Left foot turns to the left as i strike the ball?

For some reason i do this without thinking, does it cause any loss in distance or any other negatives?

If you look at the five simple keys (5SK), one of the keys is to get your weight forward on the downswing.  This is something that all pga pros do.  http://thesandtrap.com/t/55426/introducing-five-simple-keys

Not sure if that is an issue for you without seeing your swing, but given the movement in your left foot it makes me curious about Key #2 and your swing.

Some posts here talk about leaving the left foot a little open anyway.  Maybe just a little bit will relieve whatever pressure you are feeling on follow through that's making you do that.

or

stretching (lower back especially so the tension and twist is where it should be rather than delicate and terrible ala tear- ible joints)

or

just don't do that

(Even with decent pressure transfer through the swing, I do it a little bit sometimes - it doesn't seem to hurt.  but how would I know?  - I've had a few knee surgeries, I'm ok with minimizing knee twisting)

Works ok for Patrick Reed

Yes i already have the foot turned 1/4 like hogan says at address, but it turns to 3/4 just as i strike the ball

Most players who "Pivot the left foot during the swing" will have their weight on their heal with their left leg straightened.

Momentum of the rotation of the body during the swing with the hips rotating towards the target is normal.

People with bad hips or a bad knee make this movement for less joint stress and protect the joints from injury.

Other people who pivot on the heal, generally have acquired this habit and made this swing motion from early stages of their golf swing.

Other issues with balance during the swing will cause this style of swing.

Some people can play and strike the ball well, while others will struggle.

Club Rat

I'll take a look, it  may be because my clubs are too small, as when i try it with my driver (which is longer than normal) it doesn't happen plus when i move them out farther t doesn't happen though i have to bend my back and knees more to do this