What Protein, Carbs, Fat, And Fibre Actually Do

If you ask the internet what macros do, you get a wall of marketing copy and vague spiritual claims. Here’s what each macronutrient actually does, why it matters, and how much of it the human body uses.

Protein — the structural macronutrient

Protein is what gets built into things. Muscle, skin, hair, enzymes, hormones, immune cells, neurotransmitter precursors. The body cannot store protein the way it stores fat or glycogen — once protein needs are met, the surplus goes to energy (or fat).

The daily requirement for sedentary adults is around 0.8 g/kg of body weight — the bare survival minimum. For anyone training, in a deficit, or over 50, the working target jumps to 1.6–2.2 g/kg per day, distributed across at least three meals (Morton et al., 2018, Br J Sports Med, PMID: 28698222).

Figure reference: Morton RW et al. “A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults.” Br J Sports Med. 2018;52(6):376–384. PMID: 28698222. See Figure 3: protein dose-response curve, plateau at 1.6 g/kg.

Per-meal protein matters. Each meal should clear the leucine threshold (~2.5–3 g leucine, which is about 25–35 g of mixed protein) to maximally trigger muscle protein synthesis. Three meals at 30 g beat one meal at 90 g for muscle building.

Best sources: whey isolate, eggs, chicken breast, fish, lean beef, Greek yogurt, cottage cheese, tofu, tempeh, lentils + grain combos, edamame.

Carbohydrates — the performance macronutrient

Carbs are the body’s preferred fuel for high-intensity work. Brain, red blood cells, kidneys, and Type II muscle fibres run on glucose. The body can adapt to running on fat (ketosis) but performance — particularly anaerobic and high-intensity aerobic — drops measurably.

Daily carb targets depend entirely on training load:

• Sedentary: 2–3 g/kg
• Moderate training (3–4 sessions/week): 3–5 g/kg
• Heavy training (5+ sessions/week, athletes): 5–8 g/kg
• Endurance competition prep: 8–10 g/kg

Carbs also restore glycogen post-training, which directly affects how hard the next session can go. A carb-restricted lifter is a lifter who plateaus around week 6.

Best sources: oats, rice, potatoes, sweet potatoes, quinoa, fruit, vegetables, sourdough, legumes. The “advanced” sources I sometimes prescribe to GLP-1 or low-appetite clients: tapioca, honey, maple syrup, dried fruit (raisins, dates) — calorie-dense carbs that fit when total intake is otherwise too low.

Fat — the regulatory macronutrient

Fat builds cell membranes, makes steroid hormones (testosterone, estrogen, cortisol), and is the carrier for fat-soluble vitamins (A, D, E, K). It’s not just energy storage. Drop fat intake below 0.6 g/kg per day for sustained periods and sex hormones decline measurably.

Daily fat target for most adults: 0.8–1.2 g/kg/day. Below 0.5 g/kg is the line where hormonal markers start to fall. Above 1.5 g/kg, you’re using a lot of your daily calorie budget on fat — fine, but it crowds out carbs and protein.

Best sources: olive oil, avocado, nuts and seeds, fatty fish (salmon, sardines), eggs, dark chocolate, full-fat dairy in moderation. Saturated fat: not the villain it was made out to be in the 1980s, but moderation matters for cardiovascular markers.

Fibre — the structural maintenance macronutrient

Fibre isn’t a macronutrient in the calorie-providing sense — it’s a non-digestible carbohydrate. But it does enough work that it deserves the fourth slot.

What fibre does:

• Slows glucose absorption (blunts post-meal blood sugar)
• Feeds gut microbiome (short-chain fatty acid production, which improves insulin sensitivity)
• Improves satiety (volume + slowed gastric emptying)
• Improves bowel function (prevents constipation, which is rampant on low-volume diets)
• Reduces LDL cholesterol (soluble fibre specifically)

Daily target: 25–35 g per day. Most adults hit around 15 g, which is far below adequate. A 2019 meta-analysis in The Lancet by Reynolds et al. showed that for every 8 g/day increase in fibre intake, cardiovascular event rates dropped 19% and total mortality 7%.

Figure reference: Reynolds A et al. “Carbohydrate quality and human health: a series of systematic reviews and meta-analyses.” Lancet. 2019;393(10170):434–445. PMID: 30638909. See Figure 2: dose-response of fibre intake to all-cause mortality.

Best sources: vegetables (especially crucifers, root vegetables), fruit (with skin), legumes, whole grains, chia/flax seeds, oats, psyllium husk if supplementing.

The macro-distribution starting point

For most Weight Well or Lean & Strong clients:

• Protein: 1.8–2.0 g/kg
• Fat: 1.0 g/kg
• Carbs: fill the remainder of calorie budget after protein and fat are set
• Fibre: 30 g/day

Then we adjust monthly based on progress, training response, and how the food fits the client’s life.