Carbohydrates: From Simple Sugars to Starch, Fiber & Resistant Starch

Explain the chemistry, structure and physiological handling of sugars, starches and dietary fibers
Clarify why apparently small structural differences (e.g., an (\alpha) vs. (\beta) bond) translate into dramatically different nutritional effects
Review the related concepts of pre-, pro- and post-biotics (context for gut microbiota)
Detail the four defined forms of Resistant Starch (RS 1-4) and the health outcomes credited to them

Collective term for three main classes
- Sugars (mono- & disaccharides)
- Starches (digestible polysaccharides)
- Fibers (indigestible polysaccharides & lignin)
Primary origin: plant foods
- Exception: lactose in milk, trace glycogen in fresh meat/seafood (rapidly converts to lactic acid post-mortem)
Universal role: energy reserve in plants & animals

Empirical formula: ${C<em>n(H</em>2O)_n}$ (literally “hydrated carbon”)
- (n) ranges from 2 → thousands
Glycosidic Bond
- Condensation of two monosaccharides releases $H_2O$ and forms a C–O–C bridge
- Orientation of the anomeric carbon defines an (\alpha) (down) or (\beta) (up) configuration

Food-industry calculation:
${\text{Total CHO} - \text{Fiber} - \text{Sugar Alcohols}}$
Not recognized or reviewed by FDA; arose from low-CHO diet marketing
Label caveats (example from Lakanto baking mix)
- Total CHO = 11 g; Fiber = 7 g; Sugar Alcohols = 3 g → “1 net carb”
- Industry rules of thumb
- Erythritol: 100 % subtracted
- Other polyols (xylitol, maltitol, IMO): only 50 % credit

Naturally scarce; richest in legumes & dried beans
Poorly hydrolyzed by human enzymes → reach large intestine intact
Gut microbiota ferment them → gas ((\ce{CO2}), H₂, CH₄)
Commercial extraction/synthesis (soybean raffinose, fructo-oligosaccharides) used as prebiotics & low-calorie sweeteners
Non-cariogenic (do not foster dental caries)
Enzymatic aid: Alpha-galactosidase (Beano®) hydrolyzes galacto-linkages in raffinose/stachyose, mitigating gas

Digestible group

Repeating (\alpha)-D-glucose
Two fractions (always co-exist)
1. Amylose (≈20–30 %)
- Linear (\alpha)1→4 chain, minimal branching
1. Amylopectin (≈70–80 %)
- Highly branched: (\alpha)1→4 backbone + (\alpha)1→6 branch points every 24-30 residues
Visualization reference: 80 000 glucose residues per macromolecule (diagram in slides)
Botanical distribution mnemonic
- “Above ground” seeds/cereals → higher % amylose (wheat, corn)
- Exception: Waxy maize genetically modified to be ≈100 % amylopectin
- “Below ground” roots/tubers → rich in amylopectin (potato, arrowroot, tapioca)

Storage CHO in mammals; analogous to amylopectin but even more branched ((\alpha)1→6 every 8–12 residues)
Localized in liver (≈100 g) & skeletal muscle (≈400 g in fed adult)
Mobilized by phosphorylase & debranching enzymes → glucose-1-P → blood glucose or glycolysis fuel

Cellulose – linear (\beta)1→4 D-glucose; no side chains; forms microfibrils in plant walls
Hemicellulose – heterogeneous (arabinose, xylose, etc.) around cellulose core
Beta-glucans – soluble O-linked glucose in oats/barley; clinically lower LDL-C
Pectins – galacturonic acid chains between/within cell walls; gel-forming
Gums/Hydrocolloids – guar, locust-bean, carrageenan; thickening agents
Lignin – phenolic polymer; technically not a carbohydrate; confers “woodiness” (e.g., stringy aged celery)

Human digestive amylases (salivary & pancreatic (\alpha)-amylase) recognize only (\alpha) linkages
(\beta)1→4 linkages in cellulose resist hydrolysis → pass to colon intact, adding bulk & fermentable substrate

Dissolves to form viscous gels; binds bile acids & dietary fats
Slows gastric emptying → moderated glucose absorption
Clinical benefits
- ↓ Total & LDL-C (mechanism: bile sequestration ↑ hepatic LDL uptake)
- Improved post-prandial glycemia (valuable in diabetes management)
Food sources: oats/oat bran, barley, dried beans & lentils, apples, citrus, carrots, rice bran

Does not dissolve; absorbs water → ↑ stool bulk, ↓ transit time
Maintains colonic pH → environment unfavorable to carcinogen-producing microbes
Anti-constipation & potential colorectal cancer protection
Food sources: whole wheat/rye, corn bran, flaxseed, banana, vegetable & fruit skins, green beans, cauliflower, potato skins

Escapes digestion in small intestine yet supplies (~2–3) kcal·g⁻¹ (fermented SCFA energy rather than blood glucose)
Four mechanistic categories
1. RS 1 – Physically inaccessible (intact seeds, legumes, unprocessed whole grains)
2. RS 2 – Intrinsically resistant granules (raw potato, green banana, high-amylose maize)
3. RS 3 – Retrograded starch formed when gelatinized starch cools (cooled rice, pasta, potato salad, stale bread)
4. RS 4 – Chemically modified (cross-linked, etherized) for processed foods

Fermented by colonic microbiota → short-chain fatty acids (SCFA)
- Butyrate: primary fuel for colonocytes; linked to ↓ DNA damage & carcinoma risk
↑ Mineral absorption (Ca²⁺, Mg²⁺)
Improved insulin sensitivity (acute & chronic studies)
Favors beneficial bacterial taxa while suppressing pathogens
Modest ↓ serum cholesterol & triglycerides
Post-prandial effect: less caloric “availability” → reduced adipose storage

Prebiotics: non-digestible substrates (incl. oligosaccharides, soluble fibers, resistant starch) selectively utilized by host microbes → health benefit
Probiotics: live microorganisms (e.g., Lactobacillus, Bifidobacterium) that, when ingested in adequate amounts, confer benefit
Postbiotics: metabolic by-products or cell-wall components from probiotics (e.g., butyrate, peptidoglycan) exerting physiological effects without live bacteria
Carbohydrate quality (fiber & RS) directly modulates all three: feeds microbes (pre), promotes growth (pro), and yields beneficial metabolites (post)

FDA Nutrition Facts panel does not acknowledge “Net Carbs”; consumers rely on unregulated subtraction rules
Fiber & RS content of processed foods can be manipulated via chemical modification (RS 4) → labeling transparency becomes an ethical consideration
Public-health strategies (e.g., whole-grain recommendations, soluble-fiber heart-health claims on Cheerios®) hinge on robust evidence linking specific carbohydrate structures to disease risk reduction

General carbohydrate formula: ${C<em>n(H</em>2O)_n}$
“Three grams soluble fiber from whole-grain oats daily” health-claim threshold
Glycogen stores: ≈ $500\,\text{g}$ total (liver ≈100 g + muscle ≈400 g) in a well-fed adult
Butyrate production from RS fermentation: ≈15–20 % of total SCFA pool