Carbohydrates and Blood Sugar

TL;DR. All digestible carbohydrate ends up as glucose in your blood, and your body is intensely concerned with keeping that number in a narrow band. Insulin brings it down, and a handful of hormones bring it up. What determines the size and shape of the rise is not sugar-versus-starch but the physical form of the food: whether the starch is locked inside intact cell walls, how much fibre, fat, and protein arrived with it, and how fast the stomach empties. Fructose is metabolised differently from glucose, almost entirely in the liver, which is why the fructose in a bottle of soft drink and the fructose in an apple behave differently in practice.

Key takeaways

• Your entire bloodstream contains about 4 to 5 grams of glucose, roughly a teaspoon. The regulation is that tight.
• Glycaemic index describes a food eaten alone in a fixed 50 g carbohydrate dose, which is not how anyone eats. Glycaemic load, and the whole meal, matter more.
• The physical structure of food often matters more than its sugar content. Whole grains, intact pulses, and whole fruit produce far smaller glucose responses than the same carbohydrate milled or juiced.
• Fructose is handled by the liver, does not require insulin to be taken up, and at high intakes promotes fat production in the liver. In whole fruit the doses are small and arrive with fibre.
• Insulin resistance is the central metabolic problem behind type 2 diabetes, and muscle is its most important site.

What a carbohydrate is

In short: Chains of sugar units, from one unit to thousands, and the length and linkage determine everything.

Carbohydrates are built from simple sugar units. The three that matter:

Joined in pairs, they make disaccharides:

• Sucrose = glucose + fructose. Table sugar, from cane or beet. Chemically identical whatever the source.
• Lactose = glucose + galactose. Milk sugar.
• Maltose = glucose + glucose. From starch breakdown; the sugar in malt.

Longer chains make polysaccharides:

• Starch, the plant storage form, in two shapes: amylose (long straight chains, packs tightly, digests slowly) and amylopectin (highly branched, more exposed ends, digests fast). A high-amylose rice or potato produces a smaller glucose spike than a high-amylopectin one, which is a real varietal difference.
• Glycogen, the animal storage form, like a more heavily branched amylopectin, stored in liver (~100 g) and muscle (~400 g).
• Fibre, chains your enzymes cannot cut. Chapter 14.

Don't be confused: "sugar" on a label and "sugars" in nutrition science are different categories. A label's "of which sugars" counts all mono- and disaccharides, including the lactose naturally in milk and the fructose naturally in fruit. Free sugars (the WHO's category) means added sugars plus those in honey, syrups, and fruit juice, and excludes sugars inside intact fruit and in milk. The health guidance targets free sugars, which is why plain milk and whole fruit are not the problem the label numbers imply.

What the body does with it

In short: Glucose is absorbed, buffered by the liver, and driven into cells by insulin, with a whole hormonal apparatus preventing it from ever getting low.

Your bloodstream holds about 4 to 5 grams of glucose at any moment, in around 5 litres of blood at roughly 5 mmol/L (90 mg/dL). One teaspoon. A can of soft drink contains about 35 grams. Obviously it does not all arrive at once, and the reason it does not raise blood glucose sevenfold is that disposal is fast and aggressive.

Insulin, and what it actually does

Insulin is released by the beta cells of the pancreas in response to rising glucose, and to a lesser degree to amino acids and to gut hormones released just from food arriving. Its actions:

• Moves glucose into muscle and fat cells by causing GLUT4 transporters to move to the cell surface. Without insulin, muscle and fat are nearly impermeable to glucose. Brain, liver, and red blood cells are not insulin-dependent, which is why the brain keeps working in type 1 diabetes even while muscle starves.
• Tells the liver to store glucose as glycogen and to stop making new glucose.
• Stops fat breakdown. This is the underappreciated one. Insulin is the primary brake on releasing fat from fat cells, which is why it is fair to call it a storage hormone, and why the absence of insulin in untreated type 1 diabetes produces uncontrolled fat breakdown and ketoacidosis.
• Promotes protein synthesis.

