Full mechanism, in text

A liver enzyme with no brake

1 · two entries

Glucose load is shared by tissues across the body, while the liver takes up most fructose at the first pass, so the fructose load falls mainly on the liver.Glucose and fructose share the same formula (C₆H₁₂O₆), but in the body they take two different roads.

Glucose:

· Absorbed in the small intestine via SGLT1 / GLUT2 → portal vein → the liver takes a share, the rest enters the general circulation
· (controlled by insulin) → muscle and fat
· GLUT3 → brain (insulin-independent)
· GLUT1 → basic supply for tissues throughout the body
· The load is shared by tissues across the body

Fructose:

· Absorbed in the small intestine via GLUT5, a channel of limited capacity
· The small intestine has KHK of its own: in isotope-tracing experiments in mice (Jang 2018), most fructose was handled in the small intestine at low doses; at higher doses (above about 1 g/kg) the intestine could not keep up, and the excess flowed on to the liver and to colonic bacteria. The proportions in people have not been measured well
· Fructose that reaches the blood goes through the portal vein to the liver first, and the liver takes up most of it at that first pass, so muscle and fat tissue get very little

This difference drives everything that follows: the metabolic load of glucose is shared across the body, while the load of fructose falls mainly on the liver.

So what matters is how much arrives at once, and how fast:

· Whole fruit: a modest amount of fructose, wrapped in fibre and cell structure, absorbed slowly, with the small intestine taking some first
· A glass of juice: finished in minutes, the fructose arrives all at once
· A bottle of sugary soda: likewise one large dose in one go (half of sucrose is fructose)

2 · unregulated KHK

In the liver, glucose passes a gate at PFK1 that slows it when the cell has enough energy, but KHK takes fructose around it and phosphorylates however much arrives.Both sugars get a phosphate attached as their first step inside liver cells, but the regulation is completely different.

Glucose: there is a gate on the road, at PFK1

· Glucose + → G6P (hexokinase; glucokinase in the liver) → F6P
· F6P + ATP → F1,6BP, catalysed by PFK1 (phosphofructokinase 1): plenty of ATP and citrate slows PFK1, plenty of AMP / ADP speeds it up
· Then glycolysis → → ATP

· When the cell has enough energy, PFK1 slows down and more of the extra glucose is stored as glycogen
· This is the automatic throttle of glucose metabolism

Fructose: KHK takes over and goes around the gate

· Fructose + ATP → F1P, catalysed by KHK (ketohexokinase): KHK is not inhibited by ATP and binds fructose tightly, so it phosphorylates whatever arrives and ATP is used up fast
· F1P → DHAP + glyceraldehyde (aldolase B), joining glycolysis midway, just past PFK1
· Then pyruvate → acetyl-CoA → de novo lipogenesis (DNL)

· KHK has no feedback throttle: the more fructose arrives at once, the more the liver has to take all of it
· After a large sugary drink, ATP in liver cells is drawn down for a while
· The carbon skips the PFK1 control point, so it is more easily sent on to make fat

That is the chemical link between large amounts of liquid fructose and liver fat: not every sugar is the same in the body; fructose is handled mainly by the liver, and without a brake.

3 · downstream

F1P build-up and use lead to de novo lipogenesis (DNL) in the liver and to uric acid, while effects on blood pressure and satiety are not settled.F1P build-up and use have several downstream consequences:

① De novo lipogenesis (DNL)

· F1P → DHAP → glycerol-3-phosphate →
· F1P → DHAP → pyruvate → acetyl-CoA → fatty acid (palmitate)
· Triglyceride kept in the liver is part of metabolic dysfunction-associated steatotic liver disease (, formerly NAFLD); packed into VLDL and sent into the blood, it raises blood lipids

Human evidence: Stanhope 2009 (JCI, randomized trial)

