Macronutrients: Complete Guide to Proteins

Overview

Proteins are essential macronutrients composed of amino acids linked by peptide bonds. They provide 4 kcal per gram and serve structural, enzymatic, signaling, transport, and immune functions. Proteins constitute approximately 16% of total body mass.

Function	Description
Enzymatic catalysis	All enzymes are proteins (except ribozymes)
Structural	Collagen, keratin, elastin in connective tissues
Transport	Hemoglobin, albumin, transferrin
Signaling	Hormones (insulin, glucagon), receptors
Immune defense	Antibodies (immunoglobulins)
Movement	Actin, myosin (muscle contraction)
Buffering	pH regulation (plasma proteins)
Osmotic regulation	Albumin maintains plasma oncotic pressure
Gene expression	Histones, transcription factors

Amino Acids: The Building Blocks

Amino Acid Structure

All amino acids share a central carbon (α-carbon) bonded to:

• Amino group (−NH₂)
• Carboxyl group (−COOH)
• Hydrogen atom (−H)
• Variable side chain (−R)

Classification of Amino Acids

Essential Amino Acids (9)

These cannot be synthesized by the human body and must be obtained from the diet.

Amino Acid	Abbreviation	Key Functions	Food Sources
Histidine	His	Histamine synthesis, growth, tissue repair	Meat, fish, poultry, grains, dairy
Isoleucine	Ile	Muscle metabolism, immune function, hemoglobin synthesis	Eggs, chicken, pork, soy, cheese
Leucine	Leu	Muscle protein synthesis (mTOR activation), insulin secretion	Meat, dairy, soy, nuts, seeds
Lysine	Lys	Collagen formation, calcium absorption, carnitine synthesis	Meat, fish, dairy, eggs, legumes
Methionine	Met	Methylation, antioxidant synthesis (cysteine, glutathione), growth	Meat, fish, dairy, nuts, seeds
Phenylalanine	Phe	Tyrosine synthesis (precursor to dopamine, norepinephrine, thyroid hormones)	Meat, fish, dairy, eggs, legumes
Threonine	Thr	Glycoprotein synthesis, connective tissue, immune function	Meat, dairy, eggs, legumes
Tryptophan	Trp	Serotonin synthesis, niacin synthesis, sleep regulation	Turkey, chicken, milk, oats, bananas
Valine	Val	Muscle metabolism, tissue repair, nitrogen balance	Meat, fish, dairy, soy, grains

Conditionally Essential Amino Acids

These are normally synthesized but may become essential under specific conditions (illness, stress, infancy).

Amino Acid	Precursor	Condition Requiring Dietary Intake
Arginine	Glutamine	Neonates, trauma, sepsis
Cysteine	Methionine	Hepatic disease, prematurity
Glutamine	Glutamate, ammonia	Critical illness, immunocompromise
Glycine	Serine, choline	Pregnancy, rapid growth
Proline	Glutamate	Wound healing, connective tissue disorders
Serine	Glycine	Renal disease, metabolic disorders
Tyrosine	Phenylalanine	Prematurity, hepatic disease, PKU

Non-Essential Amino Acids (11)

Synthesized by the body from essential amino acids or other precursors.

Amino Acid	Precursor Pathway
Alanine	Pyruvate (transamination)
Asparagine	Aspartate (transamination)
Aspartic acid (Aspartate)	Oxaloacetate (transamination)
Glutamic acid (Glutamate)	α-Ketoglutarate (transamination)
Serine	3-Phosphoglycerate
Arginine*	Ornithine cycle (urea cycle)
Cysteine*	Methionine + serine
Glutamine*	Glutamate + ammonia
Glycine*	Serine, threonine
Proline*	Glutamate
Tyrosine*	Phenylalanine

*Conditionally essential in certain situations

Complete vs. Incomplete Proteins

Quality	Definition	Examples
Complete protein	Contains all 9 essential amino acids in adequate amounts	Meat, poultry, fish, eggs, dairy, soy, quinoa
Incomplete protein	Low in or missing one or more essential amino acids	Most plant proteins (grains, legumes, nuts, seeds)
Complementary proteins	Two or more incomplete proteins that together provide all EAAs	Rice + beans, hummus + pita, peanut butter + bread