The counter-regulatory hormones

The body treats low blood glucose as a far more urgent emergency than high, because the brain runs on it, so there are four hormones raising glucose and only one lowering it:

The asymmetry tells you what evolution optimised for. Chronically high glucose kills you over decades; low glucose kills you in minutes.

After a meal, hour by hour

Fasting glucose above 7.0 mmol/L (126 mg/dL) on two occasions, or HbA1c at or above 48 mmol/mol (6.5 percent), defines diabetes. The intermediate zone is prediabetes.

HbA1c deserves an explanation because it appears on every diabetes test result: glucose in blood spontaneously and irreversibly attaches to haemoglobin, at a rate proportional to concentration. Since red blood cells live about three months, the fraction of glycated haemoglobin is a running average of blood glucose over roughly 8 to 12 weeks, weighted toward recent weeks. You cannot fix it by behaving well for three days before the test.

Glycaemic index, and why it disappoints

In short: GI measures the glucose response to 50 g of available carbohydrate from a single food eaten alone, which is a useful research tool and a poor guide to meals.

Glycaemic index ranks foods by the area under the blood glucose curve over two hours, after eating a portion containing 50 g of available carbohydrate, relative to pure glucose at 100.

The list already shows the problem. Fructose has a very low GI and is not therefore a health food. Peanuts have a GI of 13 because they contain almost no carbohydrate. Ice cream has a lower GI than wholemeal bread, because of the fat.

Glycaemic load corrects for portion size:

$$\mathrm{GL} = \frac{\mathrm{GI} \times \text{grams of available carbohydrate per serving}}{100}$$

Watermelon has a high GI of about 76 and contains so little carbohydrate per slice that its glycaemic load is around 4, which is negligible. This is why "avoid high-GI fruits" is bad advice.

The deeper limitations:

• GI is measured on a single food eaten alone, fasted. Almost nobody eats that way.
• Adding fat, protein, acid, or fibre lowers the response substantially. Vinegar, notably, reduces the glucose response to a starchy meal by a measurable amount, and so does eating vegetables and protein before the starch in the same meal.
• Between-person variation is enormous. A landmark 2015 study by Zeevi and colleagues at the Weizmann Institute continuously monitored glucose in 800 people and found the same food produced wildly different responses in different individuals, with gut microbiome composition predicting a substantial part of the variation. One person spiked on bananas and not cookies; another the reverse.
• Ripeness, cooking, cooling, and processing shift GI hugely within a single food, as below.

The practical residue of GI is still worth having: prefer intact grains to flour, prefer whole fruit to juice, eat carbohydrate as part of a mixed meal, and cook pasta al dente. The number itself is not worth memorising.

Why physical form beats composition

In short: Starch locked inside intact plant cell walls digests far more slowly than the same starch milled into flour.

This is the most useful and least advertised fact in the whole subject.

Particle size. Wheat as intact kernels, as cracked grain, as coarse flour, and as fine white flour has an identical chemical composition per gram of starch and produces progressively larger glucose responses. Milling smashes the cell walls that would otherwise force enzymes to work slowly from the outside. This is why "wholemeal bread" made from finely milled wholemeal flour has a glycaemic index close to white bread: the fibre is present, the structure is gone. Stone-ground, coarse, and genuinely whole-grain products behave differently from fine wholemeal flour, and the label rarely tells you which you have.

Intact cell walls. Pulses (lentils, chickpeas, beans) have unusually tough cell walls that survive cooking, so a large share of their starch is physically shielded. This is why their glycaemic responses are so low, and why blended lentil soup produces a bigger response than whole lentils.

Resistant starch. Some starch escapes digestion entirely and reaches the colon as fibre:

RS3 is the interesting one domestically. Cooking and then cooling potatoes, rice, or pasta converts a portion of the starch into a resistant form, lowering the glycaemic response by roughly 10 to 30 percent depending on the food, and reheating does not fully undo it. Cold potato salad and next-day rice are genuinely metabolically different from the hot original. The effect is real and modest, and worth knowing rather than building a diet around.

