A grain of truth, a small study and suddenly a new diet can be created, especially when a celebrity is involved.

Earlier this year, a book was released in the United States called Is Gwyneth Paltrow Wrong about Everything? When Celebrity Culture and Science Clash. It was written by Timothy Caulfield, a health-science expert looking at how celebrity culture has gone from influencing our clothes and entertainment choices to our diets and views on health.

In Australia, celebrities are putting their weight behind high-profile diets such as the paleo diet, the I Quit Sugar diet and other health and wellness regimes, but at what point does celebrity skew the science?

We look into seven popular food phenomena. Are they fad or fact?

1. Paleo is the perfect diet

Paleo gets brownie points for promoting vegetables and avoiding processed food, but a slap on the wrist for outlawing legumes and wholegrains. There's good evidence that these foods are good for the heart and bowel, and help nourish friendly microbes in our gut. Does it make sense to shun barley and chickpeas that humans have eaten for at least 5000 years, but eat coconut yoghurt that wasn't in the true paleo pantry?

What about the planet? Can it sustain a few billion pretend hunter-gatherers, given that we now hunt for most of our meat in shopping centres, not the savannah? Our modern meat comes from livestock production, which is a key factor in greenhouse gas emissions.

The other question about the paleo is: how close is it to what our hunter-gatherer ancestors ate?

The clues we have from anthropology tell us that there was no single ancestral diet, but many different ones depending on the region, season and point in history, and the domesticated meat and cultivated plants we eat now are very different from the wild food ancient hunter gatherers ate, says Canadian dietitian Brenda Davis, who spoke at a seminar in Sydney on the paleo and plant-based diets earlier this year.

"Not only was the wild meat much lower in saturated fat and higher in omega-3 fats than the meat we eat now, but the wild plants would have delivered triple the fibre. It would have been like eating bark," says Davis, who looked at 2010 research by US anthropologists Stanley Boyd Eaton and Melvin Konner​, and found they estimated our ancestors ate 70 to 150 grams a day. This makes Australia's recommended daily intake of 25 to 30 grams a day look puny.

Even vegans eating an entirely plant-based diet would be flat out getting 50 to 60 grams of fibre a day, she says, which suggests that a modern paleo diet is very different from the original thing.

The idea that grains and legumes were never part of hunter-gatherers' diets and our bodies have not adapted to eating them also ignores the evidence on our doorstep – that these foods would have been part of Aboriginal diets before Europeans arrived, says Jennie Brand-Miller, Professor of Human Nutrition at the University of Sydney's Charles Perkins Centre.

"Traditional Aboriginal diets are a window into hunter-gatherer diets. Grasslands would have been sources of grains and seeds, including acacia seeds, which are a legume," she says, "and when you see early photographs of Aboriginal men living as hunter-gatherers, they were tall and muscular."

2. Bacon is a health food

That can depend on whether you ask a chef or a nutrition scientist. The idea that high-saturated-fat foods such as bacon, butter and duck fat deserve health haloes gathered steam after some studies suggested that reducing saturated fat in the diet didn't reduce the risk of heart disease.

However, that's not quite the same as saying butter, or any other saturated fat, is a health food, according to the Harvard School of Public Health. Its website, the Nutrition Source hsph.harvard.edu/nutritionsource/fats-full-story/#references, explains that a lot depends on what people eat when they replace saturated fat in the diet.

Cutting saturated fat can be good for the heart, provided that it's replaced with good fats, especially polyunsaturated fats, but reducing saturated fat isn't likely to help if it's replaced with refined carbohydrates such as white bread or white rice which, while low in fat, are as bad for the heart as too much saturated fat.

The issue of how fat in food affects our health is complicated and contentious, and we don't have all the answers, but until we do, it's smart to remember what we do know: that it's the total diet, rather than single foods or nutrients that really matters. If there's one pattern of eating that's consistently linked to less heart disease and stroke, it's the Mediterranean diet, with less saturated fat and more fat from olive oil, nuts and seeds, plenty of vegetables, fruit, legumes, whole grains, fish, a little meat, some yoghurt and cheese.

3. Gluten is the enemy

It's true that gluten, the protein found in wheat, barley and grains is bad for some people, but self-diagnosing a problem with gluten can also be barking up the wrong tree.

"With people who say, `I don't feel well when I eat bread and pasta', for instance, the real problem is sometimes a type of carbohydrate called fructans​, found in wheat and some vegetables and fruit," says Linda Hodge, an accredited practising dietitian specialising in food intolerance and allergy.

"When fructans go to the large bowel, they are fermented by gut bacteria, which helps to keep our bowel healthy, but this fermentation also produces gas, which can cause discomfort for people with irritable bowel syndrome who have very sensitive nerve endings in the gut. This doesn't mean these foods are bad, just that some people are more sensitive."

Fructans are just one possible candidate. Hodge also sees people who blame gluten for their gut symptoms, when the real cause turns out to be a sensitivity to a food chemical or a problem with dairy or soy products.

Yet because of the mud sticking to gluten, "gluten free" has become another term for "healthy" in some people's minds, yet many gluten-free processed foods aren't the best choices.

"Some gluten-free grains, such as rice, have a very high GI [glycaemic index], which means that products made with rice flour generally have a high GI too," Hodge says. "Some gluten-free products, especially many breakfast cereals, also include a lot of sugar to make them more palatable."

The people who really need to avoid gluten are the 1 per cent of Australians with coeliac disease.

