Macronutrients are a starting point, not the whole picture!

You’ve probably heard you need somewhere between 0.8 and 1.6 grams of protein per kilogram of bodyweight. Hit that number and, on paper, you’re covered. But what if the protein you’re eating is short on one of the amino acids your body can’t make on its own? The gram total says nothing about that and it’s the amino acid, not the gram total, that actually builds the muscle.

Why it matters

Protein, carbohydrate, fat and fibre are useful shorthand. They let us summarise a diet in three or four numbers, and that’s exactly why they’ve become the backbone of public nutrition messaging. But each of those categories is really an umbrella over components that behave very differently in the body different digestion routes, different metabolic pathways, different jobs. Talk only at the macronutrient level and you can hit every target on paper while still missing what actually matters physiologically. Worse, research that pools these components together can produce findings that look solid in aggregate but don’t tell you much about any specific one of them.

Protein: it’s the amino acids doing the work

“Eat more protein” is really shorthand for “supply your body with amino acids” and not all amino acids pull equal weight. Leucine in particular is the primary trigger for muscle protein synthesis (MPS), acting through the mTOR signalling pathway far more powerfully than any other amino acid (Ely et al., 2023). That’s why research keeps finding that small, leucine-rich doses can match or beat larger doses of complete protein: one study in older women found that just 1.5g of leucine-enriched essential amino acids, containing only 0.6g of leucine, stimulated muscle protein synthesis about as effectively as 40g of whey protein (Wilkinson et al., 2017), and recent work in young women found no difference in muscle protein synthesis between 1.5g of essential amino acids and 15–20g of whey protein (Apicella et al., 2025).

The practical implication: a client hitting their daily protein target from sources genuinely short on leucine, or on any of the three essential amino acids the body can’t manufacture, isn’t getting what the number implies. This is exactly why the same protein gram target can produce different training responses in different people, the total was never the whole story.

Glycine is another good example, and one that rarely gets mentioned outside specialist circles. Research from Luc van Loon’s group at Maastricht University has shown that whey protein, which is the default recommendation for most athletes, actually causes plasma glycine to fall below baseline after exercise, while co-ingesting even a modest amount of collagen (naturally rich in glycine) prevents that decline and raises glycine availability substantially (Aussieker et al., 2024). Glycine matters for connective tissue repair, not muscle fibre growth, so an athlete optimising purely for “protein grams” or even “leucine content” via whey alone may still be under-supplying the amino acid their tendons and ligaments specifically need to recover.

Carbohydrate: glucose and fructose aren’t interchangeable

Carbohydrate gets the closest to a fair single-number treatment of any macronutrient — but even here, lumping sugars together hides real differences. Glucose and fructose enter the body through different intestinal transporters, are processed by almost entirely separate organs, and are used differently once inside the body. Fructose absorption is mediated by a specific transporter (GLUT5) rather than the glucose pathway (Ferraris et al., 2018), and unlike glucose, fructose is metabolised in an insulin-independent way and produces only minor increases in blood glucose, even in people with diabetes (Tappy, 2020). The liver does most of the work with fructose specifically: deuterium imaging studies have shown a more than two-fold higher initial hepatic uptake of fructose compared with glucose, with roughly 2.5 times faster clearance (Hendriks et al., 2023).

That’s not an argument against fruit or a reason to fear fructose in normal amounts, dietary fructose in typical amounts actually works alongside glucose, synergising at several metabolic steps and even helping the liver take up and store glucose (Laughlin, 2014). It’s an argument against treating “carbohydrate” as one thing. The dose, source and context (whole fruit versus concentrated syrup, for instance) change the physiological story considerably.

Oligosaccharides, the fibre-like carbohydrates found in foods like onions, wheat and legumes, make the same point from a different angle. Restricting these (alongside related fermentable carbohydrates, together known as FODMAPs) resolves gut symptoms in roughly half to three-quarters of people with irritable bowel syndrome, but not everyone responds the same way. Research has identified distinct microbiome subtypes: people with a specific “pathogenic-like” microbial signature show a significantly greater symptom improvement from restricting these carbohydrates than people whose gut microbiome already resembles a healthy control (Vervier et al., 2021). Two clients can eat the same “carbohydrate” and have completely different digestive experiences, for reasons that have nothing to do with willpower or food quality.

Fat: omega-3 and omega-6 are groupings within groupings

“Get your omega-3s” is better than nothing, but even omega-3 and omega-6 are themselves broad categories hiding very different individual fats. Within omega-3, EPA and DHA (found in oily fish) have well-established anti-inflammatory actions, partly inhibiting multiple aspects of inflammation and giving rise to specialised pro-resolving compounds called resolvins, protectins and maresins (Calder, 2017). ALA, the plant-based omega-3 found in flaxseed and walnuts, has to be converted into EPA and DHA in the body, a conversion that’s notoriously inefficient in humans.

Omega-6 is even messier as a single category. It’s often flagged as “pro-inflammatory,” but the evidence is genuinely mixed depending on which omega-6 fat you mean: studies in healthy adults have found that increased intake of arachidonic acid or its precursor linoleic acid doesn’t raise inflammatory markers, and some epidemiological work even links them to reduced inflammation (Innes & Calder, 2018), while other reviews find that, with one specific exception (DGLA), omega-6-derived compounds generally do promote inflammation and clotting, in contrast to the inhibitory effect of omega-3s (Mariamenatu & Abdu, 2021). Even the researchers studying this admit the interaction is “complex and still not properly understood.” Painting an entire fatty acid family with one brush, pro- or anti-inflammatory, doesn’t match what happens at the level of the individual fat.

Fibre: the least understood of the four

If protein and fat get treated too simply, fibre barely gets treated at all beyond “soluble vs insoluble.” Dietary fibre subtypes, beta-glucans, pectins, arabinoxylans and others, differ in molecular structure, composition and function, but this has been largely overlooked, particularly in clinical research (Khorasaniha et al., 2023). The differences aren’t trivial: in one comparison of fibre types added to a high-fat diet, only beta-glucan reduced adiposity and improved glucose tolerance, while pectin, wheat dextrin and resistant starch had no effect on those same outcomes (Howard et al., 2024). Lignin, the structural fibre in vegetable stalks and seeds, behaves completely differently again, it’s classified as water-insoluble alongside cellulose and hemicellulose, and works mainly by speeding gastric emptying and adding bulk rather than through fermentation (Soliman, 2019).

“Eat more fibre” is true and useful as a starting point. But if the goal is a specific outcome, glucose control, cholesterol, gut microbiome diversity, the type of fibre matters as much as the total gram count, and current research is nowhere near as advanced here as it is for protein or fat.

The vitamin parallel

Here’s the interesting contrast: nobody talks about a “vitamins” macro-target. Public health messaging and research both go straight to the specific vitamin, vitamin D, vitamin C, B12, despite plenty of uncertainty about their practical benefit outside of correcting an outright deficiency. We accept that level of specificity for vitamins without a second thought. We don’t yet extend the same habit to protein, carbohydrate, fat or fibre, even though the case for doing so is arguably just as strong.

Bottom line

Macronutrient targets are a reasonable place to start, not a reasonable place to stop. Two people can hit an identical protein, carb, fat and fibre split and end up with meaningfully different physiological outcomes, because the totals say nothing about amino acid completeness, sugar type, specific fatty acids, or fibre subtype. Precision at this level isn’t about complexity for its own sake, it’s about actually delivering what your training, recovery, or health goal requires.

If you’d like your own nutrition looked at with this level of detail, not just hitting a macro number, but making sure it’s built from the components that actually match your goals, get in touch and we’ll talk through what that looks like for you.