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Nutrition &
Macronutrient Metabolism

Explore how the body breaks down carbohydrates, lipids, and proteins to harvest energy as ATP, and understand the biochemical pathways that connect them.

Biochemistry

What Are Macronutrients?

Macronutrients are the large organic molecules your body needs in substantial quantities for energy, growth, and cellular function. They fall into three major classes:

Carbohydrates
Primary energy source: sugars, starches, and fibers composed of \(\text{C}_n(\text{H}_2\text{O})_n\)
Lipids
Long-term energy storage: triglycerides, phospholipids, and sterols built from fatty acid chains
Proteins
Structural and catalytic roles: chains of amino acids linked by peptide bonds

The Energy Currency: ATP

All three macronutrients are ultimately metabolized to produce adenosine triphosphate (ATP). The hydrolysis of ATP releases energy that drives cellular work:

Caloric Energy Density

Each macronutrient provides a different amount of energy per gram. These values, measured by bomb calorimetry and adjusted for digestibility, are fundamental to nutrition science:

Carbohydrates
\(4 \; \text{kcal/g}\) : rapid fuel, preferred by the brain
Lipids
\(9 \; \text{kcal/g}\) : most energy-dense, efficient for storage
Proteins
\(4 \; \text{kcal/g}\) : used for energy only when carbs and fats are scarce

The Central Metabolic Pathway

All macronutrients funnel into a shared set of core pathways. This convergence at Acetyl-CoA and the citric acid cycle is one of the most elegant features of biochemistry.

Carbohydrates Lipids Proteins Glycolysis Amino Acid Pool (deamination) Pyruvate Beta-Oxidation (fatty acids) Acetyl-CoA Citric Acid Cycle + ETC
\( \text{ATP} + \text{CO}_2 + \text{H}_2\text{O} \)

Carbohydrate Metabolism

1
Glycolysis (cytoplasm): Glucose (\(\text{C}_6\text{H}_{12}\text{O}_6\)) is split into 2 pyruvate molecules, yielding a net of 2 ATP and 2 NADH.
2
Pyruvate Decarboxylation: Each pyruvate is converted to Acetyl-CoA by pyruvate dehydrogenase, releasing \(\text{CO}_2\) and producing 1 NADH per pyruvate.
3
Citric Acid Cycle (mitochondrial matrix): Each Acetyl-CoA generates 3 NADH, 1 FADH\(_2\), and 1 GTP per turn.
4
Oxidative Phosphorylation (inner mitochondrial membrane): NADH and FADH\(_2\) donate electrons to the ETC, driving ATP synthase to produce roughly 26 to 28 ATP.

Lipid Metabolism: Beta-Oxidation

Fatty acids are broken down via beta-oxidation in the mitochondrial matrix. Each cycle removes a 2-carbon unit as Acetyl-CoA and generates reducing equivalents:

1
Activation: Fatty acid is converted to Fatty Acyl-CoA (costs 2 ATP equivalents).
2
Transport: The carnitine shuttle moves the acyl group across the inner mitochondrial membrane.
3
Beta-Oxidation Spiral: Each cycle yields 1 FADH\(_2\), 1 NADH, and 1 Acetyl-CoA. A 16-carbon palmitate requires 7 cycles.
4
Total: Palmitate yields 8 Acetyl-CoA + 7 NADH + 7 FADH\(_2\) = approximately 106 ATP (after subtracting activation cost).

Protein Metabolism

Proteins are hydrolyzed into amino acids, which must be deaminated before their carbon skeletons can enter central metabolism. The removed nitrogen is toxic and must be excreted.

Transamination
Amino group is transferred to \(\alpha\)-ketoglutarate, forming glutamate. Catalyzed by aminotransferases.
Oxidative Deamination
Glutamate releases \(\text{NH}_4^+\) via glutamate dehydrogenase, regenerating \(\alpha\)-ketoglutarate.
Urea Cycle
Toxic \(\text{NH}_4^+\) is converted to urea in the liver: \(\text{CO}_2 + 2\text{NH}_3 \rightarrow \text{urea} + \text{H}_2\text{O}\)

Glucogenic vs. Ketogenic

When amino acids are catabolized, their carbon skeletons enter different metabolic fates. This classification is critical for understanding how the body adapts during fasting and starvation.

