The full fat diet is not as unhealthy as you think
Dr Michael Mosley on the diet myth no one wants to talk about.
I read a study the other day that found that eating yoghurt would help you lose weight. Not surprising, because it is high in calcium and protein, which keeps you fuller for longer. What was extraordinary about this study, however, was that the yoghurt had to be full fat to work.
Ever since I was a medical student, I have been convinced that fat is the enemy. Saturated fat, I've told friends and family, will clog your arteries as surely as pouring lard down them. As it's so energy-dense, eating fat will also, inevitably, mean piling on the pounds.
Recently, however, I've changed my mind. Study after study has failed to find a convincing link between heart disease and saturated fat. Research has shown that low-fat diets rarely work, that cholesterol is a poor predictor of heart disease and that eating the right kind of fat can be both good for the heart and slimming. So what's going on?
To understand the fear of fat we need to go back to 1957, the year I was born. It was then that the American Heart Association began targeting fat consumption. Out with the steak, butter and cheese; in with the pasta, rice and potatoes.
In that same year, a British scientist named John Yudkin published a study that claimed that sugar, not fat, was the driver of heart disease. In the memorable phrase of one of his peers, Yudkin was "pushed under a bus" by the anti-fat lobby. A prominent UK scientist denounced his attack on sugar as "false and misleading" and "nothing more than scientific fraud".
In 1972, a former cardiologist, Robert Atkins, pushed a similar message about the dangers of sugar and carbs in Dr Atkins' Diet Revolution. When he died, hugely overweight and with signs of heart disease, there was considerable glee among the anti-fat lobby.
In the 1980s, when the Atkins diet was at the height of its popularity, I first became concerned about my own diet. I was slim and did a lot of exercise, but I also ate quite a lot of saturated fat in the form of butter, burgers and salami. I have a family history of heart disease and strokes. Beef was replaced by chicken. Coffee came with a dash of skimmed milk. Eggs were avoided. Yoghurt was always low-fat.
Yet over the next few decades, I put on about two stone, my body fat went up to 28pc, my cholesterol soared and I became a borderline diabetic. The trouble was that as a result of eating less fat, I was now eating far more carbohydrates.
What I hadn't appreciated is the way these foods act on your body. Eating a boiled potato will push your blood glucose up almost as quickly as eating a tablespoon of sugar. Ironically, if you eat the potato with butter, the fat will slow absorption and the blood-sugar peak will be slower and less extreme.
I also hadn't appreciated that carbs, particularly refined ones, are less satiating than fat and protein. You eat them and then a few hours later you are hungry again.
Yet the anti-fat message continued, and cholesterol also took a beating. If it was common sense that eating fat would clog up your arteries, then it was also common sense that eating cholesterol would do the same.
Foods rich in cholesterol, such as eggs, were shunned. Governments warned consumers to eat no more than one egg a week and supermarkets were stacked to the rafters with foods that declared themselves "cholesterol free".
Yet it turned out that the effects of the cholesterol we eat on the levels of it in our blood is relatively small.
I now eat eggs most mornings. They are a superb source of protein, rich in vitamins and minerals. There are about 90 calories in a large boiled egg, half that of a small bowl of Frosties and a quarter that of a croissant with butter and jam. Unlike the cereal or the croissant, the protein in the eggs will keep you feeling fuller for longer.
Another persistent myth about cholesterol is that it is a powerful predictor of heart disease. I've recently had my bloods checked and my total cholesterol is higher than it should be, which, of course, is worrying. Or is it?
Cholesterol is essential to life. Made by your liver, it is transported in your arteries in a form called LDL (low-density lipoprotein). Excess amounts are carried back to your liver as HDL (high-density lipoprotein).
Broadly speaking, you want low blood levels of LDL and high levels of HDL. The ratio is more important than your total cholesterol score. But that isn't the end of the story, because even within LDL there are at least two subfractions: large, fluffy particles that don't seem to be too bad, and small, hard particles that are. To add to the confusion, there are also triglycerides, globules of fat that travel in the blood and are also a risk factor when it comes to heart disease.
Eating saturated fat raises LDL, but it also raises HDL. Triglycerides, on the other hand, are elevated by lack of exercise and eating too many calories, particularly sugary carbs.
My total cholesterol is high because my LDL levels are high, but also because my HDL levels are high. My triglycerides, on the other hand, are exceptionally low. So, am I at risk of a heart attack?
I found out by using the Qrisk Web Calculator (qrisk.org), one used by GPs. Apparently, I have an 8pc chance of having a heart attack or stroke in the next 10 years, better than average for someone of my age, gender and ethnicity.
In the 1950s, Professor Hugh Sinclair at the University of Oxford was arguing that we were eating too little fat. He had became intrigued by the high-fat diet and low rates of heart disease of the Inuit in northern Canada. He wondered whether omega-3 - an essential fatty acid found in oily fish - was protecting the Inuit from heart attacks.
Eventually, he tested his theory by putting himself on an Inuit diet, eating nothing but seal, oily fish, mollusks and crustaceans. Throughout his experiment, Sinclair measured his bleeding time - the time it took for his blood to clot - by cutting himself every week. I decided to repeat it, eating fish. Sinclair stuck to his diet for three months; I managed a few weeks. On his new diet, Sinclair's bleeding time increased from three minutes to a terrifying 50. Mine doubled. Sinclair had shown that eating fish oils reduces the risk you'll form clots that can lead to heart attack or stroke.
However, there are significant risks to going as far as he did. Inuits on such a diet are prone to bleed to death after an injury. Sinclair bled into his joints.
It is now accepted that eating oily fish reduces the risk of heart disease, yet because of pollutants such as mercury, the benefits of eating fish (or seal) are less clear than in Sinclair's day.
The campaign against fat was not exclusively because of fear that it would clog up arteries. Fat, it was widely assumed, is fattening. Gramme for gramme, fat contains more calories than either carbs or protein.
Low-fat diets were created and endorsed by the medical profession. But the success rate of such diets has generally been poor.
A poignant example of this was the Look Ahead trial. More than 5,000 overweight diabetics were put on a low-fat diet and encouraged to do more exercise, with lots of expensive support from personal nutritionists, trainers and group support sessions. After almost 10 years, the study was stopped "for futility". The patients had lost only a little more weight than a control group and there were no significant differences in heart attacks or strokes.
Where does that leave us?
The message has changed over the years from "fats are bad for you" to "saturated fats are bad for you" but even that is now being challenged.
A study recently funded by the British Heart Foundation has really put the fat in the pan and made it sizzle. Having pooled the results of 72 previous studies, researchers found that although trans fats increase the risk of heart disease, they could find no significant evidence that eating saturated fats does.
This isn't a licence to start pouring cream down your throat, because even if saturated fats don't directly harm the heart, there's no doubt that eating too many calories, whether it's fat, protein or carbs, will.
What's too many?
Officially, it's 2,500 calories a day for men and 2,000 for women. The reality is that unless you are young and active, it is going to be less than that - I find that if I eat more than 2,200 calories a day I put on weight.
Still, I have gone back to eating butter (interspersed with an olive-oil-based margarine; I'm hedging my bets), Greek yoghurt and semi-skimmed milk. I eat more fish, eggs and the occasional burger. I also eat a lot more vegetables (the fibre keeps you full). I'm not convinced that saturated fat is good for me, but nor do I now feel guilty when I eat it.
It is also time to apologise to my family for all the useless advice I've been giving them.
Dr Michael Mosley is the author of The Fast Diet, Short Books