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High fat vs. low fat diets - where do we go next?

Date added: 06/09/2017

This week, I wrote an opinion piece in the Herald defending the Scottish policy to promote the use of semi-skimmed milk in schools. A local farmer on one of the islands had wanted to supply his local school but only produced full cream milk. The school was forced to decline due to the national policy. Result: an angry farmer questioning why policy makers favoured lower fat milk when full cream milk was superior.

The 'evidence' cited by the farmer was a cross-sectional study which reported an association between use of full cream milk and lower body mass index in 2700 Canadian children.  As some of you will know, I'm not a fan of basing dietary guidance solely on epidemiology as these kinds of studies include many different factors which could influence the result, like physical activity levels or fruit and vegetable intakes. It's easy to use statistics to pinpoint a single 'cause', but it's much more complex than this.

The story generated plenty of discussion on social media, especially from the growing army of low carb/high fat enthusiasts who continually pop up to criticise official dietary guidelines.  Let's look at some of their claims.

Obesity is caused by carbs and sugar, not fat

Since the 1900s, our nation's intake of refined sugar consumption 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 used. Interestingly, since the 1980s, sugar intakes declined then remained static yet obesity levels were rising until recently. Sugar intakes are still too high - well above the 5% population target - but sugar is not the sole cause of obesity.

Whether we like it or not, obesity is down to 'calories in' exceeding 'calories out', not any one food or nutrient. The physiological equation is driven by social and environmental factors, like cheap high calorie foods, retail promotions, more purchasing opportunities, more sedentary activities like phones/iPads, and a negative attitude to exercise. These are all targets for new policies.

Turning to the evidence, Stubbs covert dietary manipulations in the 1990s showed that boosting the fat content of the diet, and therefore the energy density, increased energy intake and body weight. When the study design was changed to keep energy density constant, high fat and high carbohydrate diets had similar effects on weight. So, the fact that dietary fat naturally delivers more calories per serving (9 kcal) than carbohydrate (3.75 kcal) is the most important influence on how many calories you eat overall.

There is no evidence that saturated fat is bad for you

The original 'sat fat is bad'  view came from the Keys Seven Countries study which I think most of us now would say was pretty bad science. However, there is plenty of newer evidence from both observational and randomised trials that saturated fat isn't benign. For example, a Cochrane systematic review found a "small but potentially important reduction in cardiovascular risk on reduction of saturated fat intake". Foods high in saturated fat, like palm oil and lard, have been found to boost 'bad' LDL cholesterol while blood levels of saturated fats, particularly palmitic and stearic fatty acids, were linked with inflammation in a human study.

After carrying out a meta-analysis of randomised controlled trials (RCTs), the gold standard of evidence, Prof Dariush Mozaffarian from Tufts University in the US said of his findings: "These provide evidence that consuming polyunsaturated fats (PUFA) in place of saturated fats (SFA) reduces coronary heart disease events in RCTs. This suggests that rather than trying to lower PUFA consumption, a shift toward greater population PUFA consumption in place of SFA would significantly reduce rates of coronary heart disease". 

These studies clearly show that there is an issue with high saturated fat consumption. How much we should eat compared with total fat, polyunsaturated fat and monounsaturated fat remains to be decided and a fuller analysis of this by an Expert Panel is long overdue. We also need to be more specific about the types of saturated fat as there is growing evidence that certain high fat foods are less atherogenic than others.

Cutting fat and replacing it with carbs is not the answer

This is absolutely true if dietary fat is replaced by sugar and higher glycaemic index carbohydrates - the idea that we switch our Greek yogurt for a fat-free, sugary one. This could explain why studies comparing low fat and high fat diets give contradictory results; with some showing superiority for low fat/high carb diets (reduced LDL cholesterol, weight loss) while others suggest negative effects (high triglycerides, lower 'good' HDL cholesterol).

A 2013 meta-analysis noted such diversity of results that it concluded that the data "do not allow for an unequivocal recommendation of either low-fat or high-fat diets in the primary prevention of cardiovascular disease". Another meta-analysis decided that low carb diets performed better for weight loss over 6 months but, by a year, both low carb and low fat diets produced similar results. It is no wonder that low fat and low carb warriors are in constant conflict!

Following official advice just got us more obesity, diabetes, heart disease etc

It's a favourite cry of low carb enthusiasts that years of following official advice about low fat, high carbohydrate diets have simply produced more ill-health. Yet, if we look closely at dietary surveys, we see that people are not eating the recommended diet.

Intakes of fibre are half the 30g that we should eat, saturated fat is 2-3 percentage points above the ideal ratio of 10% calories, and sugar intakes are 2-3 times above the 5% calories target. Salt intake remains too high (8g vs. 6g). As well as this, many people have intakes of vitamin D, folate, potassium, iron, magnesium and selenium that fall well below optimal levels.

Neither are we in the UK eating a diet that in anyway resembles the recommended Eatwell Guide as evidenced by the fact that around a quarter of our daily calories comes from sweets, chocolate, soft drinks, alcohol, crisps and chips!

Where next?

In the face of such contradictory evidence, what are our options? I am personally uncomfortable with either blindly supporting government recommendations simply because they are 'official', or opting for wholesale change on the basis of cherry picked evidence and single studies. This gives us two options:

Knock heads together until a proper international expert panel takes a look at the optimal balance between fat, carbohydrate and protein in the diet. This could differ depending on the desired outcome e.g. type 2 diabetes control, weight loss, supporting high physical activity, general health. Vulnerable groups, such as children and elderly, may require different ratios. Crucially, the expert panel should be open-minded and have no financial conflicts of interest, such as Atkins Foundation, books to sell, or food industry funding.

Forget nutrients and focus on food. Our diets are much more than a collection of nutrients and few foods are 100% fat, carb or protein. Perhaps a complex scientific landscape needs a simply answer - to eat a balance of nutritious foods (wholegrain carbs, oily fish, nuts, seeds, lean meat and poultry, plenty of veg and fruit), drink tap water and keep discretionary foods and drinks (alcohol, soft drinks, sweets, chocolate, creamy desserts) to a minimum. Regular exercise is a must for maintaining a healthy body weight. This is the kind of overview promoted by the Eatwell Guide and food pyramids.

Whichever option we choose, we need to acknowledge the limitations of policy - it is designed for the average person, not the individual. However, an up-to-date, evidence-based view on the right balance of foods and nutrients in our diet is always welcome, if only to end the long-running spat of fats vs. carbs!