In an effort to contain the ever-expanding levels of obesity in the UK, the government is considering levying a tax on unhealthy foods. The so-called 'fat tax' has attracted criticism from those keen to avoid the clutches of a nanny state, while some people in the nutritional establishment have questioned how effective punitive pricing of fattening foods would be in changing our eating patterns, and have suggested subsidising more nutritious fare instead. It is likely that a strategy that makes healthy food more accessible would attract widespread support. But I wouldn't bank on the government being keen to fork out for such a scheme.

Whatever reservations there may be about the use of a fat tax, received wisdom is that it will at least tip the scales in our favour. After all, studies show that rates of obesity are higher in countries that consume plentiful amounts of fat (such as the USA, UK and Australia) compared to those where fat intake is low (such as India and China). However, some scientists have pointed out that there may be other reasons for the disparity. Higher levels of activity and limited access to food may also explain why people living in poorer countries appear to be relatively immune to the problems of obesity.

A more accurate picture of the broad relationship between fat in our diets and fat sequestered in our bodies can be gained from studies comparing countries of similar economic development. In one such study, published in the European Journal of Clinical Nutrition, the proportion of calories contributed by fat was found to range from 25 to 47 per cent across 18 countries. However, this variation in fat intake appeared to have no bearing on body weight in men. Even more surprisingly, in women, higher levels of fat consumption were clearly associated with a reduced risk of excess weight.

This evidence should make the government pause for thought before it slaps a tax on fat.

Interesting though such research is, studies of low-fat eating are better for predicting the fat tax's likely effects. Short-term studies of low-fat dieting in a controlled setting (one that may have little to do with real life) have generally found only modest weight losses resulting. The effects on weight of longer-term fat restriction are even less impressive: a 2002 review of relevant studies found that individuals eating a lower-fat diet for 18 months lost, on average, no weight at all.

With rates of obesity having trebled in the UK in the past 20 years, it's not surprising that the government feels compelled to act. However, if the policy-makers in Downing Street are serious about reversing this trend, they will need to invest in a broad, long-term strategy aimed at supporting people in their efforts to eat better and be more active. The evidence suggests that the chances of a tax on fat bringing about a reduction in our collective weight are very slim indeed.

Dear John

I suffer from dry eyes, making my contact lenses feel uncomfortable. Do you have any advice?
KS by email

A general dryness of the eyes can sometimes be related to a deficiency in certain nutrients. Of prime importance are healthy fats known as essential fatty acids (EFAs). Another common symptom of EFA deficiency is dry skin. If you need to moisturise your skin at least once a day, this suggests a lack of EFAs in the body. EFAs come in several forms and the type that appears most beneficial in combating dry eyes is called the omega-3 class. Including two or three portions of omega-3-rich fish - such as salmon, trout, mackerel, herring and sardine - in your diet should help maintain moisture in your eyes. Alternatively, supplement with one teaspoon of cod-liver oil a day.

In addition to omega-3 fats, cod-liver oil contains vitamin A - believed to be important for the health of the surface of the eye. If you don't eat fish, supplement with one tablespoon of flaxseed (linseed oil) each day. Flaxseed oil is very rich in a type of omega-3 fat (alpha linolenic acid) that is chemically related to the fats found in fish.

· If you have any issues you'd like Dr Briffa to address in his column, please email him on john.briffa@observer.co.uk. Please note that Dr Briffa cannot enter into any correspondence. You can also visit www.drbriffa.com. Before following any recommendations in this column, you should consult your own medical adviser about any medical problems or special health conditions.