Does giving blood make you lose weight?

Chris Smith put John's question to Cambridge University cardiologist, James Rudd...

James - Well it’s a great question. Thank you very much John, and it’s fantastic that you're donating blood. The NHS is always in need of blood donations at any time of the year.

To take a donation of blood the nurses will usually remove about a pint, so about 500ml of blood. So, as you say, your weight is going to go down instantaneously by around half a kilogram. But, if you have donated blood in the past, you will know that you’re given a nice large cup of tea, plenty of biscuits. So I think by the time you leave the blood donation centre about an hour afterwards, most of the weight will have been put on in the form of tea and biscuits.

The follow-up part of you question about protein and lipids is theoretically interesting but you must remember that most of our body weight is actually due to water. So I think by the time you’re rehydrated, perhaps the next morning, you’re going to be back at the starting weight. So great idea in theory but probably hasn’t got legs, I don’t think.

Chris - So your average blood samples has got 45 percent cells, 55 percent is water. So at least a couple of hundred grams of the blood donation is just water, like you say, and that’s your cup of tea bringing it back in. The biscuits got 60/70 calories; there’s quite a lot of energy in a biscuit, isn’t there? So you’ve only got to make up just a half a kilo of cells. It’s not very much. No it’s a quarter of a kilo of cells, it’s going to grow those back quite quickly so I reckon that's a no no. Go for a jog instead for the weight loss, yeah?

James - Absolutely. Go for a jog or do something else but, by all means, don't stop donating blood.