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Hello, my physician changed my basal insulin to Lantus insulin 4 months ago. The meals are covered by Humalog. Do I have to adapt the Lantus dosage if I play tennis between 4:00 and 6:00 p.m.?

Lantus (glargine) is a very long acting insulin analog (>24 h.) Experience about dosage adaptation is scarce and very recent. Nevertheless, for an exercise session not exceeding 2 to 3 hours I would not counsel to decrease the Lantus dose.
If we come back to your tennis example (4:00 to 6:00pm)
1) I would drink 20 g carbs before exercising if the blood glucose is <10.0 mmol/l (x 18 = 180 md/dl) or ingest these 20 g after 1/2 h exercise if the BG is above 10.0 mmol/l. Check for ketones if >15.0 and treat hyperglycaemia and don't exercise if positive.
2) Ingest 20 more g carbs after 1 hour playing.
3) Decrease the dinner insulin (Humalog) by 30%.
4) Eat 15 to 20 g carbs at bedtime independently of BG.
5) Check BG at 2:00 am.
6) Try to improve these advices and adapt them to your personal situation by playing at the same time in similar conditions with many BG measurments.
Consider to decrease the Lantus dose if you exercise several consecutive days