Professor Iain Broom: Fake news – low fat, high carbohydrate dietary advice
Professor Iain Broom, now an emeritus professor, recently retired as director of the Centre for Obesity Research and Epidemiology (CORE) at Robert Gordon University in Aberdeen. Previously a lecturer and senior lecturer in surgery and chemical pathology, and then clinical professor of metabolic medicine at the University of Aberdeen, he has also held various chair and directorial positions in NHS Scotland. Prof Broom sits on various research committees and advisory boards, and has authored over 200 research papers on topics including obesity, diabetes, sepsis and cancer. This talk was given at the Public Health Collaboration conference 2018 at the Royal College of General Practitioners in London.
Prof Broom begins by noting how the standard low fat, high carbohydrate dietary advice has failed to prevent and control diabetes and its associated health problems. Two limitations that can stop us from properly understanding nutrition, he says, are that modern healthcare professionals receive very little nutritional training, and that randomised controlled trials, which are designed to investigate just one variable, can be ineffective for looking at dietary interventions. As opposed to the standard low fat advice, Prof Broom says that low carbohydrate diets offer improved metabolic control that is independent of weight loss. This is a similar story to what is seen with bariatric surgery, with patients seeing improved diabetes control long before they lose any significant weight.
So what was the evidence supporting the introduction of low fat dietary guidelines in the 70s and 80s? Firstly, a prominent American physiologist named Ancel Keys published the seven countries study, concluding that saturated fat intake was linked to coronary heart disease. However, Keys arguably used his previous findings to select only countries that he knew would help to prove his hypothesis, and he also ignored available data that implied sugar was responsible. In addition to Keys’ work, the Finnish dietary study found that reducing saturated fat and increasing unsaturated fat reduced cholesterol levels. This study too looked at coronary heart disease as an outcome, but it changed more factors than just dietary fat.
For the argument against the introduction of low fat dietary guidelines, Prof Broom presents a much stronger case. Firstly, the 22 countries study, which Ancel Keys actually had the data for but did not use, found no associations between dietary fat and coronary heart disease. Instead, it suggested refined carbohydrates and tobacco were linked factors. Since then, there have been numerous studies and reviews finding no associations between dietary fat and cardiovascular disease, including the Women’s Health Initiative, a 2012 FAO review and a 2012 Cochrane review.
Mechanistically, Prof Broom explains that a low fat, high carbohydrate diet leads to inappropriately high insulin levels which drive fat production. He explains that, in insulin resistant people, insulin cannot perform its role of glucose usage in muscle as well, yet it is still able to stimulate fat synthesis in adipose tissue. What this means is susceptible individuals can both gain weight and develop insulin resistance over time by eating a high carbohydrate diet.
Prof Broom displays two MRI images to contrast the effects subcutaneous fat and liver fat can have on metabolic health. Despite both people having the same BMI and waist circumference, the person with the more liver fat has a worse lipid profile. Although obesity is not a cause of type 2 diabetes per se, Prof Broom explains that it is still the primary risk factor and that both these issues can be sorted out by changing the dietary guidelines.
On that topic, Prof Broom shares the composition of the diet he used in his obesity clinic to treat people with type 2 diabetes. This was a low calorie diet as well as a low carb ketogenic diet, and so would be appropriate for weight loss but not for weight maintenance. One common criticism of low carb diets is that they are poorly tolerated and hard to stick to. However, Prof Broom presents the completion rates of four different studies comparing low carb vs low fat diets, with the low carb diets having much higher adherence in each case. He also adds an important point on the palatability of foods, explaining that sugar and other additives are often used to make low fat foods tolerable. There does seem to be a point where people tend to stop their diet after about 6 months, and Prof Broom opines that a behaviour change programme alongside dietary interventions may help with sustainability.
Concerning type 2 diabetes and weight loss, one issue that is commonly seen is that people with diabetes will lose much less weight than non-diabetics while on dietary interventions. However, Prof Broom explains that this is only true in the context of a high carbohydrate diet with calorie restriction. Conversely, he presents data showing a low carb diet and a very low calorie, ketogenic diet to give equal weight loss for diabetics and non-diabetics alike.
Prof Broom reiterates that low carbohydrate diets improve metabolic control by decreasing insulin levels, adding that high insulin levels cause the atherosclerosis and cardiovascular disease seen in type 2 diabetes. He explains that this ‘atherogenic profile’, consisting of reduced HDL, increased triglycerides and decreases in LDL particle size (small dense LDL particles are thought to be involved in atherosclerosis), can be driven by excessive carbohydrate consumption. A study comparing low carb and low fat diets in management of severe obesity found the low carb diet to be more effective in reducing weight, triglycerides and insulin resistance. Another study found that a low carb, ketogenic diet can increase LDL particle size over 3-6 weeks. Prof Broom notes that benefits can be seen even with small reductions in carbohydrate, as the Omniheart study found reductions in blood pressure on a 48% carbohydrate diet compared to a 58% carbohydrate diet.
Prof Broom rounds the talk off by rejoicing that mainstream thinking around nutrition is gradually changing, with some countries, such as Brazil and Sweden, even going as far as to reconsider their nutritional guidelines. He echoes the words of Richard Smith, calling low fat guidelines an uncontrolled experiment with disastrous results, i.e. epidemics of obesity and type 2 diabetes.