There was a time when the adverts on radio and TV urged us all to "go to work on an egg". The famous British ad campaign, which was orchestrated by the young Fay Weldon, stated with cast-iron certainty that a breakfast egg was the best way to start the day.

Then, almost overnight, eggs went from being man's best friend to Public Enemy No 1. We were solemnly warned to limit ourselves to a maximum of two per week or risk heart attacks, strokes and a host of other self-inflicted ills caused by overdosing on deadly cholesterol.

And now we're being told to erase that last piece of scientific advice from our minds, and get cracking again on our morning eggs. The latest radio slogan from Bord Bia and the Irish Heart Foundation assures us: "An egg a day is okay."

While this may be confusing, it's nothing new. The do's and don'ts of diet and fitness have a scrambled history. Jim Fixx, the man who single-handedly kick-started the popular craze for running marathons, sold a million copies of his book extolling the health benefits of running and then dropped dead mid-jog aged just 52.

In recent times a succession of eating regimes has enjoyed massive popularity followed by a backlash of similar proportions. Three years ago, British women were asked to name their worst diet plan. The three deemed the greatest waste of time, money and effort were Slimfast, Cabbage Soup and Atkins.

Had the women of 1930s Ireland been asked to vote, the following piece of expert advice from the Irish Times may have polled strongly. It instructed: "One of the best ways to slim, according to an eminent obesity specialist, is to remain in bed, under a doctor's supervision, for two to three weeks absolutely without food, drinking only water and orange juice. But even though most women desire to be slim, few would go to such lengths to achieve it.

"Indeed, very few are in a position to afford the luxury of such a treatment. For, though it may not cost anything in food over that period, it does necessitate a domestic upheaval such as few can contemplate." (For "domestic upheaval", read "1930s husband".)

So with health and fitness guidelines seemingly as changeable as the Irish weather, what advice remains sound, and what should we take with a pinch of salt?

An egg a day is again okay?

"Yes, it is," says Dr Aine Hearty of UCD's Institute of Food & Health. She explains: "Years ago, it was realised that eggs were a source of cholesterol and people were advised to cut down. But it's now clear that eating eggs doesn't give you high cholesterol.

"Bad saturated fats of animal origin (butter, cheese, meat) are what raise cholesterol."

Janis Morrissey of the Irish Heart Foundation adds: "Eggs are actually a powerhouse of nutrition.

"People may be confused by changing messages about what's good and bad to eat, but nutrition is a young science that's still evolving. All we can do is update our advice based on the most up-to-date information coming through."

Eat five portions of fruit and veg daily

According to Dr Sarah Brewer: "Different countries suggest different quotas. It's only five portions here because so many people eat none at all." The Japanese are advised to eat 17 portions of fruit and veg a day, while a recent US study said that eight portions reduce your chance of a heart attack by 30pc.

Dr Hearty says: "The size of portions is culture specific. Guidelines from the World Health Organisation suggest 400 grams per day. An apple is around 80 grams, or one fifth of requirements. You can also take one portion as fruit juice, even made from concentrate. But only one, because you need the fibres too."

Restrict alcohol to 21 units (10.5 pints) weekly for men and 14 units for women

According to Mike Shallcross of Men's Health magazine: "These numbers err towards caution. But use common sense. If you feel fit and healthy you probably have your alcohol intake about right."

Eating carbohydrates piles on pounds

Not so, say exercise physiologists Wendy Repovich and Janet Peterson in a study which states: "Cutting carbs from your diet may have short-term weight loss benefits due to water loss from a decrease in carbohydrate stores, but eating carbs in moderation does not directly lead to weight gain. The body uses carbs for energy, and going too long without them can cause lethargy."

Rival academic Michael Eades dismissed this as "idiocy", insisting that cutting carbs produces "a nice, steady loss of fat".

Drink four litres/eight glasses of water a day

"Adults need around two litres," says Morrissey, "but some of that can be taken from fruit and veg which are high in fluids." Dr Hearty agrees, with the proviso: "If you're involved in sport, you should take on more. Tea is a good source of fluid, although I wouldn't recommend taking all your fluids as tea. Five or six cups a day is okay, as tea only has half the caffeine levels of instant coffee."

As a general rule, if your urine is pale yellow, your fluid intake is fine. If it's a darker yellow, drink more water.

Fish is brain food

"The Omega 3 fatty acids found in fish are excellent brain food," confirms Dr Hearty. "These essential fatty acids that are also good for the heart and have anti-inflammatory properties which keep joints working and combat arthritis." Unfortunately, studies show that many Irish people eat far too little oily fish to meet the recommended 450mg daily intake.

Replace butter with spreads such as olive oil

Morrissey says: "Our message is that they're all pure fat and should be spread very thin. You have more leeway with a low fat spread that comes from a source other than animals."

Dr Hearty adds: "Olive oil is high in monounsaturated fat, which controls bad cholesterol and raises good cholesterol, bringing a range of benefits."

Take 30 minutes of moderate exercise three times a week

According to fitness coach Matt Roberts: "The figure is meaningless. It assumes people know what moderate exercise is. On a scale, if one is being asleep and 10 is keeling over exhausted, you need to be hitting a nine by running, training or swimming, until you can't do much more, at least three times a week."

The latest study from the American College of Sports Medicine agrees, recommending that to keep ticking over you should supplement three vigorous workouts with an additional two sessions of weight training.

So that's all the best up-to-date advice?

For the moment. The Department of Health recently instructed the Food Safety Authority to review the current healthy eating guidelines. The review should be completed by year's end, with revised guidelines to follow.

- Damien Corless