Glucogenic Ketogenic
Carbon Skeleton Entry Pyruvate or TCA intermediates Acetyl-CoA or acetoacetate
Can Make Glucose? Yes (via gluconeogenesis) No
Can Make Ketone Bodies? No Yes
Examples Alanine, Glutamate, Aspartate Leucine, Lysine
Both Categories Isoleucine, Phenylalanine, Tryptophan, Tyrosine, Threonine

Fed State vs. Fasted State

Fed (Absorptive) Fasted (Post-absorptive)
Dominant Hormone Insulin Glucagon
Primary Fuel Glucose (dietary) Fatty acids + ketone bodies
Liver Activity Glycogenesis, lipogenesis Glycogenolysis, gluconeogenesis
Adipose Tissue Lipogenesis (storing fat) Lipolysis (releasing fatty acids)
Muscle Glucose uptake, protein synthesis Fatty acid oxidation, proteolysis

How to Calculate ATP Yield

1
Identify the substrate: glucose, fatty acid (specify chain length), or amino acid.
2
Count NADH and FADH\(_2\): Tally reducing equivalents from each pathway stage (glycolysis, pyruvate decarboxylation, TCA cycle, or beta-oxidation).
3
Apply ETC conversion factors: Each NADH yields \(\approx 2.5\) ATP; each FADH\(_2\) yields \(\approx 1.5\) ATP via oxidative phosphorylation.
4
Add substrate-level phosphorylation: Count GTP (or ATP) produced directly in glycolysis and the TCA cycle.
5
Subtract activation costs: For fatty acids, subtract 2 ATP for the initial activation step.

Example 1: Glucose ATP Yield

Calculate the total ATP from one molecule of glucose undergoing complete aerobic oxidation:

Glycolysis Pyruvate Decarboxylation (x2) Citric Acid Cycle (x2) Oxidative Phosphorylation
\( \text{NADH} \)
\( \text{FADH}_2 \)
ATP/GTP 2 0 2 2 0 0 6 2 2
\( 10 \; \text{NADH} \times 2.5 = 25 \; \text{ATP} \)
\( 2 \; \text{FADH}_2 \times 1.5 = 3 \; \text{ATP} \)
Total: 25 + 3 + 4 (substrate) = ~30 to 32 ATP

Example 2: Palmitate ATP Yield

Palmitate (\(\text{C}_{16}\text{H}_{32}\text{O}_2\)) is a 16-carbon saturated fatty acid. Let us trace its complete oxidation:

1
Activation cost: \(-2\) ATP equivalents (fatty acyl-CoA synthetase)
2
Beta-oxidation (7 cycles): \(7 \times \text{NADH} = 7\) and \(7 \times \text{FADH}_2 = 7\). Produces 8 Acetyl-CoA.
3
TCA cycle (8 turns): \(8 \times 3 = 24\) NADH, \(8 \times 1 = 8\) FADH\(_2\), \(8 \times 1 = 8\) GTP
4
ETC: \((7+24) \times 2.5 + (7+8) \times 1.5 = 77.5 + 22.5 = 100\) ATP

Example 3: Daily Calorie Calculation

A student's daily diet contains 250 g carbohydrates, 70 g fat, and 80 g protein. Calculate total caloric intake:

Common Mistakes

✕ Confusing Net vs. Gross ATP
Glycolysis produces 4 ATP but consumes 2 in the investment phase. Always report the net yield of 2 ATP.
✕ Forgetting Fatty Acid Activation
Beta-oxidation requires 2 ATP equivalents for initial activation. Failing to subtract this inflates your ATP count.
✕ Mixing Up NADH Yield Factors
NADH yields \(\approx 2.5\) ATP, not 3. FADH\(_2\) yields \(\approx 1.5\) ATP, not 2. The older values are outdated.

Macronutrient Summary

Carbohydrates Lipids Proteins
Energy Density 4 kcal/g 9 kcal/g 4 kcal/g
Primary Pathway Glycolysis Beta-oxidation Deamination
Enters TCA As Acetyl-CoA (via pyruvate) Acetyl-CoA Various intermediates
ATP per Unit ~30-32 / glucose ~106 / palmitate Variable
Storage Form Glycogen Triglycerides No dedicated storage
When Used First: immediate fuel Second: sustained energy Last resort + structural

Question 1 of 5

Which macronutrient has the highest energy density per gram?

Question 2 of 5

The net ATP yield from glycolysis (per glucose molecule) is:

Question 3 of 5

All three macronutrients converge at which key metabolic intermediate before entering the citric acid cycle?

Question 4 of 5

A student eats 200 g of carbohydrates, 50 g of fat, and 100 g of protein in a day. What is the total caloric intake?

Question 5 of 5

During the fasted state, which hormone primarily signals the liver to mobilize glucose via glycogenolysis and gluconeogenesis?