If you’ve ever tried to eat “healthy” but still felt confused about what actually happens to
the food once you eat it, you’re not alone. We’re always told to “eat more protein,” “cut
carbs,” or “watch your fat intake,” but rarely are we taught why these nutrients matter or
how they function inside the body. That’s where macronutrients (or “macros” for short) come in.
Macros are the nutrients your body needs in large amounts: carbohydrates, proteins,
and fats. Each one plays a distinct role, and despite what diet culture says, you need all
three to function properly.
In this deep dive, we’ll break down:
- What each macronutrient does in the body
- How they’re digested, absorbed, and used
- What happens when you don’t get enough (or get too much)
- How to balance your macros for energy, performance, and longevity
Let’s finally make sense of what’s really happening behind the scenes every time you eat.
What Are Macronutrients?
“Macronutrients” simply refers to nutrients the body requires in large amounts for
energy, growth, and basic survival:
Carbohydrates
- Fuel for body and brain
- 4 kcal per gram
Protein
- Building blocks for tissues and enzymes
- 4 kcal per gram
Fat
- Long term energy, hormone support, insulation
- 9 kcal per gram
Unlike micronutrients (vitamins and minerals), which support functions in small
quantities, macros make up the actual structure and fuel system of your body.
How the Body Processes Food – The Small Scoop
Before diving into each macronutrient, here’s what happens from bite to bloodstream:
- Ingestion: You eat the food.
- Digestion: Mechanical and chemical processes break it into smaller molecules.
- Absorption: Nutrients move from the digestive tract into the bloodstream or lymphatic system.
- Metabolism: Cells convert those nutrients into usable energy or structural material.
- Storage or Excretion: Excess nutrients get stored for later use or expelled.
Now, let’s break down how each macronutrient travels through this pathway.
Carbohydrates: The Body’s Preferred Source of Energy
Love them or fear them, carbs are the most misunderstood macronutrient. But
physiologically, they’re essential, especially for brain function, physical activity, and
hormone balance.
Types of Carbohydrates
| Type | Examples | Digestion Speed | Primary Use |
| Simple (Sugars) | Fruit, honey, table sugar | Fast | Quick energy |
| Complex (Starches & Fiber) | Whole grains, beans, potatoes | Slow | Sustained energy & gut health |
How Carbs Are Processed
- Digestion begins in the mouth. Salivary amylase (found in saliva) starts breaking down starches.
- Absorbed as glucose in the small intestine.
- Sent to the liver, which decides whether to use immediately as energy, store as glycogen in the muscles or liver, or convert to excess fat.
What Cards Do In The Body
- Fuel high-intensity activity and daily movement
- Power your brain and central nervous system
- Regulate blood sugar and insulin
- Support thyroid and reproductive hormones
- Provide fiber, which supports gut health
Protein: The Builder, Healer, and Enzyme Factory
If carbs are fuel, protein is construction material. Every cell in your body, from muscles
to hormones to immune cells, depends on amino acids- the building blocks of protein.
What Happens When You Eat Protein
- Stomach acid + pepsin break protein into peptide chains.
- Pancreatic enzymes in the small intestine break them into individual amino acids.
- Amino acids enter the bloodstream and are used for:
| Function | Example |
| Muscle repair & growth | Athletic recovery |
| Hormone creation | Insulin, growth hormone |
| Enzyme production | Digestive enzymes, antioxidants |
| Immune defense | Antibodies |
| Neurotransmitters | Serotonin, dopamine |
Can Protein Be Used for Energy?
Yes, but only when necessary. The body prefers carbs or fat for fuel, but in carb
deprivation, it will convert protein into glucose via a process called gluconeogenesis.
This process is not ideal for the body, it’s like burning your furniture for warmth.
Fats: Fuel, Hormones, and Cellular Support
Once demonized, fats are now recognized as essential for survival, especially for
hormone health, brain function, and long-term energy.
