Place · Level 3
Carbs & Fiber
葡萄糖是快速燃料 · 糖原是运动电池 · 纤维是肠道生态的底物
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Chapter 1
Not one thing
Not one thing
'Carbs' is at least three different things:
Sugars: glucose, fructose, sucrose — absorbed fastStarches: glucose long-chains in rice, potatoes, noodles, legumesFiber: human enzymes can't cleave it, but gut bacteria partly can
The problem usually isn't 'carbs themselves' — it's the packaging. Refined sugar + refined flour + low fiber + high fat/salt combinations are a completely different food category from legumes, whole grains, tubers, and fruit.
Sugars: glucose, fructose, sucrose — absorbed fastStarches: glucose long-chains in rice, potatoes, noodles, legumesFiber: human enzymes can't cleave it, but gut bacteria partly can
The problem usually isn't 'carbs themselves' — it's the packaging. Refined sugar + refined flour + low fiber + high fat/salt combinations are a completely different food category from legumes, whole grains, tubers, and fruit.
Glycemic load
GI (glycemic index) measures glucose rise per 50g available carb, glucose = 100. But 50g of watermelon carb equals an entire huge slice — nobody eats that — so GL (glycemic load) = GI × actual carb g / 100 is the real exposure:Watermelon: GI 72 (high), but a serving is ~150g → GL ~5 (low)White rice: GI 73, 150g serving → GL ~30 (high)Oatmeal: GI 55, one serving → GL ~13 (moderate)Lentils: GI 30, one serving → GL ~8 (low)
So 'avoid high-GI foods' is too coarse — what actually drives metabolism is GL + food matrix + processing level. The mainstreaming of CGM (continuous glucose monitors) made this layer observable in daily life, which is also why nutrition advice is shifting toward measured-over-blanket rules.
Chapter 2
Glycogen battery
Glycogen battery
The liver and muscles package glucose into glycogen:
Liver glycogen (~100 g) → maintains blood glucose, especially overnight and between mealsMuscle glycogen (~400 g) → muscle's private fuel, supplies adenosine triphosphate: The cell's universal energy currency — almost everything that costs energy spends it. rapidly during high-intensity work
The higher the intensity, the more you rely on glycogen. Long endurance bouts, intervals, ball sports, and high-volume strength training all visibly drain muscle glycogen.
So in sports nutrition, carbs aren't a moral question — they're 'do you want a charged battery for high-intensity output?'
Liver glycogen (~100 g) → maintains blood glucose, especially overnight and between mealsMuscle glycogen (~400 g) → muscle's private fuel, supplies adenosine triphosphate: The cell's universal energy currency — almost everything that costs energy spends it. rapidly during high-intensity work
The higher the intensity, the more you rely on glycogen. Long endurance bouts, intervals, ball sports, and high-volume strength training all visibly drain muscle glycogen.
So in sports nutrition, carbs aren't a moral question — they're 'do you want a charged battery for high-intensity output?'
GLUT4 translocation
Muscle takes up glucose via GLUT4 glucose transporter. At rest it sits hidden in intracellular vesicles, not on the membrane — which is why resting muscle barely responds to blood glucose.Two independent paths push GLUT4 to the membrane surface:
1. Insulin path: PI3K / Akt cascade activates → GLUT4 vesicles fuse to membrane
2. Exercise / contraction path: AMP-activated protein kinase: The cell's 'low fuel' sensor — switches on when energy is low to make energy and pause building. + Ca²⁺ + mechanical tension — does NOT require insulin
So the 30–60 min window after exercise is the 'muscle door open' period — GLUT4 stays at the membrane, and the same dose of carbs goes into muscle glycogen instead of fat.
This mechanism explains several things:
Type 2 diabetics can still control glucose with exercise (even with broken insulin path, the contraction path is independent)Post-workout carbs matter for endurance athletes' recoverySedentary lifestyle + high-carb diet → GLUT4 stays sequestered long-term — this is the cellular root of insulin resistance
(Click 'go deeper' to see the GLUT4 vesicle-to-membrane animation.)
Chapter 3
When carbs stop coming
When carbs stop coming
When carbs stop, your body does not reach for fat to make the glucose your brain is asking for — it reaches for your muscle first.
The previous two scenes were about carbs arriving. This one is about them not arriving.
The brain wants glucose continuously, and blood sugar can't pause. So the liver (joined by the kidney in prolonged starvation) opens a production line that builds glucose from other raw materials. That line is called gluconeogenesis.
