Can you gain muscle if your loosing weight ?

Possible, yes. There are exceptions. Likely, probably not for most of the general population. Exchange fat weight for muscle mass (recomp) is a LONG PROCESS. And in reality if one is gaining muscle, then weight should go up since adding muscle means adding mass. And adding mass usually means having a surplus.

A.C.E. Certified Personal and Group Fitness Trainer
IDEA Fitness member
Kickboxing Certified Instructor
Been in fitness for 30 years and have studied kinesiology and nutrition

my personal experience. is a kind of yes, based on my own results.

at the moment i have lost 18lbs in total weight since January.

on Jan 1st 2017 i was 277lbs

26% body fat. which is 72.02 Lbs of Fatty Mcfattyness.

As of today 02/03/2017 i am 259 Lbs

21% body fat. which is 54lbs

so i have lost, in total 22lbs of fat. but as above ive only lost 18lbs off my total mass.

Now , i also get my lean totals. as part of the read out when i do a weigh in.

so my lean mass, not including water weight

on Jan 1st 201lbs (72% of total mass)

As of now 02/03/2017 204lbs (77% of total mass)

now the astute among you will see their is a 2% discrepancy between my lean masses and fatty masses. im assuming this is either water, or something. but its consistent.

My understanding on the science is not really top level, but i know a bit about what it takes to build muscle. there are something like 19 different protein components and your body can produce 8 of them and you get the others from food or supplements. I.e. BCAA (branch chain amino acids.)

Now if your body is using pre-exisiting fat stores to convert to glycogen as energy. Then the proteins you consume cna be utilised by the body for building muscle.

The deficit everyone aims for is not a true deficit. its more like a deficit for the future. or a surplus from the past.

Let me explain:

if your goal is to eat 2500 calories to maintain your weight. if you eat 2000 on monday, and 3000 on tuesday. then youre not in a deficit. at all.
similarly, if you eat 2000 on monday and 2000 on tuesday, your body will seek the other energy from your fat stores. so its not a true deficit its just the excess calories you ate once upon a time. so if you add extra resistance training to your life. and say your body tries to use 500kcal extra for the exercise. it will use the protein and food you eat to build the muscles. and it will take extra fat from the pre-existing surplus to produce the glycogen hence why having more muscle, burns more fat. i mean im not an expert but that makes sense to me and the numbers from my own experience make sense.

i feel ive ranted. hopefully it makes sense.