Why weight training raises your blood sugar with type 1 diabetes. And why cardio drops it.

One note before the detail. This is how I think about it as a coach who has type 1. It isn't medical advice. Your diabetes team stays in charge of your insulin, so agree any change to doses with them.

Why cardio drops your blood sugar

During steady aerobic work like running, cycling, swimming or rowing, your muscles need a constant supply of glucose.

Inside your muscle cells are proteins called GLUT4 transporters. Think of them as glucose doors. At rest, your body needs insulin to open them.

Exercise opens them a second way. When a muscle is working hard and burning energy, a fuel sensor called AMPK switches on and sends those doors to the surface of the cell. No insulin involved.

So during cardio you have two things pulling glucose out of your blood at once: the insulin from your last dose, and the pathway the exercise itself switched on. That's why glucose can drop faster in cardio than almost anywhere else, even when your dose looked right on paper.

What to do about it

• Plan before you move. That exercise pathway stays on for the whole session, so you can't react fast enough once you're in it.
• Start from a stable number. If you're low or falling, treat that first and wait for a steady trend.
• Feed the session as you go. On longer sessions, small amounts of fast-acting carbs at regular intervals work better than one big hit beforehand. A big hit can spike you early, prompt a correction, and then crash you mid-session.
• Act on the trend, not the feeling. By the time you feel a drop, your CGM is already several minutes behind your blood.

Why lifting weights raises it

Heavy resistance training is a real stress on the body. Your nervous system responds by releasing adrenaline and noradrenaline.

Those hormones travel to your liver and tell it to release glucose. It does that two ways. It breaks down the glycogen it has stored, and it makes new glucose from other materials. Both run at the same time, so blood glucose rises quickly.

In someone without type 1, the pancreas sees the rise and matches it with insulin straight away. For you and me, that automatic correction isn't there. So the number climbs, most of all on days with squats, deadlifts and heavy pressing.

Why the crash comes after the session

The rise is temporary. When the session ends, those stress hormones clear and glucose comes back down, often faster than you'd expect.

That's because your muscles have just emptied their glycogen stores and are pulling glucose in hard to refill them. You're far more sensitive to insulin after training.

If you took a full correction mid-session, it lands in that newly sensitive body. That's where the post-training crash comes from, and it's almost entirely preventable.

Five ways to manage a weights session

1. Don't rush to correct a moderate rise. It usually comes down on its own once the session ends. In the video I go through the thresholds I use for when to wait and when a partial correction makes sense. Agree your own with your diabetes team.
2. Use your warm-up on purpose. Five to ten minutes of easy bike, walking or rowing switches on that exercise pathway before the heavy work starts. It also gives you a baseline reading and a trend arrow.
3. Breathe between heavy sets. In through your nose for four counts, then a long, slow breath out through pursed lips. Four to eight rounds helps bring your heart rate and stress hormones down.
4. Rest longer on heavy days. Two to three minutes between sets gives those hormones time to clear. Short rests keep them high for the whole session.
5. Look for patterns. If you see the same spike on heavy days for two or three weeks, that's something concrete to take to your diabetes team.

Start by collecting your own numbers

Don't change anything yet. This week, just write things down.

• After your next cardio session: how long you went, your starting glucose, and your glucose every 20 to 25 minutes.
• After your next weights session: your starting glucose, your glucose at halfway, and your glucose 30 minutes after you finish.

You can't build a reliable approach without knowing your own data first. Everyone's response is a little different.