The catch: only three raw materials can get onto it.
Carbon skeletons of glucogenic amino acids — stripped out of your own muscle proteinGlycerol — the three-carbon head on a fat molecule, which falls off when fat is broken apart; it is a small fraction of the fatLactate — leftovers from muscle, cycling back to the liver
The bulk of fat — those long fatty-acid chains — cannot get on at all.
Why not? Inside the cell, fatty acids are clipped into two-carbon units of acetyl-CoA, and acetyl-CoA has exactly one destination: pair with oxaloacetate to form citrate and enter the tricarboxylic acid (Krebs) cycle: The mitochondrial hub cycle that fully oxidizes fuel and harvests electrons for energy. cycle. But to replace the oxaloacetate it consumed, the cycle has to go all the way around — and the price of that lap is exhaling those two carbons as carbon dioxide. Once the carbon is breathed out, it isn't coming back.
So a person carrying plenty of body fat, several days into a fast, is still tearing down their own muscle. The warehouse is stacked with fuel that will never run out, and there is no road from it to the molecule the brain needs.
Ketones are the second fuel system your body evolved so it would have to tear down less muscle. The liver packages the acetyl-CoA it can't burn into ketones and ships them to the brain; once the brain runs on ketones, less glucose has to be built from scratch, so less muscle has to be taken apart. This is what Cahill spent most of his career on: ketones are the reason humans can survive prolonged starvation.
This one chain ties together three things the site has covered separately:
Why a cut costs you muscle — see protein-during-deficit on the protein side; this gluconeogenesis line is what's taking itWhy ketones only cover about two-thirds of the brain, with the rest still needing glucose — see the backup-brain-fuel page under nervousWhy the kidney takes on roughly 40% of gluconeogenesis in prolonged starvation — see renal
The chain also forces a counterintuitive conclusion: carbohydrate is the only one of the three macros that isn't essential.
There is no such thing as an essential carbohydrate — no sugar exists that you'd develop a deficiency disease without, because the line above proves the body can build its own glucose. Whereas there are 9 essential amino acids and 2 essential fatty acids (linoleic and α-linolenic). Those 11 the body genuinely cannot make, and going short causes real problems.
The site has already applied this exact ruler to alcohol (see the first scene of alcohol-metabolism: ethanol supplies energy, but it isn't essential, so it has no RDA). Measured with the same ruler, carbohydrate and ethanol land in one box; protein and fat land in the other.
But non-essential is a long way from shouldn't eat it — don't blur the two:
Very low carb is survivable; that's a direct corollary of the chain aboveThe cost is that gluconeogenesis runs continuously, amino acids keep feeding into it, and nitrogen keeps leaking out — plus the share of glucose ketones can't cover still gets billed to proteinSo this scene neither recommends nor opposes a ketogenic diet. It answers one question only: when carbs stop, who pays the bill?
The previous two scenes were about carbs arriving. This one is about them not arriving.
The brain wants glucose continuously, and blood sugar can't pause. So the liver (joined by the kidney in prolonged starvation) opens a production line that builds glucose from other raw materials. That line is called gluconeogenesis.
The catch: only three raw materials can get onto it.
Carbon skeletons of glucogenic amino acids — stripped out of your own muscle proteinGlycerol — the three-carbon head on a fat molecule, which falls off when fat is broken apart; it is a small fraction of the fatLactate — leftovers from muscle, cycling back to the liver
The bulk of fat — those long fatty-acid chains — cannot get on at all.
Why not? Inside the cell, fatty acids are clipped into two-carbon units of acetyl-CoA, and acetyl-CoA has exactly one destination: pair with oxaloacetate to form citrate and enter the tricarboxylic acid (Krebs) cycle: The mitochondrial hub cycle that fully oxidizes fuel and harvests electrons for energy. cycle. But to replace the oxaloacetate it consumed, the cycle has to go all the way around — and the price of that lap is exhaling those two carbons as carbon dioxide. Once the carbon is breathed out, it isn't coming back.
So a person carrying plenty of body fat, several days into a fast, is still tearing down their own muscle. The warehouse is stacked with fuel that will never run out, and there is no road from it to the molecule the brain needs.
Ketones are the second fuel system your body evolved so it would have to tear down less muscle. The liver packages the acetyl-CoA it can't burn into ketones and ships them to the brain; once the brain runs on ketones, less glucose has to be built from scratch, so less muscle has to be taken apart. This is what Cahill spent most of his career on: ketones are the reason humans can survive prolonged starvation.
