What's worse for you: to be under- or overweight?

We're told that being overweight is likely to shorten our lives, but I've also heard that being slightly underweight has the same effect.

We use the body mass index (BMI) to assess weight-for-height ratio. The accepted norm is a BMI between 20 and 25: above 25, and we're fatter than we should be; below 20, and we're thinner. The latest study, in the Lancet, related BMI to mortality in 900,000 adults, and threw up a big surprise - if you're in the thinner half of normal (a BMI between 20 and 22.5), you are slightly more likely to die early than if you are a bit fatter than normal (25-27.5). So it seems that carrying a little excess weight (less than a stone for a person who, for their height, should weigh 10 stone) is maybe a tiny bit healthier than being on the thin side of normal (half a stone lighter). Why this should be is unclear - maybe we need a bit of reserve fat as we age, as a buffer against future illnesses.

Choc therapy

I'm in my third trimester of pregnancy and mildly anaemic - a friend recommends plain chocolate because it contains iron.Is this too good to be true?

Probably. Chocolate doesn't just deliver iron, it delivers sugar and fat, too, which is not necessarily desirable, not least because the iron may not be so easily absorbed when bound in chocolate as in a pill. I don't know of any studies of, say, comparisons of iron absorption from chocolate and from pills, but they'd have to measure weight gain and glucose-tolerance changes, too. Many women retain fluid during pregnancy, so the definition of mild anaemia changes - we tend not to diagnose anaemia until haemoglobin levels fall well below those we think of as anaemic in non-pregnant women. Talk to your doctor or midwife about it.

A no-grapefruit zone

I love pink grapefruit but I'm on Simvastatin and the prescription says to "avoid grapefruit". Why?

Simvastatin is a "statin", a drug for people with higher than normal total blood cholesterol (fat) levels. It's one of several "statins", and all have an unfortunate relationship with grapefruit juice: chemicals in the juice (and pulp) interfere with the action of enzymes in the gut that break down certain classes of drug. This means that much more of the drug is absorbed than normal, leading to serious unwanted effects. Seville oranges and pomeloes may also contain the same chemical, so avoid them, too. Sorry.

Why do I get cramp?

I'm 65 and suffer from cramp in my limbs. What's the best treatment?

Quinine sulphate (200mg as a tablet) each evening, although this reduces the frequency of cramps by only about a quarter. There's no other clinically proven treatment for a normally mobile, healthy person (there are more complex muscle antispasmodics that complicate serious diseases such as multiple sclerosis or spinal cord injury). Look at your lifestyle - are you physically active, do you exercise regularly, eat a varied diet, keep well hydrated? Stretching muscles and keeping them warm in the evening may be the simplest solution. Cramps such as yours can sometimes be caused by a problem in the parathyroid glands (in the neck), which changes the way calcium is used by the muscles. Talk to your doctor about a blood test to assess parathyroid function.

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