Is fat really bad for you? Should we consume less salt? And what’s wrong with gluten, anyway? A science writer and a consultant cardiologist separate fact from fiction

The great diet food con

Have you ever wondered why people on diets seem to be on and off them forever? Or why it’s really only people who struggle with their weight who have extra low-fat mayonnaise in their fridge? Most processed foods branded as diet, low-fat, light or lighter aren’t what they’re cracked up to be. And if you look at their lengthy ingredients lists, you’ll realise that they’re not even very good for you.

Take low-fat mayonnaise. When you strip out the fat you have to reinject flavour with sweetness. So an emulsion of eggs and oil becomes an emulsion of water, maize starch, extra sugar and glucose syrup. Or to put another way: water, sugar, sugar and sugar. That’s an awful lot of sugar, and because the traffic lights on packets of food don’t flash red until a whopping 27g of sugar is in each 100g portion – that’s just under seven teaspoons – a busy shopper won’t think twice about choosing this “healthy” option.

Other culprits are microwaveable low-fat meals. The high glycaemic index, carbohydrates and sugars aside, manufacturers are so busy bending backwards to limbo under the threshold for each of the other traffic-light categories that they often leave out plenty of healthy foods such as positive fats, fruits, vegetables and fibres. It’s why they’re often so tasteless and don’t leave you feeling full for long.

The diet food industry is a headless beast driven by market forces: it makes good business sense to make low-quality food with effective branding. The only way we can bring it to its knees and stop this tyranny of tiny, tasteless meals is to become savvy consumers and stop buying them. If you feel yourself being seduced by an alluringly presented diet food and don’t have time to interrogate its ingredients, a good rule of thumb is to just say no.

Are calories important?

We love to ascribe value to things. It helps us make decisions about a jumble of information, like the way we use calories to help us manage what we eat.

The concept entered public consciousness during the first world war when the state used it to make sure people didn’t overeat during food scarcity. The calorie – the unit of energy needed to heat one cm3 of water by 1C – has since become the backbone of our understanding of healthy eating, but at what cost?

A healthy diet is not about restriction but inclusion of diverse and protective foods

In a perfect world it makes perfect sense. If you consume more energy than your body needs, then according to the central tenet of Einstein’s most famous equation it’ll be turned into mass – a wibbly-wobbly type of mass around your belly and thighs.

The problem is that our bodies don’t burn energy with the consistency of a Bunsen burner. “We do not absorb all the nutrients from some foods,” says Pete Wilde, a professor at the Institute of Food Research in Norwich. “By chewing and eating whole almonds, for instance, we absorb only around two-thirds of the energy listed on the label. The calcium in milk and dairy products reacts with fatty acids and again reduces the energy absorbed.”

The different rates of digestion of different foods can also affect your appetite and determine how much you end up eating. “The more slowly our food is digested the less hungry we’ll feel for longer,” Wilde adds. “The calories absorbed by two different foods could be the same, but if one food is digested more slowly, it’ll make us less hungry and less likely to snack.”

In how your body uses – and stores – energy, 140 calories of cola is not the same as 140 calories of broccoli. If you want to lose weight by starving yourself, then a calorie-restricted diet is the way to go. Though you’ll likely bounce back once you start to eat normally. If you’re after a sustainable way to be healthy it’s best to think a little more about the constituents of your food.

A healthy diet is not about restriction but inclusion of diverse and protective foods. Choosing food on the basis of only calorie content is like choosing your life partner on how quickly they can run 100 metres: it might be useful in extreme circumstances, but for your day-to-day life and general wellbeing, it’s pretty much useless.

Facebook Twitter Pinterest Look out for the mountains of salt used to make low-quality processed foods more palatable. Photograph: Jean-Christophe Riou/Riou/SoFood/Corbis

Should I lower my salt intake?

Whether your salt has been coaxed from a cave by a Tibetan monk or extracted from brine by a man called Gary, it’s always exactly the same thing – sodium chloride. Because it’s the sodium in salt that’s associated with high blood pressure (the only health concern with salt) if anyone tells you fancy salts like sel gris, fleur de sel, Hawaiian sea salt or pink Himalayan salt are any healthier than the ordinary table stuff they’re wrong.

The matter of whether or not you should lower your salt intake is less cut and dried. Some people’s blood pressure is more sensitive to salt than others’. Even scientists are divided about how to interpret the evidence on an individual level, but to get bogged down with the scientific controversies would be to miss the point: salt makes food taste good.

The first thing that MasterChef judge Michel Roux Jr comments on is a dish’s seasoning. Poor Rick Stein must live in constant fear of finally being caught by the salt police he mentions so regularly. Nobody has ever won a Michelin star with a salt-free menu.