This one chain ties together three things the site has covered separately:
Why a cut costs you muscle — see protein-during-deficit on the protein side; this gluconeogenesis line is what's taking itWhy ketones only cover about two-thirds of the brain, with the rest still needing glucose — see the backup-brain-fuel page under nervousWhy the kidney takes on roughly 40% of gluconeogenesis in prolonged starvation — see renal
The chain also forces a counterintuitive conclusion: carbohydrate is the only one of the three macros that isn't essential.
There is no such thing as an essential carbohydrate — no sugar exists that you'd develop a deficiency disease without, because the line above proves the body can build its own glucose. Whereas there are 9 essential amino acids and 2 essential fatty acids (linoleic and α-linolenic). Those 11 the body genuinely cannot make, and going short causes real problems.
The site has already applied this exact ruler to alcohol (see the first scene of alcohol-metabolism: ethanol supplies energy, but it isn't essential, so it has no RDA). Measured with the same ruler, carbohydrate and ethanol land in one box; protein and fat land in the other.
But non-essential is a long way from shouldn't eat it — don't blur the two:
Very low carb is survivable; that's a direct corollary of the chain aboveThe cost is that gluconeogenesis runs continuously, amino acids keep feeding into it, and nitrogen keeps leaking out — plus the share of glucose ketones can't cover still gets billed to proteinSo this scene neither recommends nor opposes a ketogenic diet. It answers one question only: when carbs stop, who pays the bill?
Non-essential, yet it has an RDA
IOM 2005 sets an RDA for carbohydrate of 130 g/day. Why would a non-essential nutrient have an RDA at all?The answer is in the basis IOM itself printed in the Function column of that DRI table: the RDA rests on carbohydrate's role as the primary energy source for the brain.
Which means 130 g isn't the 'you'll get sick without it' number. It's the 'enough for the brain, so gluconeogenesis doesn't have to tear down protein' number. It's a don't-eat-your-muscle voucher, not a certificate of essentiality.
Put two other rows of the same table beside it and the ruler becomes fully visible:
Linoleic acid (n-6) and α-linolenic acid (n-3): IOM gives them an AI, and the Function column reads 'essential component of structural membrane lipids, required for normal skin function'. That is what essential looks like in print.Saturated fat, trans fat, cholesterol: IOM writes that no required role other than as energy sources was identified, and that the body can synthesize what it needs. So they get no RDA and no AI.Carbohydrate: has an RDA, but the basis is the brain's consumption, not a deficiency syndrome.
Read those three rows together and it's clear what the essentiality ruler actually measures: it measures whether there's a deficiency syndrome, not whether the body has a use for it.
The body certainly has a use for carbohydrate — a large one. But has a use for and essential are two different things. This is the most commonly conflated pair in nutrition, and it's the same trap that makes the term essential amino acid misleading (there's a page on that over on the protein island).
Not one carbon? Almost
The claim above — that not one fatty-acid carbon gets into gluconeogenesis — is the textbook line; Lehninger, Stryer, and Voet all print it. Being honest, that sentence has two frayed edges.Edge one: odd-chain fatty acids. Clipping two carbons at a time, an odd chain leaves a final three-carbon propionyl-CoA behind — and that piece genuinely can enter gluconeogenesis. But almost all the fat you eat is even-chain, so the gap is narrow.
Edge two: Kaleta and colleagues, 2011, recomputed this old question on a genome-scale metabolic network and argued that routes bypassing the glyoxylate shunt may exist in humans, making fatty-acid-to-glucose theoretically feasible. But they stated it plainly in the paper: the efficiency is low.
Neither edge changes the conclusion. When you're starving, your brain can't wait on a low-efficiency theoretical bypass, and the muscle gets taken apart anyway. Saying fat can't become glucose is slightly coarse chemically, and correct metabolically.
This is also a move worth practicing when you read nutrition: the same sentence often has one answer at the is it possible level and a different answer at the is it fast enough level. For gluconeogenesis, the one that decides whether you lose muscle is the second.
kaleta-2011-fat-to-glucose
Chapter 4
Fiber · microbiome fuel
Fiber · microbiome fuel
Soluble vs insoluble can't answer the question you actually have. Both oat and inulin are soluble — so why does oat blunt a glucose spike and inulin doesn't? Why does inulin bloat you and psyllium doesn't?