Ripeness. A green banana is mostly resistant starch with a GI around 30; a heavily spotted one is mostly free sugars with a GI above 50. Same fruit, ten days apart.

Fructose: the one that is different

In short: Fructose is absorbed by a different transporter, does not need insulin, is metabolised almost entirely in the liver, and in large doses drives fat production there.

Glucose can be used by nearly every cell in the body. Fructose cannot: it goes to the liver, which converts it to glucose, glycogen, lactate, or fat.

The metabolic difference that matters is regulatory. Glucose metabolism has a control step (phosphofructokinase) that slows down when the cell has enough energy. Fructose metabolism enters the pathway below that control point, via fructokinase, which has no such brake. So a large fructose load is processed regardless of need, and the excess carbon is converted to fat in the liver by de novo lipogenesis.

At high habitual intakes, this contributes to:

• Non-alcoholic fatty liver disease, now the most common liver disease in the world.
• Raised blood triglycerides.
• Increased uric acid, because fructose metabolism consumes ATP rapidly and generates uric acid as a by-product, which is why sugary drinks are associated with gout.

But dose and context decide, and this is where the argument goes wrong in both directions. A medium apple contains roughly 10 g of fructose, wrapped in fibre, inside intact cells, absorbed over an hour, alongside water and potassium and polyphenols. A 500 mL bottle of soft drink contains roughly 25 to 30 g of fructose in solution, absorbed in minutes, with nothing else. Cohort studies consistently find whole fruit intake associated with lower risk of type 2 diabetes while fruit juice and sugary drinks are associated with higher risk. The molecule is the same. Everything else differs.

High-fructose corn syrup is worth a specific note because it attracts disproportionate blame. HFCS-55, used in soft drinks, is about 55 percent fructose and 42 percent glucose. Sucrose is 50 percent fructose and 50 percent glucose, and is split into exactly those in the gut. The two are close enough that no convincing metabolic difference between them has been demonstrated in controlled human studies. HFCS is a problem because it made sugar cheap and ubiquitous in the US food supply, not because it is a uniquely toxic molecule.

Insulin resistance and type 2 diabetes

In short: Cells stop responding well to insulin, the pancreas compensates by making more, and eventually cannot keep up.

Insulin resistance means a given amount of insulin produces less glucose uptake than it should. The pancreas responds by making more insulin, so blood glucose stays normal for years while insulin levels climb. That compensated phase can last a decade or more, is invisible on a standard fasting glucose test, and is where most of the damage begins. Eventually beta cells fail to keep up, glucose rises, and type 2 diabetes is diagnosed.

Where resistance happens:

• Muscle is quantitatively the most important, taking up the majority of post-meal glucose. Muscle insulin resistance appears early and is strongly improved by exercise.
• Liver resistance means the liver keeps producing glucose when it should stop, which is why fasting glucose rises.
• Fat tissue resistance means fat keeps being released into the blood, raising free fatty acids, which worsens resistance elsewhere.

What drives it, in rough order of evidence:

1. Excess fat stored in the wrong places, particularly inside liver and muscle cells and around the viscera. Ectopic fat, not fat as such: this is why some people with a normal BMI are insulin resistant and some with obesity are not.
2. Physical inactivity. Muscle contraction moves GLUT4 to the cell surface without insulin, which is why a single walk after a meal measurably lowers the glucose response and why exercise improves insulin sensitivity within days.
3. Chronic energy surplus.
4. Genetics. Substantial, with far higher risk in South Asian, East Asian, African, and Indigenous American ancestries, at lower BMIs.
5. Sleep deprivation. A few nights of short sleep measurably reduces insulin sensitivity in healthy volunteers. This is a genuinely underrated factor.
6. Some drugs, notably corticosteroids and some antipsychotics.

