Forget football rivalry. The most polarising issue on social media seems to be whether we should follow low fat or low carb diets. Doctors, nutritionists, academics and authors of self-help books line up on either side of the argument. Fat is bad. Fat is good. Carbs and sugar are evil. And so on. But hang on a minute... is there really good evidence that the amount of fat or carbs we have in our diet makes a difference to our health? Let’s take a look.
What are carbs and fat?
Carbs, short for carbohydrates, are the family name for starchy foods, like potatoes, pasta, rice, bread, cous cous and pasta, and sugars, like glucose, fructose and sucrose. This includes both the sugars that you find naturally in sweet foods, such as honey or fruit, as well as those added by manufacturers or in the home. Carbs contain 4 calories (kcal) per gram.
The fats group includes oils (olive oil, sunflower oil), butter, margarine, cheese, the fatty bits on meat and poultry, fish oils and nut oils. Fats contain 9 calories (kcal) per gram – that means they are twice as energy dense as carbs. Despite this, we need a reasonable amount of fats in the diet to protect our body cells, keep us warm, and provide fat soluble vitamins.
Within the fat and carb families, there are healthier and less healthy options. For example, many experts say we should eat more ‘unsaturated’ fats from vegetable, nut and fish sources and less ‘saturated’ fats from animal and dairy sources. This is because of historic data linking saturated fats with blood cholesterol levels – but more on this later.
In terms of carbs, experts say we should limit our sugar intakes (though fruit is fine) and eat much more of the starchy sources. This is the major point on which low carb advocates disagree with official diet guidelines.
Calories make us fat, not just carbs
Since the 1900s, our nation's intakes of refined sugar rose and obesity levels followed suit. Surely this means that obesity can be blamed on our sweet tooth? Not exactly as, at the same time, we did less manual labour, bought more cars, increased our sedentary leisure time and ate more calories than we burned. Interestingly, since the 1980s, sugar intakes declined then remained static yet obesity levels continued to rise. Sugar intakes are still too high – more than double the current target - but sugar is not the sole cause of obesity.
Instead, years of research show that weight gain is dependent on 'calories in' versus 'calories out'. A study in 1995 (source 1) found that secretly boosting the fat content of people’s diets (which raised energy density) increased their calorie intakes and body weight. When the study design was changed to keep energy density constant (2), high fat and high carbohydrate diets had similar effects on weight. So, the fact that dietary fat naturally delivers more calories per serving than carbs seems to be the most important influence on how many calories you eat overall.
Saturated fat isn’t all bad
The original 'saturated fat is bad' theory came from a study in the 1950s which compared fat intakes, blood cholesterol levels and heart disease rates in different countries (3). Nowadays, we would say this is pretty limited science. However, at the time it was ground breaking and had a massive impact on dietary advice.
Since then, several scientists and doctors have challenged the evidence claiming that cholesterol is just one of several risk factors for heart disease, and pretty unimportant at that. They point out that more recent studies show only a tiny impact of saturated fats on heart disease risk and no impact on mortality (death rates) (4).
Another complicating factor is that saturated fat is not one substance but a group of unique fatty acids, such as stearic, myristic and palmitic acids, which may have different effects on our health ranging from neutral to negative (5).
However, there’s still worrying evidence that eating certain saturated fats, like those in palm oil and lard, boost 'bad' LDL cholesterol (6) and increase inflammation (7).
Carbs aren’t killing us
A recent study reported in the press (8) claimed that low fat diets were killing us! The research looked at diets across the globe, finding that people who ate very high intakes of carbs were more likely to die early. However, critics (9) of the study pointed out that the very high carb diets were in developing countries which also had a major issue with malnutrition. Indeed, the optimal balance of carbs and fats was similar to what is already recommended in the UK.
Turning to sugars, a UK expert group reported (10) that eating a lot of sugars was linked to poor dental health in children and increased calorie (energy) intakes in adults (although not obesity). Sugary soft drinks were linked to calories, weight gain and type 2 diabetes. This led the UK government to recommend a maximum sugar intake of 7 cubes of sugar (30 grams) (11). Note that nowhere does it say that sugar should be completely avoided as some commentators would have you believe.
What should I eat?
It’s hard enough for people to decide what to eat every day without the need to plough through the evidence on fat and carbs. So, here are some tips:
· Carbohydrates have advantages and disadvantages. On the whole, the advantages (energy for the brain and body) outweigh the disadvantages (raising blood sugar levels) but only if you choose higher fibre options. These include wholegrain bread and rice, any type of pasta, potatoes with skins, quinoa, cous cous et cetera.
· Fats also have advantages and disadvantages. Studies are pretty clear that eating lots of fatty foods boosts calories and facilitates weight gain but the jury is still out on how much fat, and what types, are optimal for heart health. That will be the subject of a new UK expert report due out in 2018 (12). In the meantime, choose healthier fats such as oily fish, nuts, seeds, olive oil and rapeseed oil. If you prefer butter to margarine, that’s fine but watch the portion size.
· Sugars add extra calories into the diet and don’t contain any vitamins or fibre so go easy on them, especially if they are in ‘treat’ foods such as cakes, biscuits, desserts, soft drinks and confectionery. Low calorie sweeteners are a helpful option for hot drinks and baking at home.
· There is some evidence that low carb diets help control diabetes in the short term but there seems to be no advantage to using them long term (13). Talk to your health professional if you are thinking of following a low carb diet for this reason.
· There is also evidence that low carb diets perform better for weight loss in the short-term but that low fat diets are just as good in the long term (14). So, both types of diet seem effective and it’s up to you which one you think you can maintain. If you’re physically active, the higher carb, low fat diet may be more helpful.
· Finally, remember that lots of diets are healthy – there is no perfect diet for everyone despite what people on social media claim.