4. Grains kill your brain

On the one hand, health authorities tell us wholegrains are good for us, and on the other, a US neurologist claims that grains are the fast track to dementia. Who is right?

The idea that grains cause Alzheimer's disease comes from US neurologist David Perlmutter​, author of Grain Brain: The Surprising Truth about Wheat, Carbs, and Sugar – Your Brain's Silent Killers, who argues that grains raise levels of blood glucose, which in turn damage the brain.

What we do know is that having diabetes, which develops when the body cannot control levels of blood glucose, increases the risk of Alzheimer's. It's also true that too many refined carbohydrates can, over time, raise blood glucose levels.

"But to say that grains harm the brain is drawing a very long bow," says Professor Henry Brodaty, co-director of the Centre for Healthy Brain Ageing at the University of NSW. "The evidence just isn't there."

There's also a big difference between the refined grains in, say, a doughnut and a wholegrain such as traditional oats that's slowly digested and a good source of nutrients. Wholegrains are also one of 10 food groups identified as protective against Alzheimer's by Rush University Medical Center in Chicago. The others are green, leafy vegetables, other vegetables, nuts, berries, beans, fish, poultry, olive oil and wine.

5. Raw is better than cooked

That depends on what it is and how it is being cooked. Cooking can kill nutrients in some foods, or create nasty substances, but in the case of vegetables, cooking sometimes releases more nutrients. Studies have found that tomatoes provide more of the antioxidant lycopene when they're cooked, for instance, and steaming broccoli can increase the availability of glucosinolates​, compounds believed to protect against cancer.

On the other hand, vegetables cooked in water can lose water-soluble nutrients such as vitamin C and some B vitamins (although steaming, roasting and stir-frying usually help preserve them) – a good reason to eat a mix of cooked and raw vegetables.

6. Low carb is the one true path

If short-term weight loss is all that matters, a low-carbohydrate diet can work, but it might kill you sooner.

When it comes to weight loss and keeping weight off, low-carb diets with moderate amounts of protein are a good bet, says Brand-Miller. Examples of this approach include the CSIRO Total Wellbeing Diet and what became known as the World's Best Diet – an eating plan based on a large Danish study called the Diogenes Diet, which compared five different diets to see which worked best at maintaining weight loss, and found that the low-carb, high-protein formula worked best.

However, the emphasis is on modestly lower amounts of carbohydrate and modestly higher amounts of protein, as opposed to low-carb, high-protein diets such as the Atkins diet, says Brand-Miller, who co-authored the Australian edition of World's Best Diet.

"I think the low-carb, high-protein approach is playing with fire. I wouldn't recommend it for more than six months, because there's evidence linking it to higher mortality from cardiovascular disease and a higher risk of type-2 diabetes," she says.

7. Coconut oil fixes everything

Coconut oil is a shining example of how a small amount of research can trigger a tsunami of health claims, from improving weight loss to helping prevent heart disease and treating dementia, but what's the evidence to back these claims?

Coconut oil's reputation for being heart healthy, despite being high in saturated fat, is largely based on the fact that it's high in a fatty acid (lauric​ acid) that raises "good" HDL cholesterol, says Margaret Allman-Farinelli, Professor of Dietetics at the University of Sydney.

"When a New Zealand study compared the effects of a polyunsaturated oil (safflower oil) on cholesterol levels with two saturated fats, coconut oil and butter, it found that safflower oil was best at lowering `bad' LDL cholesterol, while butter raised it, but coconut oil was in the middle. However when it came to raising `good' HDL cholesterol, coconut oil raised it a more than safflower oil."

Does that give it the green light for heart health?

"To know that, we need long-term studies over many years to understand its effects, which is what has happened with olive oil, but with coconut oil and the heart, there hasn't been enough research to be able to make any recommendations, partly because, unlike olive oil, it hasn't been a big part of the Western diet."

THE HOW-TO GUIDE TO 'SUPER' FOOD STARDOM

Here's a plan for creating a food fad. Take a small study linking consumption of the xixalaca seed (found only in the Amazon rainforest) to the lean physiques and longevity of an indigenous Indian tribe. Add a segment on Dr Oz spruiking xixalaca's unique health benefits and a celebrity claiming that xixalaca smoothies helped shift those last five kilograms. Toss in a new book, The Xixalaca Miracle, by a well-known chef whose website sells special oven trays for dry-roasting xixalaca at home, and you're done. The internet and word of mouth will do the rest. Your supermarket will be stocking xixalaca muesli in no time.

Just kidding, but only about xixalaca. It doesn't exist, but the trajectory by which health fads take wing isn't far off the mark. It starts with a grain of truth – a study or two linking a particular food to an important health benefit or new research questioning a health message, such as "eat wholegrains", that has been standard advice for years. Spurred on by celebrity testimonials and self-styled experts with books to sell, what starts as speculation soon becomes fact, especially if the food industry jumps on board with products that tap into the trend.

To really understand why so many health-food movements clash with mainstream nutritional advice, it helps to know how health authorities arrive at this advice in the first place.

Messages, such as "eat five serves of vegetables a day" or "limit processed meats like ham and salami", aren't based on a couple of studies that popped up online last week. They're the result of a painstaking process of sifting through hundreds of research papers and making recommendations based on the best-quality evidence, says Margaret Allman-Farinelli, Professor of Dietetics at the University of Sydney.

"Confusion happens when a single study comes up with a finding that's different from the current thinking. It all seems very exciting, because everyone wants a magic bullet for better health or weight loss, and the usual recommendations seem boring, because we've been hearing them for years," she says.