Because what a fiber does is set by two independent axes, and they act in two different organs:
Viscosity — in the small intestine: some fibers take on water and thicken the chyme, so sugar and bile acids diffuse to the gut wall more slowly. That's how oat blunts a glucose spike and lowers LDL.Fermentability — in the colon: some fibers get eaten by bacteria, yielding short-chain fatty acids and, incidentally, gas. That's the feed-your-microbes mechanism — and also the bloating mechanism.
The axes are orthogonal, so there are four quadrants (McRorie 2017):
Psyllium = high viscosity + low fermentation: the gel reaches the rectum intact and works as a lubricant (Marlett 2000). It works precisely because the bacteria can't eat it — so it relieves constipation and lowers LDL without the gasInulin = low viscosity + high fermentation: feeds microbes hard, makes gas, does nothing to a glucose spikeWheat bran = low viscosity + low fermentation: pure physical bulkOat β-glucan = high viscosity + moderate fermentation: a bit of both
So when you see the words high fiber, ask: which axis? Constipation wants high-viscosity, low-fermentation. Feeding microbes wants high fermentation. If fiber bloats you, avoid the fermentable end.
Fiber targets aren't reached by suddenly maxing out in one day — add gradually week over week and drink enough water.
Because what a fiber does is set by two independent axes, and they act in two different organs:
Viscosity — in the small intestine: some fibers take on water and thicken the chyme, so sugar and bile acids diffuse to the gut wall more slowly. That's how oat blunts a glucose spike and lowers LDL.Fermentability — in the colon: some fibers get eaten by bacteria, yielding short-chain fatty acids and, incidentally, gas. That's the feed-your-microbes mechanism — and also the bloating mechanism.
The axes are orthogonal, so there are four quadrants (McRorie 2017):
Psyllium = high viscosity + low fermentation: the gel reaches the rectum intact and works as a lubricant (Marlett 2000). It works precisely because the bacteria can't eat it — so it relieves constipation and lowers LDL without the gasInulin = low viscosity + high fermentation: feeds microbes hard, makes gas, does nothing to a glucose spikeWheat bran = low viscosity + low fermentation: pure physical bulkOat β-glucan = high viscosity + moderate fermentation: a bit of both
So when you see the words high fiber, ask: which axis? Constipation wants high-viscosity, low-fermentation. Feeding microbes wants high fermentation. If fiber bloats you, avoid the fermentable end.
Fiber targets aren't reached by suddenly maxing out in one day — add gradually week over week and drink enough water.
SCFA trio
Colonic bacteria ferment soluble fiber into short-chain fatty acids (short-chain fatty acids: Small molecules (acetate/propionate/butyrate) gut bacteria make from fiber — they feed the gut lining and calm inflammation.) — three workhorses with completely different functions:Acetate (~60%): enters systemic circulation, reaches liver for cholesterol/fat synthesis, reaches brain for satiety regulationPropionate (~20%): almost entirely captured by liver, suppresses hepatic gluconeogenesis, lowers fasting glucose and cholesterol synthesisButyrate (~20%): the preferred fuel of colonic cells (~70% of their energy), doesn't enter blood. It's the repair signal for the colonic epithelial barrier, anti-inflammatory, and negatively correlated with colon cancer risk
Worth remembering: oral butyrate can't reach the colon — gastric acid plus small-intestine absorption mean it never makes it there. To deliver butyrate to the colon, you must feed gut bacteria fiber and let them produce it. The big butyrate producers are *Faecalibacterium prausnitzii* (a marker of gut health), *Roseburia*, and *Eubacterium rectale* — they love resistant starch (RS), β-glucan, and inulin.
So 'eat probiotics for gut health' is only half right — feeding the bacteria (prebiotics) is usually more reliable than swallowing them (probiotics), because most swallowed strains don't colonize.
Chapter 5
Glucose curve
Glucose curve
The same 50 g of carbs can produce completely different glucose curves.
Influencing factors include: food particle size, processing level, fiber, protein, fat, acidity, resistant starch formed when cooked food cools, and whether you just exercised.
In practice, 'carbs + protein + vegetables / legumes / whole grain' usually produces a steadier curve than sweet drinks or white bread alone. After exercise, muscle GLUT4 transport is upregulated and the same carbs more readily replenish glycogen.
