The Diet Wars: Low-Carb vs. Low-Fat

The Diet Wars: Low-Carb vs. Low-Fat

Great medicine, like great journalism is about finding truth. Once that truth is uncovered, journalists seek to share it with their audience in a compelling way. Doctors must do the same with their patients. In the same way that a great journalist tailors their writing to the reader a doctor must tailor treatment to their patient. Unfortunately, when it comes to the science of diet and nutrition, uncovering truth is as elusive as trying to find shoes that you like in your size in the January sales. In other words, close to impossible.

The task is made even harder by the often extremely dogmatic stance adopted by many ‘experts.’ Extreme viewpoints on diet are understandable. Many take up their position with absolute sincerity and set upon a crusade to improve the health of the population. Others are merely eyeing the considerable commercial opportunities. Perhaps the fiercest current debate in diet and nutrition are the relative merits of ‘Low carb’ and ‘Low fat’ diets. Add in the term ‘weight loss’ and this debate escalates into all out war.

The long-held view that ‘low-fat’ was best for health and weight loss had been railed against by ‘low carb’ advocates who highlight the failures of the prevailing dietary guidelines in controlling obesity and chronic disease. The proponents of the ‘low fat’ dogma counter that the ‘low fat’ dietary guidelines are valid, just poorly adhered to. The two sides seemed locked in scientific combat, unwilling to give ground. Bullets and shells have been replaced by meta-analysis and randomised controlled trials.

The great thing about a meta-analysis, making them a much more palatable weapon than artillery shells, is that they pool lots of data together. In medicine, having big data means that trends that might be mere blips in small trials becomes clear and obvious. The drawback of this pooling of lots of data is that not all the trials will have been done in the same way. Sometimes the included trial may not even have compared exactly the same thing. This unhelpfully skews the results.

The current advice for people who are overweight or obese is to adopt a low fat diet. A recent meta-analysis sought to establish which type of diet was best for weight loss. The study included 17 trials involving some 1797 overweight and obese adults. The studies had a variety of durations from 8 weeks to 24 months with an average duration of only 35 weeks. So this is not particularly helpful in informing about long-term, sustained weight loss.  A further drawback was that there is no single definition of exactly how much carbohydrate constitutes a low carb diet. So the amounts of carbohydrate consumption in different studies varied. In common with many studies on diet, the outcome was suggestive of publication bias. This occurs where studies are more likely to be published if they demonstrate a more compelling result. Many of the included studies require participants to remember what they have eaten, often several weeks previously. An unenviably tricky task. Indeed, such was the strain on both appetite and memory that many participants in both groups dropped out.

The results showed that the low carb diet came out on top. But only just. Indeed, although the low carb diet was statistically better it was not better by so much as to be practically relevant to every day life. The average difference in weight loss between the types of diet was 2kg. Furthermore, the greater the weight loss the less likely it was to be due to one or other of the two diets. This is unsurprising given that the average calorie difference between each diet was not significant. In the past 50 years the waistline of the population has expanded in symbiosis with the concurrent increase in the sizes of the portions we eat and the packaging and tableware we eat from. In other words, we eat many more calories now than we used to and concurrently obesity has, well, expanded.

The proponents of the low carb would argue that the definition of low carb diet was simply not stringent enough. However, a meta analysis comparing very low carbohydrate diets and low fat diets yielded even less impressive results. Here the amount of carbohydrates in the low carb diets was restricted to less than 50g or less than 10% of daily energy intake from carbohydrate. The people in the low fat group were restricted to less than 30% of their daily energy consumption from fat. Despite this the average difference in weight loss was only 0.91kg. The adherence to the very low carbohydrate diet whilst good in the short term, quickly started to wane.

There have also been two helpful randomised controlled trials on low carb diets. In the first of these, lasting two years, low fat, low carb and Mediterranean diets were pitted against each other. The patients in the low fat group lost an average of 2.9kg, in the Mediterranean diet lost an average of 4.4kg and in the low carb diet an average of 4.5kg. The low carb group had the highest rate of dropout. However, low carb dieting in this trial seemed to have a beneficial effect on participants’ cholesterol profiles. In the Mediterranean diet group blood sugar control improved.

A year later, a randomised control trial in the same journal compared low calorie diets with differing compositions of macronutrients (protein, fat, carbohydrate) ranging from low to high levels of carbohydrate. The results showed that in each diet participants lost around 3.3-3.5kg at two years. So it seems likely that the most important factor in losing weight through diet is reducing calorie consumption, despite claims to the contrary. Given that, at present, in Western diets the dominant macronutrient is carbohydrate it is unsurprising that limiting this should lead to successful weight loss. Indeed, head to head this is the most successful dietary intervention to loose weight. However, limiting fat or carbohydrate is likely to lead to weight loss. The important question, is which is the most suitable way to loose weight for you?

To answer this question it is important to consider your current and past health, activity levels, age, eating patterns, taste, beliefs and lifestyle. From a doctor’s perspective, particularly important is health. With this in mind it is worth remembering that high intake of sugar causes the body to produce insulin. Over time with prolonged intake of large amounts of sugar the body becomes less sensitive to the effects of insulin, so called “insulin resistance.” This condition is the cornerstone of many chronic diseases such as type 2 diabetes and coronary heart disease. The diet that beats both low carbohydrate and low fat diets in terms of real health outcomes though, is the Mediterranean diet. This has been found to have beneficial effects in reducing the risk of heart disease and Alzheimer’s disease as well as being on a par with the low carbohydrate diet in terms of weight loss. The Mediterranean diet itself places emphasis on eating lots of fresh vegetables and fruit and healthy fats such as those found in olive oil and oily fish, nuts and pulses.

So in choosing a diet to help you loose weight make sure you choose the one that is right for you. Keep in mind the positive impact that the right diet can have on your long-term health aside from it’s weight reducing benefits. If you can stick to whole, non-processed food, mainly vegetables, and a little high quality meat and fish you are on the right track.

Dr Hugh Coyne

Private GP, Parsons Green