ONE of my colleagues shouted at me: "There's folic acid in bread". "What about chlorine being put in the water? Are you going to take that out?" hollered another.

"You don't have to eat bread; and chlorine is there to make the water safe," I replied.

All friendly banter, this took place on Monday in the melee at one end of the Commons chamber where we move either into the "Aye" or the "No" lobby to vote.

Normally, the only thing I have to check when I get to the Commons to vote is "which side we are on". The Whips will tell me. I go into the correct lobby, give my name to the clerk who checks me off the register, get counted by the tellers, and that's it. There's nothing cynical or unthinking about this.

Governments, of any party and in any nation, can only operate if their members accept a collective responsibility. That, of course, is easy if you happen to agree anyway with the proposition; more testing -- but more important -- if you happen to have had reservations or a different point of view.

The deal is that you work out the issues and the arguments internally, and then are loyal to the conclusions, and to your colleagues.

Without it, parties in government would find it hard to impossible to stick to their contract with their electorate to deliver on what they said they would.

But there are some issues on which we all agree that there should be no instructions how to vote, since the issues raised are ones of conscience, which transcend party, like hanging and abortion, and -- on Monday -- whether we should or should not allow fluoride to be added to our water supplies.

So with another member of the Cabinet, Hilary Benn, the International Development Secretary, I found myself in the "Aye" lobby along with 179 other MPs of all parties to vote for a complete ban on compulsory fluoridation. We lost by 103 votes. When that failed I voted for another amendment, to give the power to decide to local authorities.

We lost that by 43 votes, so the status quo -- by which the decision is taken by the health authorities, continues.

It's a pity that there's not more proper reporting of Parliament, because it was a good debate, with much passion. Those in favour of fluoridation argue that it is a substance found naturally in many water supplies, and that it has been shown conclusively to improve dental health, especially of children.

It's a substantial argument, which needs to be taken seriously. But I and colleagues of a like mind take the view that there's a higher argument, which is that it is just plain wrong compulsorily to medicate people through the water supply. It's true that all sorts of trace elements are found in our water; my supply in London is full of calcium and furs up the kettle; in Blackburn it's soft, and tastier.

It's also true -- as one of my pals was reminding me -- that chlorine is added to supplies. But this is to help purify the supply; and without the chlorine, or similar chemicals, there could be too many bugs in our supply. But with fluoride there's a choice.

It doesn't have to be put in the water.

I accept that dental health is not good enough and that fluoride may help. But fluoride can be taken in other ways - not least in toothpaste, where the individual (or the parent) makes the choice for themselves, but not for everyone else.

So the Commons made its decision. I didn't agree with it, but it was a free vote, supported by MPs of our parties.

However, I remain very uneasy about the idea; and I also think that if we want seriously to tackle tooth decay we have to get a change in diet -- less sugar, more fruit and vegetables, and in habits, like teeth cleaning. And that way people's personal choice -- a really important principle -- can be preserved.