In a human resistance-exercise study, consuming carbohydrate together with essential amino acids before exercise resulted in about 160% greater net uptake of an amino acid by working muscle than consuming the same nutrients after exercise. Read the study
That is a surprising result if you've been taught that carbohydrate in a protein shake is simply something to avoid.
It raises a much more interesting question:
What does carbohydrate actually do when it is consumed alongside protein and amino acids?
The answer involves insulin, glucose, blood flow, amino-acid delivery, muscle protein metabolism and exercise.
And it changes the way we should think about the words "low sugar" on a protein powder label.
The "less sugar is better" assumption
When people choose a protein powder, the nutrition panel often gets reduced to three numbers:
More protein.
Less carbohydrate.
Less sugar.
The lower the carbohydrate number, the better the product is assumed to be.
That logic is understandable.
Sugar has a poor reputation, particularly when the goal is healthy ageing, maintaining muscle and managing body weight.
But carbohydrate is a biological fuel, not simply a sweetener.
Carbohydrate is digested into glucose and other simple sugars. Glucose can be used directly for energy and can be stored as glycogen in muscle and liver. During exercise, muscle glycogen provides an important source of fuel, while carbohydrate consumed after exercise contributes to replenishment of those stores. Review of carbohydrate and exercise recovery
Carbohydrate also interacts with protein metabolism.
That is where the story becomes considerably more interesting.
Protein doesn't simply "turn into muscle"
When you consume protein, digestion breaks it down into amino acids and smaller peptides.
Those amino acids enter the bloodstream and become available to tissues, including skeletal muscle.
Resistance exercise provides a stimulus for muscle adaptation.
The muscle then responds by changing the balance between two ongoing processes:
Muscle protein synthesis builds new muscle proteins.
Muscle protein breakdown breaks existing muscle proteins down.
The difference between these processes determines net muscle protein balance.
A major review by Kevin Tipton and Arny Ferrando explains that muscle protein accretion is influenced by resistance exercise, amino acids, protein type, nutrient timing and the combination of nutrients consumed. Tipton & Ferrando review
This gives us an important starting point:
Protein nutrition is about more than the amount of protein on the label.
The body responds to the entire nutritional environment.
What happens when carbohydrate availability is low?
Here's another part of the biology that changes the way we should think about carbohydrate and protein.
Your body can use carbohydrate, fat and protein as sources of energy. When carbohydrate availability is reduced, the body adapts by increasing its reliance on other fuels.
One of those fuels is amino acids.
A 2023 review by Margolis and Pasiakos examined the effects of low carbohydrate availability on muscle metabolism, resistance exercise and hypertrophy. Their review found evidence that low carbohydrate availability can increase protein oxidation, including oxidation of branched-chain amino acids such as leucine, isoleucine and valine. The authors concluded that increased amino-acid oxidation can limit the availability of essential amino acids needed to support muscle protein synthesis and recovery.
The response can occur relatively quickly. The review describes research in which just 24 hours of low-carbohydrate feeding increased BCAA oxidation and altered molecular signals involved in muscle adaptation compared with a mixed-macronutrient diet. Longer periods of low carbohydrate availability, lasting 8–12 weeks, were associated with changes in anabolic signalling, protein synthesis and myogenesis that may limit the hypertrophic response to resistance training.
This creates an important distinction.
Protein provides amino acids.
Those amino acids can be used to build and repair tissue, support metabolic processes and participate in signalling.
But amino acids can also be used as fuel.
When carbohydrate is available, it provides glucose that can be used as an energy source. When carbohydrate availability is low, the body can increase its use of other fuels, including amino acids.
That means the question isn't simply:
"How much protein am I eating?"
It is also:
"What is happening to those amino acids once I've eaten them?"
This is one reason carbohydrate has a legitimate biological role alongside protein.
The objective is not simply to put as much protein as possible into a formulation while driving carbohydrate as low as possible.
The objective is to provide nutrients that work together within the body's metabolic environment.
Protein itself stimulates insulin
Insulin is commonly associated with sugar.
But amino acids can stimulate insulin release too.
Leucine is particularly important because it acts both as an essential amino acid used in protein synthesis and as a signal involved in the regulation of muscle protein metabolism.
Human research has demonstrated that leucine stimulates insulin secretion. In one controlled study, researchers compared glucose, leucine and the combination of leucine and glucose. Leucine alone produced an insulin response, while the combination produced a substantially greater insulin response. Human study of leucine, glucose and insulin
Why does this matter?
