I would like to stop using commercial antiperspirants/deodorants. A friend recommended using bicarbonate of soda, applied direct. It certainly works, but is it safe to use in this way?

I don't see that applying bicarbonate of soda to the skin would cause any harm: it isn't a recognised irritant or allergen. However, it is soluble, which means it would quickly be dispersed and spread away from the area to which you apply it. So I don't know how often you would need to do so, or how much you'd need to apply, to maintain any kind of effect.

I worry that my children, aged five and four, aren't eating enough - both are fussy and rarely finish their meals, and one refuses to eat meat. They are slim and generally healthy, though they get lots of colds. To what extent do eating habits at this age affect their immunity?

Your children won't starve themselves and their immune systems won't suffer. They are just exerting their right to be independent little people. Let them be. You choose what they want to eat, and if they don't eat it, then tough. It isn't always a bad thing for children not to finish their meal. It suggests they know when they have had enough. That's better for them than always being hungry and always eating. Four- and five-year-olds get lots of colds because they are in contact with hordes of other children. There are hundreds of different cold viruses around, and by catching most of them in their first decade, they will eventually become immune to them.

· Do you have a question for Dr Smith? Email doctordoctor@theguardian.com