What does your fasting blood sugar tell you about your risk of a heart attack?
A fasting blood sugar in the prediabetes range is linked to a slightly to moderately raised risk of a heart attack, and that risk increases as the value rises. You can actively lower that risk through more exercise and healthier eating.
A fasting blood sugar in the normal range, roughly below 5.6 mmol/L, is not linked to a higher risk of a heart attack. That changes as the value rises. At 5.6 to 6.1 mmol/L, the risk of a heart attack is around 11% higher than in someone with a normal blood sugar. At 6.1 to 7.0 mmol/L, it climbs to around 17%. People with diabetes have, on average, twice the chance of a heart attack, even when other factors such as blood pressure and smoking are taken into account.
Even so, fasting blood sugar only tells part of the story. If your doctor already knows your blood pressure, weight and smoking status, fasting blood sugar adds little extra predictive value for people without known diabetes. In other words, a slightly raised fasting value on its own does not shift the risk estimate much when the other factors are already in the picture.
In people who already have heart or vascular disease, prediabetes carries more weight. There, the extra risk of a new heart event or death is not 11 to 17% but around 36 to 37%. Detecting a raised blood sugar early is therefore especially worthwhile in that group.
Good news: with prediabetes, there are things you can do yourself. Getting at least 150 minutes of exercise a week and eating fewer calories substantially lowers the chance of progressing to full diabetes. That works better than medication. The drug metformin is a second option, and works best in people under 60 who are overweight, or in people who have previously had gestational diabetes. Talk to your doctor about what makes most sense in your situation.
This answer draws on several large summaries of the available evidence, the largest of which covers data from more than 10 million people. The findings on prediabetes and heart and vascular risk are consistently seen across those summaries, but they always reflect associations, not proven cause-and-effect relationships. The researchers behind the most comprehensive overview (95 analyses combined) rated the certainty of the evidence for the link between prediabetes and heart and vascular disease as 'moderate', meaning we can be reasonably confident but not confident enough to call the relationship definitively causal. For depressive symptoms and cognitive decline in prediabetes, the certainty was low to very low.