The two most effective interventions are weight loss and exercise, and the evidence for both is unusually strong. The DiRECT trial showed that substantial weight loss through a structured low-calorie programme put a large share of people with type 2 diabetes into remission at one year, with remission strongly related to how much weight was lost. Exercise improves insulin sensitivity even without weight loss. Neither is easy; both work.

How much carbohydrate should you eat?

In short: There is no essential dietary requirement for carbohydrate, and that fact is much less useful than it sounds.

Unlike essential fatty acids and essential amino acids, there is no essential carbohydrate: your liver can make all the glucose your brain needs from amino acids and glycerol. Very low carbohydrate diets are therefore physiologically possible, and ketogenic diets have genuine medical uses, notably in drug-resistant epilepsy, where the evidence is decades old and solid.

For general health, the honest state of the evidence:

• Both very low and very high carbohydrate intakes are associated with higher mortality in large cohort studies, with the lowest risk around 50 to 55 percent of energy from carbohydrate. The ARIC study and its associated meta-analysis is the most cited version of this U-shaped finding. Observational, with all that implies.
• What replaces the carbohydrate matters more than the reduction. In those same analyses, low-carbohydrate diets replacing carbohydrate with animal fat and protein were associated with higher mortality; those replacing it with plant fat and protein were associated with lower.
• Low-carbohydrate diets produce faster early weight loss, largely water (each gram of glycogen holds about 3 g of water), and at 12 months the difference against other diets in controlled trials is small. Adherence predicts outcome better than macronutrient ratio.
• Type of carbohydrate dominates. Whole grains, pulses, fruit, and vegetables are associated with lower disease risk across essentially every large cohort. Refined grains and sugary drinks with higher. This finding is far more consistent than anything about total quantity.

The WHO recommends free sugars below 10 percent of energy, with a conditional suggestion of below 5 percent, which is about 25 g a day for an average adult. One standard can of soft drink exceeds that.

The bottom line

• All digestible carbohydrate becomes glucose. Your blood holds about a teaspoon of it at a time, defended by one hormone that lowers it and four that raise it.
• Glycaemic index is a laboratory measure of a food eaten alone; glycaemic load and, far more, the composition of the whole meal are what matter in practice. Individual variation is large.
• The physical structure of a food matters more than its sugar content. Milling destroys the structure that slows digestion, which is why fine wholemeal flour behaves much like white.
• Fructose is metabolised in the liver without a regulatory brake, so high loads promote liver fat and raise uric acid. Whole fruit does not deliver those loads; drinks do.
• Insulin resistance is the central process behind type 2 diabetes, muscle is its most important site, and exercise and weight loss are the two interventions with the strongest evidence.

Sources and notes

Carbohydrate chemistry, insulin action, and counter-regulatory hormones follow standard endocrinology texts. Glycaemic index and load values follow the international tables compiled by Atkinson, Foster-Powell, and Brand-Miller, Diabetes Care, 2008 and later editions. Individual variation in glycaemic response is Zeevi et al., Cell, 2015, from the Weizmann Institute. Resistant starch classification follows Englyst's typology. Particle size effects on glycaemic response follow Jenkins' and Wolever's work from the 1980s onward. Fructose metabolism, de novo lipogenesis, and uric acid generation follow Lustig's and Havel's reviews, with the sugar-sweetened beverage epidemiology from Malik and Hu. HFCS versus sucrose equivalence follows controlled feeding comparisons summarised by White, American Journal of Clinical Nutrition, 2008. The U-shaped carbohydrate-mortality relationship is Seidelmann et al., The Lancet Public Health, 2018, from ARIC. Type 2 diabetes remission is the DiRECT trial, Lean et al., The Lancet, 2018. WHO free sugars guidance is the 2015 guideline.

Open questions. Whether fructose is metabolically harmful at intakes achievable from whole food, rather than from beverages, is not settled. The U-shaped mortality curve for carbohydrate intake is observational and its shape depends heavily on what replaced the carbohydrate, which the studies handle inconsistently.

👉 Next: protein, the nutrient with the clearest practical case and the noisiest supplement industry.