For years I had a cough without apparent reason (no infection). I then noticed if I ate liver the cough completely disappeared within 24 hours. Further investigation showed that I just has a Vitamin A deficiency and the problem was resolved eating liver once a week or taking a Vitamin A supplement. I have found that Vitamin A retinol works much better the carrots (carotin) but be aware not to have too much retinol. I have tested this by stopping the Vitamin A and the cough returns. This is worth a try for anyone having a chronic cough or any mucus membrane problems.