Influencing factors include: food particle size, processing level, fiber, protein, fat, acidity, resistant starch formed when cooked food cools, and whether you just exercised.
In practice, 'carbs + protein + vegetables / legumes / whole grain' usually produces a steadier curve than sweet drinks or white bread alone. After exercise, muscle GLUT4 transport is upregulated and the same carbs more readily replenish glycogen.
Personalization
Weizmann Institute 2015 (Zeevi et al., *Cell*) put CGMs on 800 people eating the same foods, and found person-to-person variability exceeds food-to-food variability:Same banana: person A's glucose spikes 80 mg/dL, person B's rises only 20Same slice of white bread: person A reacts 3× higher than person B
Drivers of individual variance, in weight order:
1. Gut microbiome composition: largest weight, explains ~25% of variance
2. Baseline insulin sensitivity / HbA1c
3. Circadian rhythm: same food at breakfast triggers 20-40% lower response than at dinner
4. The previous meal's 'second-meal effect'
5. Sleep duration: one bad night cuts next-day insulin sensitivity ~25%
So 'what's this food's GI, can I eat it?' is slowly giving way to 'how does *my body* respond to it?'. CGMs are increasingly common in prediabetes and metabolic syndrome.
But watch out: mainstreaming CGM also breeds 'any glucose spike is bad' over-anxiety. In healthy people, mild postprandial swings under 7.8 mmol/L are physiologic — no need to panic.
Chapter 6
Postprandial slump
Postprandial slump
30-90 min after a high-carb meal you feel sleepy, foggy, can't focus, want to nap — almost everyone has experienced it, but few understand the cause correctly.
It's not a single mechanism — it's 4 pathways stacking simultaneously:
1. Glucose roller coaster: high-GI meal → insulin overshoot → 90-120 min later glucose rebounds below pre-meal (reactive hypoglycemia), brain senses energy shortage → fatigue + thirst + poor focus
2. Tryptophan enters brain: insulin shuttles branched-chain amino acids (BCAA: Leu / Ile / Val) into muscle, free plasma tryptophan rises relatively, the blood–brain barrier: The 'security gate' on brain vessels that blocks most substances in blood from entering the brain. transporter favors tryptophan → brain 5-HT and melatonin synthesis rise → drowsiness arrives
3. Orexin silenced: glucose directly inhibits lateral hypothalamic orexin / hypocretin wake neurons (this is the core narcolepsy pathway) → arousal drive drops
4. Blood-flow diversion: postprandial visceral blood flow rises 30-40%, intracranial perfusion shifts slightly — small effect alone, but stacks on the other three
So blaming postprandial slump on 'didn't sleep enough' or 'getting old' misses the mark — it's normal physiology amplified by the wrong meal structure. Once you understand the mechanism, the fix isn't blanket 'eat fewer carbs' — it's (next page) restructure the meal, walk 10 min after eating, and front-load heavy carbs to breakfast/lunch when physiologic insulin sensitivity is high.
It's not a single mechanism — it's 4 pathways stacking simultaneously:
1. Glucose roller coaster: high-GI meal → insulin overshoot → 90-120 min later glucose rebounds below pre-meal (reactive hypoglycemia), brain senses energy shortage → fatigue + thirst + poor focus
2. Tryptophan enters brain: insulin shuttles branched-chain amino acids (BCAA: Leu / Ile / Val) into muscle, free plasma tryptophan rises relatively, the blood–brain barrier: The 'security gate' on brain vessels that blocks most substances in blood from entering the brain. transporter favors tryptophan → brain 5-HT and melatonin synthesis rise → drowsiness arrives
3. Orexin silenced: glucose directly inhibits lateral hypothalamic orexin / hypocretin wake neurons (this is the core narcolepsy pathway) → arousal drive drops
4. Blood-flow diversion: postprandial visceral blood flow rises 30-40%, intracranial perfusion shifts slightly — small effect alone, but stacks on the other three
So blaming postprandial slump on 'didn't sleep enough' or 'getting old' misses the mark — it's normal physiology amplified by the wrong meal structure. Once you understand the mechanism, the fix isn't blanket 'eat fewer carbs' — it's (next page) restructure the meal, walk 10 min after eating, and front-load heavy carbs to breakfast/lunch when physiologic insulin sensitivity is high.
