A large new study in BMC Medicine has started to spell it out.
Scientists at UCL and Imperial College London found two clear chemical patterns in the blood, one tied to an ultra-processed diet and one to a minimally processed diet.
The patterns also tracked people's heart and diabetes risk over the next 20 years.
The ultra-processed blood signature explained roughly half of the link to later heart disease and close to half of the link to type 2 diabetes.
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Ultra-processed food is a broad category. It covers industrially made products with ingredients you would not find in a home kitchen. Examples include soft drinks, packaged snacks, sweetened cereals, instant noodles, ready meals and reconstituted meats.
What the blood pattern actually looks like
The team measured 233 small molecules in the blood of 6,010 British adults in the Whitehall II study, a long-running UK health survey. A machine-learning tool picked out 34 molecules that lined up with heavy ultra-processed eating.
Another 50 lined up with a minimally processed diet. Eight molecules showed up on both lists.
The ultra-processed pattern looked unhealthy on familiar measures. Blood levels of omega-3 fats were lower. HDL cholesterol, the "good" cholesterol, was lower too.
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Triglycerides carried by very-low-density lipoprotein, a type of fat linked to clogged arteries, were higher. The minimally processed blood pattern pointed in the opposite direction.
How much of the heart and diabetes risk showed up in the blood
Here is where the new work adds something. The researchers tested how much of the link between an ultra-processed diet and later disease was carried by the blood signature itself.
The blood pattern explained about 51 percent of the connection to cardiovascular disease over the 20-year follow-up. It explained about 43 percent of the connection to type 2 diabetes.
Put differently, roughly half of the diet-to-disease link seems to run through changes a doctor could already see in a standard blood test.
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People who ate the most ultra-processed food had a higher risk of heart disease over the two decades. Those who ate the most minimally processed food had lower heart risk and sharply lower diabetes risk.
The authors wrote that processing itself may be part of the reason, though more research is needed before anyone can call it cause and effect.
What the study cannot tell you
The design is observational. It can show that a pattern of eating travels with a pattern of blood changes and with a pattern of disease. It cannot prove one caused the other.
The blood signature did not catch everything either. The authors flag gut microbes, low-grade inflammation and signals that affect appetite as other possible pathways that were not measured in this study.
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That fits with earlier research showing that two meals with identical calories can act very differently in the body when one is ultra-processed and the other is not.
The practical advice has not changed. The American Heart Association advised in August 2025 that most ultra-processed foods pile up saturated fat, added sugar and sodium.
The group recommended replacing them where possible with vegetables, fruit, whole grains, legumes, nuts, seeds and lean proteins.
About 55 percent of the calories American adults eat currently come from ultra-processed products, and the share among children and teenagers is higher still.
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This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.