Does milk sugar raise liver fat like table sugar?
Yes. In this crossover trial, peak liver fat production doubled after a lactose drink compared with a glucose-based drink, 22% versus 11%, and the lactose result was statistically indistinguishable from table sugar at 23%.
Lactose is the sugar in milk. For a long time it was grouped with the “safe” carbohydrates, the ones built out of glucose, rather than with fructose. Fructose is the half of table sugar that has the worst reputation, because it drives the liver to build new fat. The authors state plainly that the effects of galactose-containing sugars like lactose on this process were simply not known.
How the liver turns sugar into fat
The liver can make fat out of carbohydrate. Scientists call this de novo lipogenesis, which just means “building fat from scratch.” Some carbohydrates push the liver to do this harder than others. Glucose gets used up all over the body, so relatively little reaches the liver’s fat-building machinery. Fructose goes almost straight to the liver and gets converted.
Lactose is not made of two glucose units. It is glucose plus galactose. Galactose, like fructose, is handled mainly by the liver. That is the reason this study was worth running, and the reason the result is not a surprise once you look at the chemistry.
What the data show
Twenty-four adults without obesity, 12 men and 12 women, each completed three separate lab visits in random order, with an average washout of about a month between visits (33 plus or minus 14 days). At each visit they drank a beverage containing 50 g of fat plus 100 g of carbohydrate. The carbohydrate was either maltodextrin (a glucose polymer, the control), lactose (the milk sugar being tested), or sucrose (table sugar, included as the active comparator).
Liver fat production was not estimated from a blood marker. It was measured directly using the deuterated water method, and labelled palmitate was added to the drink so the researchers could track where the swallowed fat actually ended up.
After maltodextrin, peak liver fat production was 11%, plus or minus 3%. After lactose it rose to 22%, plus or minus 4%. That difference had less than a 1 in 1,000 chance of being a fluke (p<0.001). After sucrose it was 23%, plus or minus 4%, and the gap between lactose and sucrose was well within the range of ordinary noise (p=0.54).
Blood triglycerides followed the same pattern. The triglyceride response after maltodextrin was 0.85 mmol per litre over 360 minutes. After lactose it climbed to 1.63, again with less than a 1 in 1,000 chance of being coincidence. After sucrose it was 1.50, and lactose and sucrose were statistically indistinguishable there too (p=0.41). Tracing the labelled fat showed that more of the swallowed fat ended up in circulating triglyceride particles after lactose than after maltodextrin.
Dr. Kumar’s Take
The honest headline is chemistry, not dairy. Galactose behaves more like fructose than like glucose inside the liver, and this trial is the first to show it with direct measurement rather than inference.
The dose matters enormously. Participants drank 100 g of lactose in a single sitting, far more than anyone gets from a normal serving of milk or yogurt, and they drank it alongside 50 g of fat with no protein-rich food, no fiber, and no meal structure around it. That is a metabolic stress test, not breakfast. This trial tells you what the pathway does when you push it hard. It does not tell you that a glass of milk raises your liver fat.
What it does dismantle is the lazy shortcut that only fructose deserves attention. Sugar guidelines are written almost entirely around fructose-containing sugars. If galactose does the same thing to the liver, then the mental category of “sugars that matter” was drawn in the wrong place. That is worth knowing before the next round of dietary advice gets written.
How strong is the evidence?
The design is the strongest part. A randomised crossover means every person served as their own control, which removes the genetic and lifestyle differences that muddy most nutrition research. Three visits, randomised order, a month of washout between them, and a direct tracer measurement of the outcome rather than a proxy.
The limits are equally clear. Twenty-four healthy adults without obesity is a small, fit sample, and the people most worried about liver fat are usually neither. Each visit measured a single day’s response to a single drink, so nothing here speaks to what happens over months or years of ordinary eating. And an isolated sugar in water is not the same food as milk, which arrives with protein, fat, and calcium attached.
Practical takeaways
- Treat large single doses of any concentrated sugar, including lactose powder, as a real metabolic load rather than assuming milk sugar is a free pass.
- Check the label on protein powders, meal-replacement drinks, and “milk sugar sweetened” products, where lactose can appear in amounts far beyond what whole dairy delivers.
- Nothing in this trial argues against ordinary milk, yogurt, or cheese, where lactose comes in small amounts packaged with protein and fat.
- If you are tracking liver fat, the useful question is total concentrated sugar in a sitting, not which specific sugar it happens to be.
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Frequently Asked Questions
Should I stop drinking milk because of this study?
No. The trial gave people 100 g of pure lactose dissolved in a fat-containing drink, which is not how anyone consumes dairy. Milk delivers its lactose slowly, in small amounts, alongside protein and fat that change how fast the sugar reaches the liver. This study was designed to answer a mechanism question about galactose, not to evaluate dairy foods as they are actually eaten.
Does lactose-free milk avoid this problem?
The trial did not test lactose-free milk, so there is no direct answer here. Worth understanding, though, is that lactose-free milk usually contains the same sugars in split form: the lactose has been broken into glucose and galactose by an added enzyme. The galactose is still there. Since galactose is the part of lactose that the liver converts, removing the lactose molecule while keeping its two halves would not obviously change the pathway this study measured.
Is the liver making fat always a bad thing?
No, it is a normal process that happens in everyone, every day. The concern is chronic overactivity, which is one of the routes to fat building up inside liver cells. The number reported here, 11% rising to 22%, is the peak share of newly made fat during a single test, not a measure of how much fat the liver is storing. A one-day spike after a large sugar drink is a signal about the pathway, not a diagnosis.
Bottom Line
Milk sugar does not behave like the harmless carbohydrate it was assumed to be. Given as a large single dose alongside fat, lactose doubled peak liver fat production compared with a glucose-based carbohydrate, 22% versus 11%, and produced a result indistinguishable from table sugar at 23%. Blood triglycerides followed the same pattern. The dose was far larger than a normal serving of dairy, so this is a statement about how galactose behaves in the liver rather than a warning about milk. It does mean the long-standing assumption that only fructose-containing sugars deserve scrutiny was built on a gap in the evidence, and that gap has now been filled.
