Contrary to conventional advice, eating more of some fats may be good for our health, says Michael Mosley.
It really is the sort of news that made me want to weep into my skinny cappuccino and then pour it down the sink. After years of being told, and telling others, that saturated fat clogs your arteries and makes you fat, there is now mounting evidence that eating some saturated fats may actually help you lose weight and be good for the heart.
Earlier this year, for example, a systematic review, funded by the British Heart Foundation and with the rather dry title "Association of dietary, circulating and supplement fatty acids with coronary risk" caused a stir.
Scientists from Oxford, Cambridge and Harvard, amongst others, examined the links between eating saturated fat and heart disease. Despite looking at the results of nearly 80 studies involving more than a half million people they were unable to find convincing evidence that eating saturated fats leads to greater risk of heart disease.
In fact, when they looked at blood results, they found that higher levels of some saturated fats, in particular a type of saturated fat you get in milk and dairy products called margaric acid, were associated with a lower risk of heart disease.
Although there were critics, NHS Choices described this as "an impressively detailed and extensive piece of research, which is likely to prompt further study".
Some academics queried the paper, others worried that this sort of research would confuse people and the message they would get would not be "it's OK to eat more of some forms of fat" but that "it's OK to eat lots more saturated fat, even if it is in pies". We know that current levels of obesity have been fuelled, at least in part, by snacks like muffins, crisps and cakes, all high in fat, sugar and calories.
When I talked to one of the researchers behind this paper, Prof Kay-Tee Khaw of the Department of Public Health at Cambridge University, she was quite clear that her research was not a licence to fill up on junk food but she also accepted that new research made the dietary picture more complicated.
"It's complicated in the sense that some foods which are high in saturated fats seem very consistently to reduce heart disease."
Khaw told me that there is good evidence that eating a handful of oily nuts a few times a week will reduce your risk of heart disease, despite the fact they are contain saturated and unsaturated fats. She said the evidence for full-fat dairy is less strong, but she is quite happy to eat butter and drink milk.
She is also happy to eat red meat, although she is concerned about where it comes from.
"It's very clear that cattle that are fed on pasture have very, very different fatty acid patterns from cattle that are corn-fed. So I think how the animal has been reared probably has a big impact in its nutrient profile and, presumably, on health outcomes, which may be why there's such conflicting evidence, because it depends on where the source of your food comes from."
In the US, where most cattle are corn-fed, there is good evidence that higher rates of red meat consumption are associated with higher rates of heart disease. In Europe, where cattle are more commonly raised on grass, the association seems much less clear.
Although eating some types of fat may not be as damaging as we once thought, surely fat is bad for you because it makes you fat? Not necessarily.
A recent study, this time published in the Scandinavian Journal of Primary Health Care, "High dairy fat intake related to less central obesity", certainly questioned the link.
In this study, researchers followed 1,589 Swedish men for 12 years. They found that those following a low-fat diet (no butter, low-fat milk and no cream) were more likely to develop fat around the gut (central obesity) than those eating butter, high-fat milk and whipping cream.
One reason for this may be that fat is extremely satiating, so when people cut it out of their diet they consciously or unconsciously replace the calories with something else, often refined carbohydrates like white bread or pasta.
This may help to explain the failure of the Women's Health Initiative, a study which was run by the National Institute of Health in the US.
In this study 48,835 postmenopausal women were randomly allocated to either a low-fat or a control group. It was the largest long-term randomised trial of a dietary intervention ever conducted and over an eight-year period the women did manage to cut their fat consumption by over 8%. Unfortunately, this made almost no difference to their weight, when compared with the control group, and no difference at all to their risk of heart attack or stroke.
By contrast, in another study, published last year in the New England Journal of Medicine where 7,500 men and women were randomly allocated to either a low-fat diet or a much higher fat Mediterranean diet, the high-fat group clearly came out tops. On the Med diet, along with fruit, vegetables, meat and fish, the volunteers were encouraged to eat oily nuts, olive oil and have a glass of wine with their meal.
Not surprisingly the drop-out rate was much lower for those on the Mediterranean diet than those on the low-fat diet (4.9% versus 11.3%) and they also had much better health outcomes. There were fewer strokes and a 30% lower risk of having a heart attack.
This isn't a licence to start eating fry-ups or pouring cream down your throat, because even if the fat doesn't harm your heart, there's no doubt eating too many calories will. I still think most saturated fat, particularly if it comes from processed food, is unhealthy, but I have gone back to butter, Greek yoghurt and semi-skimmed milk, as well as cramming in lots more nuts, fish and vegetables.
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