Limiting Amino Acids in Plant Proteins

Plant Food	Limiting Amino Acid	Complementary Partner
Grains (wheat, rice, corn)	Lysine	Legumes
Legumes (beans, lentils, peas)	Methionine + Cysteine	Grains, nuts, seeds
Nuts and seeds	Lysine	Legumes
Corn	Lysine, Tryptophan	Legumes
Vegetables	Methionine, Cysteine (generally)	Nuts, grains, dairy
Soy	(Complete)	—

Protein Combining (Complementation)

The concept of consuming complementary proteins at the same meal (once thought necessary) has been updated. Current evidence indicates that consuming a variety of plant proteins throughout the day is sufficient to meet essential amino acid requirements.

Protein Digestion and Absorption

Digestive Pathway

Site	Enzyme	Substrate	Product
Stomach	Pepsin (activated from pepsinogen by HCl)	Proteins	Large polypeptides
Stomach	HCl	Denatures proteins, activates pepsinogen	Unfolded protein chains
Small intestine (lumen)	Trypsin (activated from trypsinogen by enteropeptidase)	Polypeptides	Smaller peptides
Small intestine (lumen)	Chymotrypsin	Polypeptides	Smaller peptides
Small intestine (lumen)	Elastase	Elastin, polypeptides	Smaller peptides
Small intestine (lumen)	Carboxypeptidase A and B (from pancreas)	C-terminal end of peptides	Free amino acids, smaller peptides
Small intestine (brush border)	Aminopeptidase	N-terminal end	Free amino acids
Small intestine (brush border)	Dipeptidases	Dipeptides	Free amino acids
Small intestine (brush border)	Tripeptidases	Tripeptides	Free amino acids

Absorption Mechanisms

Transport Form	Transporter	Type	Location
Free amino acids	Various sodium-dependent (e.g., B⁰AT1) and sodium-independent transporters	Active transport and facilitated diffusion	Enterocytes (small intestine)
Dipeptides and tripeptides	PepT1 (H⁺-coupled)	Active transport (cotransport with H⁺)	Enterocytes (small intestine)
Intact proteins	(Minimal, except in neonates — pinocytosis)	Endocytosis	Enterocytes

Nitrogen Balance

State	Nitrogen Intake vs. Excretion	Clinical Context
Equilibrium	Intake = Excretion	Healthy adult maintenance
Positive balance	Intake > Excretion	Growth (childhood, pregnancy), muscle building, recovery from illness
Negative balance	Intake < Excretion	Starvation, illness (cancer, burns, trauma), inadequate protein intake

Protein Quality Measurement

PDCAAS (Protein Digestibility Corrected Amino Acid Score)

The PDCAAS is the current FDA/WHO standard for evaluating protein quality.

PDCAAS = (mg of limiting amino acid in 1 g test protein / mg of same amino acid in 1 g reference protein) × True Fecal Digestibility

Protein Source	PDCAAS Score	Limiting Amino Acid
Casein (milk)	1.00	None (complete)
Egg white	1.00	None (complete)
Soy protein isolate	1.00	None (complete)
Whey protein	1.00	None (complete)
Beef	0.92	None (complete)
Chickpeas	0.78	Methionine + Cysteine
Black beans	0.75	Methionine + Cysteine
Kidney beans	0.68	Methionine + Cysteine
Lentils	0.52	Methionine + Cysteine
Peanut butter	0.52	Lysine
Wheat gluten	0.25	Lysine
Brown rice	0.50	Lysine

DIAAS (Digestible Indispensable Amino Acid Score)

DIAAS is the newer, more accurate standard recommended by FAO (2013). Unlike PDCAAS, DIAAS uses ileal digestibility rather than fecal digestibility and can have scores above 1.00.