The relationship between salt intake and blood pressure is complicated, says Anthony Heagerty, professor of medicine at the University of Manchester. “Lowering salt is advisable on a population level because many people will respond favourably to an overall lower salt intake,” he says, “but teasing out the effect on an individual is more difficult. An unhealthy diet, excess weight and alcohol play a large part in an individual’s risk of hypertension, so if you’re concerned about blood pressure you can lower your salt, but it’s perhaps more important to focus on your overall food and alcohol intake.”

Fruit and veg rich in potassium, such as mushrooms, spinach and bananas, can help lower blood pressure. Look out for the mountains of salt used to make low-quality processed foods and snacks more palatable. Using salt in a sensible way can make the food you cook at home taste better, and, importantly, give you a better handle on what you’re putting in your body.

The complicated history of fats

It started in the 1950s with a case of mistaken identity. Saturated fats, researchers said, were the main cause of heart disease. The smoking gun? Fatty deposits in patients’ arteries and studies showing that people in Mediterranean countries – where people tend to eat more unsaturated fats like olive oil than saturated fats like butter – had a lower risk of disease.

By the 1970s the food industry responded. Their answer was trans fatty acids, or trans fats: unsaturated fats transformed by hydrogenation to fit convenient criteria. Once hydrogenated, for example, cotton or vegetable oils would be solid at room temperature to make margarine. Fats could be chemically tweaked to increase the shelf life of biscuits and cakes, or to avoid them from breaking down during the repeated reheating done in chip shops and restaurants.

That these trans fats were made from unsaturated ones, the thinking went, meant they were healthier than saturated fats. It was a misconception supported by the medical community; and one which proved to be deadly.

The increased consumption of trans fats contributed to a peak of heart attacks and strokes during the 1980s and 90s. Eating them, it turns out, can also increase your risk of type-2 diabetes, so there’s little wonder that manufacturers, at the behest of governments, have been reducing the amount of trans fats in their products.

“There’s still no legal requirement for companies to label trans fats as such,” warns Chloe Miles, of the British Dietetics Association. “It’s important to check the ingredients lists. Look out for anything with partially hydrogenated oils or fats. The usual suspects are takeaways, pastries, pies, fried foods, cakes, biscuits and hard margarines.”

Ding dong, pseudoscience nutritionists proclaimed at the outing of trans fats, the witch is dead! But they fell into a dangerous trap: they started saying that saturated fats such as butter and coconut oil are good for you. This is by no means true. Though angelic compared with trans fats, eating too much saturated fat will damage your health by displacing healthier foods. Trying to replace saturated fats completely with low-fat options, though, means you’ll be eating more sugar and refined carbohydrates, which will do you worse, so saturated fats should be used wisely to make some healthy foods more satisfying.

The heroes are unsaturated fats from whole food sources. Olive oil, seeds, nuts, oily fish and avocados, if prioritised over saturated fats, can help you lose weight and avoid heart disease. And they may even protect against neurological disorders such as depression. The health benefits of these fats is a simple message that should not get lost in a complicated history.

Facebook Twitter Pinterest People who eat excessive amounts of processed meats can also have unhealthy lifestyles. Photograph: Norman Hollands/Getty Images

Should I eat processed meat?

Bacon sandwiches are one of life’s little pleasures. The only problem is that the World Health Organisation has classified bacon, along with all other processed meats, as a group-1 carcinogen – up there with cigarettes, plutonium, asbestos and uranium.

But the picture is a little more nuanced. Traditionally we lumped red and processed meats together in these scientific analyses. Only recently have the two been separated out, showing that for cancer, as well as cardiovascular disease, a sensible amount of red meat is probably fine – processed meat, it seems, had been the stronger contributor to disease and had been tipping the balance. But, in terms of risk, is a 30% meat sausage really the same as a cured, air-dried Iberico ham? The truth is that nobody knows.

“There are some mechanistic explanations for why processed meats might cause colorectal or bowel cancer,” says Louis Levy, head of nutrition science at Public Health England (PHE), “such as something carcinogenic that happens during the cooking process or something used to preserve them.”

Given the incomplete picture, PHE advises a limit of 70g of red or processed meat a day. The reason for 70g? It’s to ensure that the people who eat meat will still get the iron benefits of nutrient-dense red meats. But this seemingly sensible guideline falls foul of a familiar problem – it lumps all red and processed meats together.

Another possibility for the link between processed meat and disease, Levy says, is confounding: studies have shown that people who eat excessive amounts of processed meats have otherwise poor diets and unhealthy lifestyles which increase their likelihood of disease; given the severity of the potential consequences, only a fool would be flippant about the risk.

