2026

That response does not explain the difference between the nitrogen result and the two adjusted amino-acid estimates. I’d still like to see the finished-product data behind the label.

What about BCAAs and leucine?

The total is useful, but the profile matters too. BCAAs are leucine, isoleucine and valine. Leucine is part of that total, so you should not add the two figures together.

Optimum Nutrition led this set on BCAAs and leucine per 100 g of powder. MyProtein and Protein Works followed. Bulk and Applied Nutrition were close on total BCAAs, and the two plant-containing blends had lower concentrations.

I spoke to Dr Paul Morgan, Senior Lecturer in Human Nutrition at Manchester Metropolitan University, about what that means. His explanation was that protein sources differ in both their amino-acid composition and their digestibility. What you consume and what becomes available to the body are related, but they are not identical.

He also made the point that soy can have a good amino-acid profile. A lower concentration per scoop does not tell the whole story when a product is much cheaper, or when it is being added to an already varied diet.

That is why I would not use a BCAA chart alone to declare one protein “best”. It leaves out the rest of the essential amino acids, the rest of your diet and how well the product suits you.

Now put the price next to the results

This is where the order changes. I divided the project’s recorded price per kilogram by the adjusted first-lab amino-acid estimate. That gives the cost of reaching 100 g of the estimate, rather than 100 g of powder or 100 g claimed on the label.

Serious Protein came out cheapest overall at £1.61. Bulk was the cheapest whey at £3.65. MyProtein came next at £3.90 using the project’s £26.75/kg assumption, followed by Optimum Nutrition at £4.71. Protein Works and Applied Nutrition were £5.58 and £5.69. Free Soul was £8.54.

Most of the available price histories swing substantially. At the recorded low and high prices, Bulk’s cost runs from about £2.98 to £4.63 per 100 g of the estimate. Optimum’s runs from £3.99 to £6.05. A sale can change the order.

Pack size matters too: Serious Protein was a 4 kg tub; Free Soul was a 600 g pouch. And Free Soul includes additional ingredients. This comparison asks what the protein component costs, not whether every buyer values those extras.

The project also explored estimated digestibility. I have kept that out of the headline ranking because absorption was not measured in these products or in the people using them. Applying an assumed percentage can be useful for a scenario; calling it measured “absorbed protein” would overstate what we know.

Found a different price? Put it in here

Protein is constantly on offer. Use the pack price and weight you’re actually looking at, and this will calculate the cost against the tested product’s composition.

Heavy metals: look at the amount

Optimum Nutrition, Bulk and MyProtein came back below the reporting limit for all four metals. Applied Nutrition, Free Soul and Serious Protein had detectable cadmium and lead. Protein Works had detectable total arsenic.

Serious Protein had the highest detected cadmium and lead in this set: 0.029 mg/kg and 0.018 mg/kg. In 30 g of powder, that is 0.87 micrograms of cadmium and 0.54 micrograms of lead.

For context, the food-supplement reference limits used in this project are 1 mg/kg for cadmium, 3 mg/kg for lead and 0.10 mg/kg for mercury. Serious Protein’s detected cadmium and lead were 2.9% and 0.6% of those respective limits. These are concentration comparisons, not percentages of a daily exposure allowance. Product classification matters when deciding which legal limit applies. UK Committee on Toxicity reference.

Protein Works contained 0.074 mg/kg of total arsenic, equivalent to 2.22 micrograms in 30 g. The test did not separate organic and inorganic arsenic. California’s Proposition 65 cancer-warning benchmark for inorganic arsenic is 10 micrograms per day; even treating the whole measured amount as inorganic would put that one serving below that particular benchmark. That is not a UK limit or an assessment of all your daily exposure. OEHHA benchmark.

And “not detected” needs context. Mercury was reported below 0.050 mg/kg for every powder. In 30 g, that means less than 1.5 micrograms, not zero. If you want to compare smaller exposures than the method can quantify, you need a more sensitive test.

What I make of it

If I were choosing on label agreement and amino-acid concentration, Optimum Nutrition stands out in this set. If I were choosing whey on the project’s recorded price per gram, Bulk leads, with MyProtein close behind under the price assumption shown. I would check the price on the day.

Serious Protein is remarkably cheap per gram of the amino-acid estimate. Even with its lower concentration, the cost advantage is large. But it contains soy, wheat and milk. It is not vegan or dairy-free, and it is not nutritionally identical to whey.

Free Soul has the lowest amino-acid concentration and the highest cost on this comparison. Its extras may matter to some customers. For someone buying it principally to hit a protein target, the difference between the label and both amino-acid estimates deserves an explanation.

I personally avoid dairy because I find it flares my rosacea, so whey is not an option for me. I use soy protein. That is my experience and preference, not a recommendation that everyone should copy it, and it does not make this milk-containing Serious Protein blend suitable for me.

What I want from brands is straightforward: test the finished product, explain the method and show customers the results for the batch they are buying. People should be able to work out what they’re paying for.

This project cost £6,187 in lab testing. I own Natural Foundation Supplements, which helps fund this work. We do not sell protein at the time of this project. If you want to support more testing, that’s where to find us. But the results are here for you to judge for yourself.

Results describe the named batches, flavours and methods. Prices are historical or explicitly stated assumptions. This testing does not measure clinical outcomes, individual absorption, taste or mixability. Brands can send batch-matched evidence or corrections through the contact page.

Reports, calculations and sources

The calculation sheets below explain the conversions and tryptophan estimates used for the adjusted amino-acid totals.

Calculation sheets

• First-round results, raw amino acids and method notes
• Results with the second test
• BCAA and leucine calculations
• Value analysis and price assumptions

Calculations are based on the project sheets, lab reports and photographed labels. Expert discussion is paraphrased from my interview with Dr Paul Morgan. Free Soul’s response is from my email correspondence with the brand.