4 pathways · deep dive
Path 1 · Glucose roller coaster (reactive postprandial hypoglycemia)High-GI meal (white rice + sugary drink + refined bread) at 30-60 min sends glucose to 8-11 mmol/Lβ cells overshoot insulin release (healthy people too, but slow-meal phenotypes plus insulin resistance overshoot more)90-120 min later glucose drops below pre-meal (sometimes <3.9 mmol/L) — brain reads energy shortage → drowsy, irritable, sugar-craving (the classic 'I want milk tea at 3pm')CGM signature is a sharp spike + steep drop + rebound (swing >4 mmol/L), not a smooth wave
Path 2 · Tryptophan + LNAA competition (the Wurtman classic)
5-HT (serotonin) and melatonin precursor is tryptophan (Trp), a large neutral amino acid (LNAA)Trp crosses the blood–brain barrier: The 'security gate' on brain vessels that blocks most substances in blood from entering the brain. via the LAT1 transporter, competing with BCAAs (leucine / isoleucine / valine) for the same channelA high-carb meal causes insulin to push BCAAs into muscle for protein synthesis → plasma BCAA temporarily dropsPlasma Trp/LNAA ratio rises 30-50% → more Trp enters the brain → brain 5-HT and melatonin synthesis riseThis is why pure-carb meals make you sleepier than high-protein meals — and why 'turkey makes you sleepy because of tryptophan' is a myth (turkey has no more tryptophan than chicken; the holiday sleepiness comes from the high-carb side dishes)
**Path 3 · Orexin / Hypocretin wake system inhibited (Burdakov 2006 *Neuron*)**
Lateral hypothalamic (LH) orexin / hypocretin neurons are the brain's master wake switch, projecting to LC / VTA / TMN / BF to maintain arousalGlucose directly inhibits these neurons: glucose enters → closes tandem-pore K⁺ channels → neuron hyperpolarizes → silencedThis is the core mechanism of narcolepsy type 1: autoimmune destruction of orexin neurons → arousal collapse, postpartum sleepiness amplifiedHealthy people have this effect too, just milder — but a high-carb meal stacked with path-1 glucose spike inhibits orexin neurons more deeply
Path 4 · Postprandial mesenteric vasodilation
30-60 min postprandial, visceral (gut + pancreas + liver) blood flow rises 30-40% for absorption and metabolismCardiac output redistributes — in healthy people cerebral perfusion barely changes (autoregulation protects), but in the elderly or those with autonomic neuropathy, mild cerebral hypoperfusion can occurThis pathway alone has small effect, but layered on the previous three it adds the 'a bit sleepier' baseline
So postprandial slump is 4 paths firing at once. This is why changing only one thing (e.g., cutting carbs alone) has limited effect, while restructuring the meal + post-meal walk (next page) — hitting paths 1 + 3 together — works clearly.
Fix the meal, not the carb
Common misconception: 'post-meal slump = I ate carbs, just stop carbs.' Blanket carb-cutting actually makes hunger + sleepiness worse (early starvation ketosis is also sleepy), and training-day performance crashes.The 4 things that actually solve the problem, by effect:
1. Restructure the meal: vegetables / protein first, carbs last (food order)
Order is vegetables → protein + fat → carbs. Mechanism: eating vegetables + protein first triggers glucagon-like peptide-1: A gut hormone released after eating that makes you feel full and helps lower blood sugar. + CCK, slows gastric emptying, halves the rate at which the same carbs enter the small intestine, drops postprandial glucose peak 30-40% (Shukla 2015 *Diabetes Care*, classic RCT in T2D). No need to refuse carbs — just change the order. Simple enough to execute immediately.
**2. Walk 10-15 min after the meal (DiPietro 2013 *Diabetes Care*)**
N=10 elderly prediabetic, CGM for 48 hours. Intervention group walked 15 min at moderate intensity after each meal (starting within 30 min); controls sat continuously or did equal total exercise but not postmeal. Result: postprandial peak dropped, 24-h glucose variability halved, effect significantly outperformed an equal-volume morning workout. Mechanism: muscle contraction activates the non-insulin GLUT4 path (atlas `carbs-fiber/glycogen` L4 has the AMP-activated protein kinase: The cell's 'low fuel' sensor — switches on when energy is low to make energy and pause building. details) — the same carbs go directly to muscle glycogen rather than piling up in blood. Executable version: walk after dinner to take out trash / collect a delivery / walk the dog — no need to change clothes or shoes. Lower the threshold, higher the adherence.
