Kribou:
For most (if not all) T1's, Levemir is certainly not a once-daily insulin. The emc website has some information which indicates once daily in conjunction with oral agents (i.e. for T2's) or once or twice daily as part of a basal-bolus regime
Levemir Cartridge 100 U/ml, Levemir Pre-filled Pen 100 U/ml , SPC from the eMC
If you look at the action curve (which I have attached) you can see that at at low or moderate doses, it is not even a 12 hour insulin. I tried a switch from Lantus (again twice daily) to Levemir and found that I couldn't even get the Levemir to last for 12 hours. Of course everyone seems to react differently and some T1's love Levemir.
My (entirely personal) take on the Lantus/weight gain thing is that it is a bit of a red herring. The "problem" with Lantus is that it is
very very easy to take too much - you then end up having to eat more to keep up with your insulin. Worse, as you increase your dose, you start to become less insulin sensitive and you then need even more - this leads to a vicious circle.
If you have your basals set properly you should not be covering ANY of your food with the Lantus, that's the job of your bolus. Getting a correct basal dose requires work, time and a lot of testing but it is worthwhile and if you are serious about better numbers I would recommend you do this regardless of whether you use Levemir or Lantus. IMHO correct basals are the KEY to good control.
A good description of how to do this can be found in two books (both available through Amazon.co.uk):
Think Like a Pancreas by Gary Scheiner
Using Insulin by John Walsh
Alternatively, you could talk to your Doctor about going onto a DAFNE course.
Joel