Two shakes, two curves: why protein products can act so differently

The number on the tub is not the whole story

Nutrition tables are useful, but they answer a narrower question than most people assume. They state how many grams of something a product contains. They say nothing about how quickly that something reaches the blood, what it triggers on the way and how the body answers.

With carbohydrates this difference is familiar: four grams of sugar in a soft drink act differently from four grams in wholemeal bread. With protein products the same thought is rarely followed through – yet it applies just as much. Two powders with identical tables can be composed differently, be digested at different speeds and trigger different hormonal responses.

First: the protein source itself

The biggest difference between two powders often lies not in the carbohydrates but in the protein. Whey is digested quickly, floods the blood with amino acids within a short time and thereby stimulates insulin release strongly. The branched-chain amino acids leucine, isoleucine and valine are principally involved, along with the gut hormones GIP and GLP-1.³

Casein behaves differently. It curdles in the stomach, is released more slowly and produces a flatter, longer response. In direct comparisons whey stimulated insulin release more strongly than casein – explained by its faster digestion and higher content of branched-chain amino acids.⁶

For blood sugar this is initially good news: more insulin at the same sugar content usually means a flatter curve. Whey even measurably lowers glucose after a meal, partly because it additionally slows gastric emptying.⁵ Two products with different whey-to-casein ratios can therefore behave differently without the nutrition table revealing it.

Second: the type of carbohydrate

Here lies a detail you cannot see on the label. Four grams of carbohydrate fully dissolved as powder in liquid meet the gut differently from four grams floating in the liquid as a crunchy piece.

Products with visible pieces – biscuit crumbs, brittle, chocolate chips – usually bring those carbohydrates in the form of starch from flour. Starch is broken down to glucose by the body’s own enzymes, and the more finely processed it is, the faster that happens. A powder whose carbohydrates come mainly from milk components or fibre behaves differently, although the table shows the same number.

Third: fat and fibre as a brake

Both delay gastric emptying, and gastric emptying is one of the most important pacemakers for the height of a glucose peak.⁵ A shake with more fat or more soluble fibre tends to produce a flatter curve – but a longer one, because the material arrives later.

That leads to a result that looks paradoxical at first: a product with more calories can produce a lower peak than a lighter one, because the fat brakes. Anyone looking only at the maximum reaches a false conclusion here. What matters is the area under the curve, not the single point.

Fourth: the sweeteners

Protein powders are almost always sweetened, and the differences between the agents are considerable. Erythritol is barely metabolised and hardly raises blood sugar at all. Maltitol, by contrast, has a glycaemic index of around 35 – roughly half that of table sugar – and in larger amounts can certainly become visible.

Then there is an ingredient frequently overlooked: isomalto-oligosaccharides, usually abbreviated to IMO. They are often listed as fibre because a laboratory test classes them as indigestible. Human gut enzymes, however, break them down largely to glucose – just more slowly. As a result they drop out of the net-carbohydrate calculation and raise blood sugar anyway.

Fifth: the biggest factor is you

The most important point comes last because it puts everything else in perspective. An Israeli group monitored 800 people continuously for a week, capturing 46,898 meals. The central finding: the blood sugar response to identical, standardised meals differed considerably between participants. For the same meal there were both strong and weak responders.¹

A model incorporating clinical data and gut flora composition predicted individual responses markedly better than general rules. The authors concluded that blanket dietary recommendations are of limited use – which argues for personal observation, but also for not adopting other people’s results unchecked.

Honesty requires the counter-voice: a published response questioned how much of the observed spread really reflects stable person-level differences and how much day-to-day fluctuation.² That people respond differently is well documented. How reliably a personal profile can be derived from that is less clear than the popular reception suggests.

What protein itself does to blood sugar

A common misconception holds that protein is irrelevant for blood sugar. With an intact insulin response that is largely true: the liver compensates for the amino acids arriving and the curve stays flat. With pronounced insulin resistance it looks different. There, amino acids can stimulate gluconeogenesis via glucagon and deliver small amounts of glucose with a delay.⁷

In practice this means: anyone seeing a rise after a protein shake does not necessarily have a carbohydrate problem. It can also be the delayed response to the protein itself – and that shows up later than a carbohydrate peak, often only after an hour and a half to two hours.

How to test a product on yourself properly

None of this means you should give up. It means your own measurement is worth more than any table – provided it is done cleanly. Four rules decide whether a conclusion or merely an impression stands at the end.

Change one thing only. Same time, same amount, same training before and after, comparable sleep. Anyone changing two things at once cannot attribute the result to a cause afterwards. That is the most common mistake in self-testing, and it renders the whole effort worthless.

Repeat at least three times. A single morning contains so much noise – sleep, stress, heat, the previous day – that a difference of ten or fifteen points cannot be explained by it. Only when the same pattern appears repeatedly does it become interesting.

Look long enough. The peak alone says little. Equally important is when the value comes back and whether it falls below the starting point. Two hours is the minimum; with fatty products it is worth looking longer still.

Write it down rather than remember it. Starting value, product, amount, time, course. Anyone relying on memory preferentially recalls the results that match their expectation. That is not a character flaw but normal – and the reason why notes make the difference.

What a self-test cannot do

It tells you how you responded to a product. It says nothing about how another person responds – that follows directly from the variability between individuals. And it says nothing about which ingredient was responsible, as long as the products differ in several respects.

Anyone wanting to know whether a particular component is the trigger would need products differing only in that respect. With ready-made retail products that is practically never the case. Two flavours from the same brand differ in aroma, sweetening, pieces and often in the protein blend all at once. You can establish that a difference exists – the cause remains open.

What I take from this

Two products with similar tables can act differently, and there are enough documented mechanisms for it: the protein source, the form of the carbohydrates, fat and fibre as a brake, the sweeteners. Which of these tips the balance in an individual case cannot be said without a controlled comparison.

The most honest way to handle that is the most modest one: measure your own response, note it cleanly, generalise nothing from it. What produces a flat curve in me may look different in you – not because one of us is doing something wrong, but because that is what the research leads you to expect.