John Campbell, of Christchurch, asks :-

Why is human urine sometimes yellow and sometimes colourless?

Rob Walker, a kidney specialist at Otago University's School of Medicine, responded.

The colour of the urine reflects what you have eating and the consequences of metabolism. It is also affected by how much fluid an individual has consumed.

If you have drunk a lot of water, then your urine tends to be dilute and usually clear. If you have only consumed a small amount of fluid, then your kidneys conserve water and produce a concentrated urine which is more likely to be yellow in colour. The yellow colour is due to chromogens or pigments which are breakdown products from your metabolism. This is best illustrated by looking at the first morning urine which usually is very yellow. This is because you have produced a concentrated urine overnight. You have had little or no water over the previous 8 hours, so your kidneys have actively retained the amount of water your body requires, producing a concentrated urine.

Later in the day as you have drunk a lot of fluid, the colour of the urine tends to become more clear. However this will depend upon just how much water you have drunk.

The colour of these products can also be affected by how acidic the urine is. For example, most people eating a normal diet containing animal protein produce an acidic urine. Strict vegetarians produce an alkaline urine.

Sometimes the pigments that are in the urine can act as an indicator of the urine pH (acid or alkaline) and therefore change colour accordingly. For example - If you were to consume a large quantity of beetroot, this could colour your urine, as the pigment from the beetroot can act as a pH indicator.

The yellow colour of the urine does not reflect any potential problems with your kidney function. The only colour change that would be important to note and report to your doctor is, if there is any blood in the urine. This turns the urine a pink-red colour through to a dark almost coca-cola colour.