· Overweight or obese adults aged 40–72 drank fructose- or glucose-sweetened drinks supplying 25% of their daily energy needs for 10 weeks; 32 completed
· Weight gain was similar in both groups, but visceral fat rose only in the fructose group; the fructose group had higher liver DNL, post-meal triglycerides, fasting apolipoprotein B and , and lower insulin sensitivity
· One result ran the other way: fasting triglycerides rose by about 10% in the glucose group
· How to read it: a few dozen people, 10 weeks, a very high dose. It shows that large amounts of fructose and the same amount of glucose go to different places in the body; how much that matters for disease needs larger and longer studies

② Uric acid

· ATP drops to ADP and then to AMP; AMP → IMP → hypoxanthine → xanthine → uric acid
· After an acute fructose load, blood uric acid rises soon after, by an amount that varies with dose and population
· Choi 2008 (BMJ, a cohort of US male health professionals): compared with less than 1 serving a month, 1 sugary soda a day came with about 1.45 times the gout risk, and 2 or more a day about 1.85 times
· Choi 2010 (JAMA, a cohort of US female nurses): 1 sugary soda a day about 1.74 times, 1 orange juice a day about 1.41 times
· These are associations seen in cohort studies and cannot prove cause on their own; but the chemistry and the population data point the same way

③ Blood pressure and blood vessels

· High uric acid often travels with poorer endothelial function and high blood pressure; whether it is a cause is still debated

④ Fullness signals

· Fructose barely stimulates insulin release; some researchers think it also sends a weaker fullness signal to the brain, but this is not settled in people

Where it can lead: MASLD can progress step by step to MASH and cirrhosis; there is also metabolic syndrome, gout, and the risk of type 2 diabetes when the liver and pancreas carry too much fat.

4 · whole fruit vs HFCS

Whole fruit brings fructose to the liver slowly, while a sugary drink delivers a large dose within minutes; the difference is how much arrives at once and how fast.It is the same fructose, yet whole fruit is a small matter and sugary drinks are the problem: the difference is how much arrives at once, and how fast.

① Fibre and the structure of whole food

· Whole fruit: the sugar is held inside cells, together with fibre, needs chewing, and is released slowly
· Juice: the fibre is mostly gone, and the sugar arrives as soon as you drink it

② How fast it arrives

· Whole fruit: chewing, stomach emptying and intestinal absorption bring fructose to the liver slowly, and the small intestine takes some first (mouse experiments)
· Liquid fructose: a large dose in minutes, more than the small intestine can handle, flowing on to the liver

③ A built-in ceiling

· Whole fruit: a few pieces and you are full (fibre and volume)
· Juice: one glass takes several fruits to press, yet hardly fills the stomach

Population data (Muraki 2013, three US cohorts, observational): people who ate more whole fruit had a lower risk of type 2 diabetes (most clearly for blueberries, grapes and apples); people who drank more fruit juice had a higher risk; swapping juice for whole fruit went with lower risk.

Natural sugar is not healthy sugar:

· WHO counts the sugars in honey, syrups and fruit juice as free sugars, the same as sugar added to food; sugars that are naturally part of whole fruit, vegetables and milk are not counted
· Honey is about 38% fructose, not very different in the body from sucrose (half fructose); agave syrup and coconut sugar are added sugars too
· High-fructose corn syrup () is often singled out as the villain, but the Tappy 2010 review notes there is no direct evidence that it is worse than sucrose; what matters is the total amount of added sugar

Hall 2019 does not speak to fructose: in this inpatient randomized trial the ultra-processed diet led people to eat about 500 kcal more a day, but the two diets were matched for sugar, so the result cannot be put down to fructose.

WHO's sugar advice: free sugars below 10% of daily energy (strong recommendation), and below 5% is better (conditional recommendation).

In practice:

· First swap sugary drinks and juice for water, tea, coffee or sparkling water
· Then watch for added sugar hidden in breakfast cereal, flavoured yogurt and sauces
· Eat whole fruit as usual, just not a big pile at once

Full mechanism, in text

1 · two entries

2 · unregulated KHK

3 · downstream

4 · whole fruit vs HFCS

KHK pathway · the chemistry of why fructose differs

Full mechanism, in text

1 · two entries

2 · unregulated KHK

3 · downstream

4 · whole fruit vs HFCS

KHK pathway · the chemistry of why fructose differs