Eat a small snack containing 15 to 30 grams of carbohydrates, such as fruit juice, fruit, crackers or even glucose tablets before you begin your workout. 100 to 250 mg/dL (5.6 to 13.9 mmol/L). You're good to go. For most people, this is a safe pre-exercise blood sugar range. 250 mg/dL (13.9 mmol/L) or higher. This is a caution zone â Your blood sugar may be too high to exercise safely. Before exercising, test your urine for ketones â substances made when your body breaks down fat for energy.