Fats & metabolism: the late spike – and why quality matters

Fat is the macronutrient with the smallest immediate effect on blood sugar: unlike carbohydrates, it delivers no direct glucose. Yet its effect on metabolism is anything but neutral.

Why fat changes the curve

Fat slows gastric emptying. As a result, glucose from the same meal is absorbed more slowly: the blood-sugar curve becomes flatter, the rise starts later and drags out longer.¹ With carbohydrate-rich meals, this can soften the spike.² For people on insulin therapy, this later peak (the so-called “pizza effect”) is worth planning for.

Fig. 1: More fat with the same carbohydrates leads to a flatter, but later and more drawn-out, blood-sugar curve.

Over the day: too much fat can dampen insulin action

Larger amounts of fat across the day can temporarily worsen insulin sensitivity – via raised free fatty acids in the blood.¹ So what matters isn’t only the amount, but above all the type of fat.

Fat quality is decisive

A large meta-analysis of randomised nutrition trials (102 studies, over 4,200 adults) showed: unsaturated fats (especially polyunsaturated, PUFA) in place of carbohydrates or saturated fats improved HbA1c and insulin resistance (HOMA-IR).³ Saturated fat, by contrast, tends to promote insulin resistance.³ Good sources of unsaturated fats are olive oil, nuts, avocado and oily fish.

Key takeaways

Fat barely raises blood sugar directly but slows absorption.
Result: a flatter, but later and longer, blood-sugar curve.
The “pizza effect” matters with insulin therapy.
Lots of fat across the day can temporarily dampen insulin action.
Unsaturated fats are more favourable than saturated.

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Sources

Sources as of: 21 Jul 2026.

Fat slows gastric emptying and flattens the postprandial curve; free fatty acids and insulin sensitivity. Review. pmc.ncbi.nlm.nih.gov (opens in a new window)
Fat softens the glucose spike of carbohydrate-rich meals. Study. pubmed.ncbi.nlm.nih.gov (opens in a new window)
102 RCTs: unsaturated (PUFA) fats vs. carbs/saturated improve HbA1c and HOMA-IR. Meta-analysis, PLOS Medicine. journals.plos.org (opens in a new window)
Meta-analysis (30 RCTs, 2023): replacing SFA with MUFA/PUFA did not significantly change insulin sensitivity in the short term. Am J Clin Nutr. ajcn.nutrition.org (opens in a new window)