Protein Source	DIAAS Score (adult)	Limiting Amino Acid
Whey protein isolate	1.09	None
Whole milk powder	1.14	None
Soy protein isolate	0.90	Methionine + Cysteine
Pea protein concentrate	0.82	Methionine + Cysteine
Cooked rolled oats	0.54	Lysine
Cooked brown rice	0.52	Lysine
Cooked kidney beans	0.52	Methionine + Cysteine
Cooked chickpeas	0.51	Methionine + Cysteine

Protein Quality Comparison

Method	Advantages	Disadvantages
PDCAAS	Easy to use, current regulatory standard	Truncated at 1.00 (penalizes high-quality proteins), uses fecal (not ileal) digestibility
DIAAS	More accurate (ileal digestibility), no truncation, accounts for anti-nutritional factors	More expensive, requires ileal cannulation in pigs (standard model)
PER (Protein Efficiency Ratio)	Historical measure	Outdated, uses rats, less relevant to humans
BV (Biological Value)	Measures retained nitrogen	Nitrogen retention ≠ health outcomes
NPU (Net Protein Utilization)	Uses retained nitrogen	Same limitations as BV

Dietary Recommendations

Protein Intake Recommendations

Population	RDA / Recommendation	Basis
Adult (≥19 years)	0.8 g/kg body weight/day	IOM RDA
Sedentary adult (75 kg)	~60 g/day	Calculated from RDA
Pregnant women	1.1 g/kg/day	Additional tissue growth
Lactating women	1.3 g/kg/day	Milk production
Infants (0-6 months)	1.52 g/kg/day	Rapid growth
Children (1-3 years)	1.05 g/kg/day	Growth and development
Children (4-8 years)	0.95 g/kg/day	Growth and development
Adolescents (9-18 years)	0.85 g/kg/day	Growth and development
Older adults (>65 years)	1.0-1.2 g/kg/day (expert consensus)	Sarcopenia prevention
Athletes (endurance)	1.2-1.4 g/kg/day	ACSM/AND/DC joint position
Athletes (strength)	1.6-2.0 g/kg/day	ACSM/AND/DC joint position
Critically ill	1.2-2.0 g/kg/day	Clinical guidelines (ASPEN, ESPEN)

AMDR (Acceptable Macronutrient Distribution Range)

Population	Protein (% of total calories)
Adults (≥19 years)	10-35%
Children (1-3 years)	5-20%
Children (4-18 years)	10-30%
Pregnancy	10-35%

Protein Per Meal (Muscle Protein Synthesis)

Aspect	Recommendation
Optimal per-meal dose	20-40 g high-quality protein
Distribution	Evenly across 3-4 meals (spares vs. skewed distribution better for MPS)
Leucine threshold	~2-3 g leucine per meal (activates mTOR/p70S6K)
Timing	Post-exercise window (0-2 hours)
Overnight	Pre-sleep protein (30-40 g casein) may enhance overnight MPS in elderly

Food Sources

Animal Protein Sources

Food	Serving	Protein (g)	Calories	Protein per 100 kcal (g)
Chicken breast (skinless, cooked)	100 g	31	165	18.8
Turkey breast (cooked)	100 g	29	135	21.5
Beef sirloin (lean, cooked)	100 g	26	190	13.7
Pork loin (cooked)	100 g	27	155	17.4
Salmon (cooked)	100 g	25	208	12.0
Tuna (canned, drained)	100 g	26	130	20.0
Eggs (whole, cooked)	2 large (100 g)	13	155	8.4
Egg whites	2 (66 g)	8	33	24.2
Milk (whole)	1 cup (244 g)	8	149	5.4
Greek yogurt (plain, nonfat)	200 g	20	125	16.0
Cottage cheese (low-fat)	1 cup (226 g)	28	163	17.2
Cheddar cheese	30 g	7	113	6.2
Whey protein isolate	30 g (1 scoop)	25	110	22.7
Casein protein	30 g (1 scoop)	24	110	21.8