Because insulin has a much broader biological role than simply responding to sugar.
Insulin regulates glucose metabolism and helps tissues handle circulating nutrients. In muscle, insulin contributes to amino-acid uptake and influences protein metabolism.
This means that when you consume a substantial amount of protein, the body is already managing an interaction between amino acids, insulin and glucose.
That brings us to an important question:
Where does the glucose come from?
Glucose is part of the metabolic picture
The body maintains blood glucose within a relatively narrow range because glucose is an important fuel, particularly for tissues with high glucose requirements.
When blood glucose falls sufficiently, the body activates counter-regulatory mechanisms.
Adrenaline and cortisol are among the hormones involved in restoring glucose availability. Human studies of hypoglycaemia demonstrate significant increases in epinephrine and cortisol as blood glucose falls. Human research on counter-regulatory hormones during hypoglycaemia
This is a normal physiological response.
Ray Peat's discussion of glycaemia also highlights an important relationship between protein, insulin and glucose availability. He notes that amino acids such as leucine can stimulate insulin and describes the counter-regulatory response that occurs when glucose availability becomes inadequate. Ray Peat: Glycemia, starch, and sugar in context
The practical biological principle is straightforward:
Protein supplies amino acids.
Carbohydrate supplies glucose.
Insulin helps regulate the handling of both.
These nutrients participate in interconnected metabolic processes.
What happens when carbohydrate and amino acids arrive together?
This is where the evidence becomes particularly relevant to exercise nutrition.
In the human study mentioned at the beginning of this article, participants consumed a drink containing 6 g of essential amino acids and 35 g of sucrose either immediately before or immediately after resistance exercise.
The researchers measured phenylalanine uptake across the exercising leg as an indicator of net amino-acid utilisation.
When the drink was consumed before exercise, net phenylalanine uptake across the leg was approximately 209 mg, compared with approximately 81 mg when the same drink was consumed after exercise.
That's roughly a 160% greater net uptake in the pre-exercise condition. Read the original study
The researchers attributed the greater response primarily to increased amino-acid delivery to the exercising muscle.
Exercise increases blood flow to working muscle.
Providing amino acids before exercise means those amino acids are available as the muscle's blood flow increases.
The carbohydrate component forms part of the metabolic environment accompanying those amino acids.
The result is a powerful demonstration of a principle that gets lost in the simple "sugar is bad" conversation:
The timing and combination of nutrients can influence how the body delivers and uses amino acids during exercise.
Carbohydrate and protein perform different jobs
Think of muscle nutrition as a coordinated system.
Protein provides amino acids.
Amino acids provide the raw materials required to build new proteins.
Leucine provides both an essential amino acid and an anabolic signal.
Leucine is involved in activating pathways that regulate muscle protein synthesis.
Resistance exercise provides the stimulus.
It tells the muscle that adaptation is required.
Carbohydrate provides glucose.
Glucose supplies readily available energy and contributes to the restoration of muscle glycogen following exercise.
Insulin helps coordinate nutrient metabolism.
It regulates glucose handling and participates in the metabolic environment in which amino acids are handled.
These functions overlap and interact.
The Tipton and Ferrando review describes amino acids as potent stimulators of muscle protein synthesis and carbohydrate as a nutrient that can influence muscle protein balance, particularly through effects on protein breakdown. Tipton & Ferrando review
This is why a protein powder cannot be fully evaluated by looking at protein content alone.
Why this matters after 40
The nutritional environment becomes increasingly important as we age.
Muscle becomes less responsive to some anabolic stimuli with age. This phenomenon is commonly referred to as anabolic resistance.
In practical terms, the nutritional signal that produced a strong muscle-building response when you were younger can become less effective as you age.
Research by Breen and Phillips examined anabolic resistance in ageing muscle and the role of resistance exercise and nutrition in maintaining muscle protein synthesis. Breen & Phillips review
That makes the quality and context of protein nutrition increasingly relevant.
The question becomes more sophisticated than:
"How much protein am I consuming?"
It becomes:
"How does my body receive, process and use that protein?"
That's where the interaction between protein, amino acids, carbohydrate, exercise and energy availability becomes important.
So what does this mean when choosing a protein powder?
This brings us back to the nutrition panel.
If you've been taught that the best protein powder is the one with the lowest possible carbohydrate content, you may automatically see carbohydrate as a negative.
The evidence gives you another way to look at it.