3. Front-load heavy carbs to breakfast / lunch, not dinner
Physiologic insulin sensitivity is high in the morning, low at night — same food causes 20-40% lower response at breakfast than dinner (Jakubowicz et al., multiple chrono-nutrition studies). Insulin, GLP-1, and hepatic gluconeogenesis all have 24 h rhythms. Practical version: put rice, noodles, potatoes at breakfast and lunch; make dinner protein + vegetables + modest carbs. Shift workers / night shift don't get this benefit because their suprachiasmatic nucleus: The brain's master clock — set by light, it runs the body's day–night rhythm. is disrupted (atlas `shift-work-circadian` covers it).
4. CGM for personalization (Weizmann 2015, mentioned earlier)
Same banana causes person A 80 mg/dL spike vs person B 20 — individual variance exceeds food variance. Prediabetic, T2D, or repeated-slump people can wear CGM for 2 weeks to identify their top 3 trigger foods. Not recommended for healthy people just for fun — it tends to spawn 'any glucose spike is bad' over-anxiety.
Watch out: if postprandial slump is severe enough to include orthostatic dizziness, palpitations, cold sweats, near-syncope (not just sleepiness), it may not be ordinary slump but reactive hypoglycemia (true low glucose), dumping syndrome (post-gastric-surgery), hypotension, arrhythmia, or autonomic dysfunction. See a doctor — don't self-treat.
Chapter 7
Three rules
Three rules
First, on sedentary days, swap carbs for 'structured' versions: legumes, whole grains, tubers, fruit, vegetables.
Second, on training days don't fear carbs. Before and after hard training, carbs support output and recovery — they don't automatically become fat.
Third, add fiber slowly. Going from 10g to 35g overnight makes the gut protest; adding one serving of legumes or whole grains per week is more realistic.
Second, on training days don't fear carbs. Before and after hard training, carbs support output and recovery — they don't automatically become fat.
Third, add fiber slowly. Going from 10g to 35g overnight makes the gut protest; adding one serving of legumes or whole grains per week is more realistic.
Fiber ramp
DGA US dietary target: 25 g (women) / 38 g (men) fiber per day. Actual US average is ~15 g; urban Chinese ~10-15 g — gap is large.3-week ramp to avoid bloating and IBS triggers:
1. Week 1: 1 daily serving of whole grain replacing 1 serving of refined rice/flour (oats, brown rice, quinoa, whole wheat)
2. Week 2: 1 daily serving of legumes into main meals (rajma red kidney beans, chickpeas, lentils, edamame) — any form
3. Week 3: 1 daily serving of fruit-with-skin + 30 g raw nuts (almonds / walnuts / chia)
Also +500 ml water — fiber needs water; high fiber without water causes constipation.
Contraindicated populations: active IBS flare, active IBD, early post-gastric-surgery, severe gastroparesis. In these situations high fiber worsens symptoms — adjust under medical guidance.
References · 12
- Institute of Medicine. (2005). Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. National Academies Press. nap.nationalacademies.org/catalog/10490/dietary-reference-intakes-for-energy-carbohydrate-fiber-fat-fatty-acids-cholesterol-protein-and-amino-acids
- U.S. Department of Agriculture & U.S. Department of Health and Human Services. (2020). Dietary Guidelines for Americans, 2020-2025 (9th ed.). www.dietaryguidelines.gov/sites/default/files/2020-12/Dietary_Guidelines_for_Americans_2020-2025.pdf
- Hargreaves, M., & Spriet, L. L. (2020). Skeletal muscle energy metabolism during exercise. Nature Metabolism, 2(9), 817–828. 10.1038/s42255-020-0251-4
- Melkonian, E. A., Asuka, E., & Schury, M. P. (2023). Physiology, Gluconeogenesis. In StatPearls. StatPearls Publishing. www.ncbi.nlm.nih.gov/books/NBK541119
- Cahill, G. F., Jr. (2006). Fuel metabolism in starvation. Annual Review of Nutrition, 26, 1-22. 10.1146/annurev.nutr.26.061505.111258
- Owen, O. E. (2005). Ketone bodies as a fuel for the brain during starvation. Biochemistry and Molecular Biology Education, 33(4), 246-251. 10.1002/bmb.2005.49403304246
- Gerich, J. E., Meyer, C., Woerle, H. J., & Stumvoll, M. (2001). Renal gluconeogenesis: its importance in human glucose homeostasis. Diabetes Care, 24(2), 382-391. 10.2337/diacare.24.2.382
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