Try to favour red meat over processed meat and choose quality over quantity because there are other benefits to cutting back on meat. Your shopping will be cheaper. You’ll be able to eat more oily fish, vegetables, nuts and fibre. And, given the enormous amount of carbon the meat industry pumps into the air, if you go veggie for a few days a week you’ll be doing your bit for the planet.

The gluten-free con

Food packaging that yells about what is not in it and only whispers about what is should always be a red flag. This is nowhere more true than with gluten-free foods.

About 1% of the population have coeliac disease. They are allergic to gluten, a mixture of proteins found in grains, and must adhere to a strict, lifelong diet if they want to live without stomach pains, diarrhoea, vomiting, anaemia and fatigue.

Then there are people who are gluten intolerant. It’s tricky to define but it probably affects about another 1% of the population who have the milder symptoms of coeliac (it can’t be medically diagnosed) without lasting damage to their gut.

Interesting, then, that about one in five of us – or 20% vs the 2% who would actually benefit – buys or has bought gluten-free food. “Only people with coeliac disease are likely to benefit from a gluten-free diet,” says Dalvinder Mandair, a consultant gastroenterologist in the NHS. “For the others there’s no physiological mechanism by which not eating gluten will help them, but because gluten intolerance is such a subjective thing it can be open to persuasion.”

That 18 in every 20 customers are buying gluten-free products because of a perceived but non-existent health benefit is certainly good marketing. But that’s no reason to do down people’s attempts to feel healthy – as long as their decisions are well-informed. It’s at this point that things start to break down for gluten-free foods.

If you’ve not got coeliac disease, you should ask yourself whether it’s really worth going gluten-free

They’re often more expensive than their gluten-containing counterparts, and to compensate for the elasticity gluten gives to things like cakes and bread, manufacturers spoon in more sugar, E-numbers and additives. It’s a long way from the healthy, natural image they like to invoke.

If you keep avoiding food with gluten in it you’ll not be eating much fibre, and a diet lacking in fibre can increase your risk of lower-bowel cancer and cardiovascular disease.

If you’ve not got coeliac disease, you should ask yourself whether it’s really worth going gluten-free.

Facebook Twitter Pinterest In how your body uses energy, 140 calories of cola is not the same as 140 calories of broccoli. Photograph: Tastyart Ltd Rob White/Getty Images

Understand cholesterol and control it with diet

Cholesterol is another wrongly maligned component of our diet. Because it’s a waxy substance it can’t dissolve in our blood, so it’s packaged into little ferries along with fat and protein to be transported to where it is needed for things like making hormones. It’s cholesterol’s inclusion in these transportation ferries where the misunderstanding about its role in ill-health comes from.

Low-density lipoprotein, or LDL, cholesterol ferries are known as “bad cholesterol”. These are the packages of cholesterol, fats and proteins made in the liver and sent off around the body. En route, and under specific conditions, they can get trapped in the lining of your arteries and leave fatty deposits that can lead to cardiovascular disease.

Then there’s “good cholesterol”, or high-density lipoprotein (HDL) cholesterol ferries. On their way back to the liver to be unpacked and excreted, these ferries help to pick up the fatty deposits and thus lower your risk of disease. Whether your total cholesterol, therefore, is high or low is an oversimplification – it’s the balance of good and bad cholesterol ferries that’s important.

Think of this balance, known as your lipid profile, as a gauge of your general health. The canary down the proverbial coalmine is a bad profile: high levels of bad LDL cholesterol, high levels of trigylcerides (another type of fat in your blood) and low levels of good HDL cholesterol. The good news is that many people can reverse a bad profile by changing their diet and lifestyle rather than by popping more pills.

Counter to what you might have heard, cholesterol in your diet from things such as eggs and prawns is almost irrelevant. Instead, watch out for excess sugar and refined, low-fibre carbohydrates (both cause spikes in your blood sugars, your liver responds to the surge by packaging the sugar into triglycerides and bad cholesterol and sending them out into the body). Avoid trans fats, eating more saturated fats than unsaturated fats from whole food sources, and drinking too much alcohol.

To improve your profile you can actively introduce foods with lots of soluble fibre: pectin in gummy fruits and beta-glucan in porridge protect the rest of the food you’ve eaten from digestive enzymes, meaning it’s digested more slowly and you get fewer sugar spikes. Prioritising unsaturated fats from whole foods such as nuts and oily fish will reduce your bad cholesterol and increase your good cholesterol. And in case you thought you could eat your way along a delicious path to a healthy heart: exercise is key, so get those walking shoes on and earn that brunch.

Dr Ali Khavandi is a consultant interventional cardiologist at Royal United Hospital NHS Foundation Trust Bath