Per
slice!!!
You'd definitely need to test for spikes if you're going to try it as it's certainly
not low in carbs at all - most bread, white or brown, has around 40g carbs per 100g weight, and this is no different at 41.8g carbs. Burgen in comparison is nearly half this - at 25g carbs per 100g, so a better choice.
If you want the benefits of chia seeds, then I think you'd be much better off buying them separately and using them as sprinkes or as ingredients in suitable lower carb recipes like chia pudding or muffins.
Robbity