Types of Fats
| Type | Examples | Role |
| Saturated | Butter, cheese, coconut oil | Structural stability |
| Monounsaturated | Olive oil, avocados | Heart & metabolic health |
| Polyunsaturated (Omega-3 & 6) | Salmon, flax, nuts | Anti-inflammatory, brain health |
| Trans fats | Processed snacks | Inflammatory (recommended to avoid) |
Fat Digestion & Absorption
- Starts in the small intestine — Bile from the liver emulsifies fat.
- Enzymes break fat into fatty acids & glycerol.
- Absorbed into the lymphatic system, then bloodstream.
- Used for energy during rest & low-intensity activity, hormone production (estrogen, testosterone, cortisol), brain insulation & nerve communication, and cell membrane structure
What Happens When You Overeat (or Undereat) Macros?
| Macro | If Too Low | If Too High |
| Carbs | Fatigue, brain fog, poor performance, hormone issues | Blood sugar spikes, fat storage |
| Protein | Muscle loss, weak immunity, brittle hair/skin | Over 2.5g/kg may strain kidneys |
| Fat | Dry skin, hormone crashes, anxiety, vitamin deficiency | Excess calories → stored as body fat |
How the Body Chooses Energy Sources
Your body is always prioritizing between carbs, fat, and protein depending on activity
levels and availability:
| State | Primary Fuel |
| Rest / Sleeping | Fat |
| Walking or low activity | Mostly fat |
| High-intensity exercise | Carbs |
| Extreme carb restriction | Protein (gluconeogenesis) |
Fiber: The Forgotten “Fourth” Macronutrient
Fiber isn’t digestible, so it technically provides zero calories, but it plays a massive role
in metabolism, gut health, and blood sugar control.
| Fiber Type | Function |
| Soluble Fiber | Forms gel, slows digestion, lowers cholesterol |
| Insoluble Fiber | Adds bulk to stool, improves bowel movements |
| Fermentable Fiber | Feeds gut bacteria → produces anti-inflammatory compounds |
How to Balance Your Macros
There’s no one-size-fits-all formula, but here are common evidence-based starting
points:
| Goal | Carb | Protein | Fat |
| General Health | 40–50% | 20–30% | 25–35% |
| Weight Loss | 30–40% | 30–40% | 25–35% |
| Muscle Gain | 40–50% | 25–35% | 20–30% |
| Endurance Athletes | 50–65% | 15–20% | 20–30% |
As always, check with your dietitian to see how you should split your macros. This can
be a very individualized item that is crucial to your nutrition journey!
So… What’s the Best Macro Split?
The truth? The best macronutrient balance is the one you can maintain without
sacrificing energy, mood, or health.
Instead of obsessing over numbers:
- Make protein a priority at each meal.
- Choose high-fiber carbs over refined ones.
- Include healthy fats in moderation.
- Eat in a way that supports your lifestyle and goals.
Final Takeaway
Macronutrients aren’t just numbers, they’re biological workers, constantly fueling,
repairing, and protecting you.
- Carbs give you energy
- Protein builds and heals
- Fats sustain and regulate
When you understand how they work in the body, you’ll stop labeling foods as good or
bad and start making choices based on function, not fear. If you have more questions
about how macro nutrients work, set up a talk with one of our registered dietitians!
Resources:
Academy of Nutrition and Dietetics. (2021). Position of the Academy of Nutrition and
Dietetics: Dietary protein for optimal health. https://www.eatright.org
Hargreaves, M., & Spriet, L. (2020). Exercise metabolism and the role of carbohydrate.
Journal of Applied Physiology, 128(4), 891–898.
https://doi.org/10.1152/japplphysiol.00666.2019
National Institutes of Health. (2022). Nutrient recommendations: Macronutrients.
https://www.nhlbi.nih.gov
World Health Organization. (2020). Healthy diet. https://www.who.int/news-room/fact-
sheets/detail/healthy-diet
Harvard T.H. Chan School of Public Health. (2023). The Nutrition Source:
Carbohydrates, protein, and fat. https://www.hsph.harvard.edu/nutritionsource