Ask why the carbohydrate is there.
Carbohydrate can provide glucose.
It can contribute to muscle glycogen restoration.
It can influence insulin and nutrient handling.
And when consumed with amino acids around resistance exercise, it can influence amino-acid delivery and utilisation. Read the resistance-exercise study
That makes carbohydrate a functional nutrient within the right formulation.
One example is 40UP
40UP is a protein and nutritional formulation developed by CollagenX for people over 40.
A 60 g serve provides:
35.7 g protein
14 g carbohydrate
3 g creatine
3.5 g leucine
4.8 g glycine
3 g MCT
The carbohydrate component forms part of the overall nutritional architecture of the formula.
It provides carbohydrate alongside protein, amino acids and the other nutrients in the formulation.
The research we've examined provides the biological context for that decision.
The body uses carbohydrate as a source of glucose.
Exercise increases the demand for energy and changes blood flow to working muscle.
Protein supplies amino acids.
Leucine contributes to anabolic signalling.
Insulin participates in nutrient handling.
And the combination of carbohydrate and amino acids can influence amino-acid delivery and utilisation around resistance exercise. Read the original resistance-exercise research
This is the biological reason carbohydrate has a place in the formulation.
The relevant question is therefore not simply whether a protein powder contains carbohydrate.
It's what role that carbohydrate plays within the complete formula.
The 14 g on the label needs context
A 60 g serve of 40UP contains 14 g of carbohydrate.
That number needs to be understood in the context of the complete serving.
It is consumed alongside 35.7 g of protein, including 3.5 g of leucine, as part of a formulation designed around the nutritional requirements of people over 40.
And there is an important distinction between carbohydrate and sugar.
Carbohydrate is the broader nutritional category.
Sugars are one type of carbohydrate.
The nutrition panel therefore needs to be read in context rather than treating every gram of carbohydrate as though it were a gram of table sugar.
What about low-sugar and zero-sugar protein powders?
The popularity of low-sugar protein powders has created another interesting development.
When manufacturers remove carbohydrate while maintaining sweetness, they often turn to artificial or non-nutritive sweeteners.
This produces an attractive nutrition-panel message:
High protein.
Low carbohydrate.
Low sugar.
But nutritional quality extends beyond the carbohydrate number.
CollagenX chooses a different formulation philosophy, avoiding artificial sweeteners such as erythritol.
That position is informed by emerging research into the metabolic effects and health associations of individual sweeteners.
For example, a study published in Nature Medicine reported an association between higher circulating erythritol concentrations and increased risk of major adverse cardiovascular events, alongside experimental evidence involving platelet reactivity and thrombosis. The authors identified the need for further investigation. Nature Medicine erythritol study
The broader lesson is simple:
A lower sugar number tells you what has been removed. It doesn't tell you whether the replacement improves the formulation.
We've examined the research surrounding erythritol and artificial sweeteners in more detail here:
Read: Is Your Collagen Safe? The Research Behind Erythritol
Carbohydrate is a nutrient, not a verdict
The word "carbohydrate" has become almost synonymous with "bad" in parts of the nutrition industry.
Biology is more precise.
Carbohydrate provides glucose.
Glucose provides readily available energy.
Carbohydrate contributes to glycogen restoration.
Carbohydrate influences insulin.
Insulin participates in nutrient handling.
And carbohydrate consumed alongside amino acids can influence amino-acid delivery and utilisation during exercise. Tipton & Ferrando review
The effect of carbohydrate depends on how much is consumed, what form it takes, when it is consumed and what it is consumed with.
That's why the number on a nutrition panel tells only part of the story.
The real question isn't "How little carbohydrate can I get?"
A protein powder can be designed around a very simple objective:
Maximise protein. Minimise everything else.
That's one approach.
Another approach is to consider how the nutrients work together.
For people over 40, that means looking at protein quality, amino-acid availability, leucine, creatine, carbohydrate, energy availability and exercise as interconnected parts of the nutritional picture.
That's the philosophy behind 40UP.
The carbohydrate is not an incidental ingredient.
It has a biological purpose within the formulation.
So, is sugar bad?
The answer depends on what you mean by "sugar."
Large amounts of added sugar consumed regularly have a very different nutritional context from a measured amount of carbohydrate incorporated into a protein formulation and consumed as part of an active lifestyle.
The body doesn't classify food according to whether an ingredient has a good or bad reputation.
It responds to molecules, concentrations, timing, energy requirements and metabolic context.