Plant Protein Sources

Food	Serving	Protein (g)	Calories	Protein per 100 kcal (g)
Soybeans (edamame, cooked)	1 cup (155 g)	18	188	9.6
Tofu (firm)	100 g	11	76	14.5
Tempeh	100 g	20	193	10.4
Seitan (wheat gluten)	100 g	25	147	17.0
Lentils (cooked)	1 cup (198 g)	18	230	7.8
Chickpeas (cooked)	1 cup (164 g)	14	269	5.2
Black beans (cooked)	1 cup (172 g)	15	227	6.6
Kidney beans (cooked)	1 cup (177 g)	15	225	6.7
Quinoa (cooked)	1 cup (185 g)	8	222	3.6
Peanut butter	2 tbsp (32 g)	8	191	4.2
Almonds	¼ cup (35 g)	7	204	3.4
Hemp seeds	3 tbsp (30 g)	10	166	6.0
Chia seeds	2 tbsp (28 g)	5	138	3.6
Pumpkin seeds	¼ cup (30 g)	9	178	5.1
Peas (green, cooked)	1 cup (160 g)	9	134	6.7
Spirulina	1 tbsp (7 g)	4	20	20.0

Protein Comparison: Animal vs. Plant

Aspect	Animal Proteins	Plant Proteins
Essential amino acid profile	Complete	Usually incomplete (except soy, quinoa)
Protein quality (PDCAAS)	0.92-1.00	0.25-1.00
Digestibility	90-99%	70-90% (varies by food and processing)
Saturated fat content	Higher (varies by cut/processing)	Lower (except coconut, palm)
Fiber content	None	Variable (often high)
Phytochemicals	None	Present (beneficial antioxidants)
Cost per gram protein	Higher (generally)	Lower (generally)

Proteins and Health

Positive Health Associations

Protein Intake Pattern	Health Outcome	Evidence Level
Adequate intake	Muscle mass maintenance	Strong
Higher intake (older adults)	↓ Sarcopenia risk	Strong
Plant protein dominant	↓ Cardiovascular disease risk	Strong
Replacing red/processed meat with plant protein	↓ Colorectal cancer risk	Strong
Higher intake (weight loss)	↑ Satiety, preserved lean mass	Strong
Leucine-rich meals	↑ Muscle protein synthesis	Strong

Health Concerns with Excessive Protein

Concern	Evidence	Context
Kidney damage in healthy individuals	Not supported (no evidence in those with normal kidney function)	Myth debunked
Kidney damage in CKD	Well-established	Restrict protein in advanced CKD
Bone health (calcium loss)	Not supported (higher protein may improve bone density)	Myth debunked
Dehydration	Minimal effect in practice	Hydration generally adequate
Cancer risk (red/processed meat)	Strong association	Specific to red/processed meat, not all protein
Gout (purine-rich proteins)	Association with animal purines	Individual susceptibility

Branched-Chain Amino Acids (BCAAs)

BCAA	Structure	Unique Role
Leucine	Branched side chain	Primary mTOR activator, key regulator of muscle protein synthesis
Isoleucine	Branched side chain	Glucose uptake, muscle metabolism
Valine	Branched side chain	Muscle energy, nitrogen balance

BCAAs are unique in that they are metabolized primarily in muscle rather than liver. They constitute ~35% of muscle protein.

Key Takeaways

• Complete proteins containing all 9 essential amino acids are found primarily in animal foods and soy
• Plant proteins can meet all needs through variety throughout the day (no need to complement at every meal)
• RDA is 0.8 g/kg/day; optimal intake ranges from 1.2-2.0 g/kg/day depending on activity level and age
• Protein quality is measured by PDCAAS (current standard) and DIAAS (newer, more accurate)
• Leucine content (2-3 g per meal) is critical for maximizing muscle protein synthesis
• Distribute protein evenly across 3-4 meals rather than skewing toward one meal
• Older adults likely benefit from intakes above the RDA (1.0-1.2 g/kg/day)
• Excessive protein is not harmful in healthy individuals but should be avoided in advanced kidney disease