That's why the better question is not:
"Does this protein powder contain carbohydrate?"
It is:
"Why does it contain carbohydrate?"
Once you understand the biology, the answer becomes much clearer.
Carbohydrate supplies glucose.
Protein supplies amino acids.
Leucine contributes to anabolic signalling.
Exercise provides the stimulus for adaptation.
Insulin helps regulate nutrient handling.
And the combination of nutrients influences how the body responds.
That's why 40UP contains carbohydrate.
Not because the goal is simply to add sugar.
Because the goal is to provide a nutritional formulation in which the ingredients have a biological purpose.
So the next time you see carbohydrate on a protein powder label, pause before deciding that it's a negative.
Look at the whole formula.
Understand the purpose of the ingredients.
And ask the question that matters:
Why is it there?
References
-
Tipton KD, Rasmussen BB, Miller SL, et al. (2001).
Timing of amino acid-carbohydrate ingestion alters anabolic response of muscle to resistance exercise. American Journal of Physiology-Endocrinology and Metabolism, 281(2), E197-E206.
DOI: 10.1152/ajpendo.2001.281.2.E197
This is the key study showing greater net phenylalanine uptake when an essential amino acid-carbohydrate drink was consumed immediately before rather than after resistance exercise.
PubMed record -
Tipton KD, Ferrando AA. (2008).
Improving muscle mass: response of muscle metabolism to exercise, nutrition and anabolic agents. Essays in Biochemistry, 44, 85-98.
DOI: 10.1042/BSE0440085
Particularly important for the article's broader explanation that resistance exercise and nutrition interact, amino acids stimulate muscle protein synthesis, carbohydrate can attenuate muscle protein breakdown, and nutrient timing and co-ingestion can influence protein accretion.
PubMed record -
Kalogeropoulou D, Lafave L, Schweim K, Gannon MC, Nuttall FQ. (2008).
Leucine, when ingested with glucose, synergistically stimulates insulin secretion and lowers blood glucose. Metabolism, 57(12), 1747-1752.
DOI: 10.1016/j.metabol.2008.09.001
This supports the article's explanation that leucine itself can influence insulin secretion and that leucine and glucose interact metabolically.
PubMed record -
Breen L, Phillips SM. (2011).
Skeletal muscle protein metabolism in the elderly: Interventions to counteract the 'anabolic resistance' of ageing. Nutrition & Metabolism, 8, 68.
DOI: 10.1186/1743-7075-8-68
This supports the section explaining anabolic resistance and the changing nutritional response of muscle with ageing.
PubMed record -
Ivy JL. (2004).
Regulation of muscle glycogen repletion, muscle protein synthesis and repair following exercise. Journal of Sports Science & Medicine, 3(3), 131-138.
This supports the discussion of carbohydrate as a substrate for muscle glycogen restoration and its relationship with post-exercise recovery and protein metabolism.
PubMed/PMC record -
Beelen M, Burke LM, Gibala MJ, van Loon LJC. (2010).
Nutritional strategies to promote postexercise recovery. International Journal of Sport Nutrition and Exercise Metabolism, 20(6), 515-532.
DOI: 10.1123/ijsnem.20.6.515
Useful for the discussion of carbohydrate and glycogen restoration during recovery.
PubMed record -
Witkowski M, Nemet I, Alamri H, et al. (2023).
The artificial sweetener erythritol and cardiovascular event risk. Nature Medicine, 29, 710-718.
DOI: 10.1038/s41591-023-02223-9
This is the study cited in the artificial-sweetener section. It reported associations between circulating erythritol concentrations and major adverse cardiovascular events and included experimental findings relating to platelet reactivity. The study does not, by itself, establish that consuming erythritol causes cardiovascular events.
Nature Medicine article -
Human counter-regulatory response to hypoglycaemia.
The human study cited for the discussion of counter-regulatory hormones demonstrates increases in epinephrine and cortisol during experimentally induced hypoglycaemia. -
Low carbohydrate availability impairs hypertrophy and anaerobic performance Lee M. Margolis, Stefan M. Pasiakos. Current Opinion in Clinical Nutrition & Metabolic Care July 2023.
Additional explanatory source
10. Peat R.
Glycemia, starch, and sugar in context.
This is the source you identified and is useful for explaining the relationship between amino acids, leucine, insulin, glucose availability and counter-regulatory responses. I would treat it as an explanatory/background source rather than a primary scientific reference in the article.
Ray Peat article
