If anyone’s interested in the public health nutrition debate around the dietary guidelines, then here’s a summary and critique of our latest jousting round(s) with conventional wisdom.
In late 2016, an article from New Zealand in defense of the current dietary guidelines was published in the renowned medical journal The Lancet. While the authors (who included Prof Jim Mann, Dr Lisa Te Morenga, Prof Rod Jackson, and Prof Boyd Swinburn) ranged far and wide over the justifications for current guidelines, they cited no research critical of them, and ignored the most trenchant criticisms, which allowing them to exaggerate the importance of the evidence that it suited them to address.
An example of their straw man approach is here:
“But the case for reducing carbohydrate in general centres on whether there are benefits associated with reduction of starches and non-starch polysaccharides.”
Non-starch polysaccharides are, in general, what we call fibre. Yet there is no case being made for a weight loss benefit from reducing fibres that we have ever seen (di- and mono-saccharides, or sugars, would be the correct term to include instead). This seems like a rather childish game – “if you’re restricting carbohydrate you must be restricting fibre too, because that’s a carbohydrate”. It also betrays a poor awareness of food composition – a low carbohydrate fruit or vegetable will have more fibre per calorie than a high carbohydrate food.
They claim that dietary guidelines now include a high-fat diet; “a high-fat, high-carbohydrate Mediterranean diet, which is associated with a fairly low risk of many NCDs” but this is about 40% fat. This definition of high fat allows them to make the claim that “Meta-analyses of trials in people not attempting to lose weight show moderately lower bodyweight loss among those on diets fairly low in fat (30% or less total energy) than those on carbohydrate-reduced higher fat diets.”
But in fact not one of the control arms in those studies was “carbohydrate-reduced” in a medical sense, except for the fact that they were a little higher-fat than the interventions, merely representing the normal diets of their day, replete with white flour, partially hydrogenated oils, and sweets. Food quality is an important confounder in diet trials, and in most of the old low-fat and saturated-fat reduced studies the intervention groups were told to eat nuts and fish, whole grains, fruits and veges, and cut back on sugar, flour, and foods made with hydrogenated shortening, making it hard to attribute any benefits to increased carbohydrate or polyunsaturated vegetable oil.
The article re-iterated claims that saturated fat should be replaced with polyunsaturated fat, then stated that pitting one macronutrient against another risks confusing the public.
But what is wrong with telling everyone to eat healthy higher-carb diets?
There are three main problems that we can see:
Firstly, and most importantly, a large, and growing, proportion of the population is carbohydrate-intolerant. They have obesity, metabolic syndrome, excessive TG/HDL ratio, rising HbA1c, if not frank diabetes, and restricting carbohydrate is the most effective way to reverse this cluster of chronic disease associated with hyperinsulinaemia, which is increasing their odds of dying young from heart disease, cancer, diabetic complications and so on. Let us also be clear – carbohydrate intolerant means that these people have difficulty of disposing of dietary carbohydrates without advert metabolic effects of high triglycerides, high blood glucose and hyperinsulinemia.
Secondly, there is no evidence that full-fat dairy is anything but beneficial compared to low-fat (and there’s very little evidence comparing lean and fatty meats). If dairy fat, the most saturated animal fat in existence, doesn’t cause heart disease, then the basis for saturated fat restriction is very weak. There may be benefits from optimal intakes of certain polyunsaturated fats, but there’s no evidence that oils are the best source of these fats, nor that replacing other fats (which will always tend to limit the percentage of fat in the diet) is essential for benefit.
Thirdly, this “virtuous” diet advice might disadvantage the poor. Fat-free milk or (plain) yoghurt is the same price as full-fat milk or yoghurt, yet supplies half as much energy and fewer vitamins. Nuts, fish, and lean meat are more expensive per calorie than cheese, fatty cuts, and eggs. Light coconut cream, cream cheese, or sour cream is the same price as full-fat. If someone makes these low-fat choices, they need more energy from other sources (i.e. are left hungry), but have less money left to ensure its quality. Fruit and vegetables are relatively expensive, and a good whole grain bread costs about four or five times as much as white bread.
We tried to unpick some of these contradictions in a letter to the Lancet, which that journal was gracious enough to publish last week.
Dietary guidelines are not beyond criticism
Mann and colleagues (Aug 27, p 851) claim that criticisms of the dietary guidelines are not evidence-based. However, even by their own account, the promotion of reduced-fat dairy products in existing guidelines is not evidence-based, in view of the lack of association of dairy fat with cardiovascular risk, and the strong protective associations that exist between ruminant fatty acids and type 2 diabetes. This evidence contradicts the theory that the effect of dietary saturated fat on serum cholesterol is the cause of the association between serum cholesterol and cardiovascular disease.
Carbohydrate intolerance is increasing in developed and developing countries, as indicated by growing rates of diabetes, obesity, and metabolic syndrome, with the consequent expansion of health costs. Evidence is emerging that a major nutritional cause of modern chronic disease is the glycaemic environment created by the interaction between insulin resistance and foods with a high glycaemic load (GL), increased consumption of which has been a natural consequence of advice to limit dietary fat.
Mann and colleagues cited two meta-analyses [4,5] excluding weight loss trials, in which low-fat diets were only compared with low quality, high GL control diets. However, in view of the disappointing results in most trials in which a low-fat diet has been compared with alternative dietary interventions, the evidence is unclear on whether a fat-restricted bias in dietary advice is justified. Population dietary guidelines should be adapted to include advice on carbohydrate restriction, which is likely to be beneficial or protective for a large, but growing, proportion of people.
[References are in link]
That’s all. Seems uncontroversial enough right? What we’re saying is that some people do well with low carb advice, and there are today more than enough people in this category to justify including it as an option in dietary guidelines. We’re also saying that the evidence for fat restriction is not so strong that it needs to be a barrier to low carb diets, nor to good nutrition in general.
We weren’t just trolling (or The Lancet wouldn’t have published our letter – The Lancet is harder to get into than the Auckland housing market). We really hoped to be having a discussion about how the low carb idea can be incorporated into guidelines for the people who need it. The UK’s Public Health Coalition showed how this can be done last year, and we started the ball rolling with our own Real Food Guidelines in 2014.
But instead Mann et al. doubled down on their claims.
In particular, they pretended not to understand the idea of carbohydrate intolerance.
We find the link proposed by Henderson and colleagues between “carbohydrate intolerance” and “diabetes, obesity, and metabolic syndrome” puzzling. Carbohydrate intolerance is characterised by abnormal carbohydrate digestion as in lactose intolerance, and is not associated with abnormalities of glucose metabolism.
This from the people who think that carbohydrate restriction means fibre restriction.
Their new justification for low-fat dairy is interesting.
Low-fat, as opposed to full-fat, dairy products are generally recommended to promote consumption of essential nutrients and to allow intakes of food sources of unsaturated fatty acids without promoting excess energy intake.
Basically, we’ve been telling you to eat low-fat dairy so we can feed you extra oil without making you fat. Well guess what people, it’s not working and there is no “totality of evidence” as you always call it for this. In fact, such a body of evidence just doesn’t exist.
This passage makes a point which is not without substance, but deserves further comment:
However, we are unaware of any deleterious effects of minimally processed wholegrains or fibre-rich intact vegetables (notably legumes and pulses) and fruits—which are protective against diabetes, useful in its management, and with additional benefits in terms of cardiovascular and gastrointestinal disease.
On reading this reply, Nina Teicholz, author of The Big Fat Surprise: Why Butter, Meat and Cheese Belong in a Healthy Diet, which is an engrossing and indeed exciting (and deservedly best-selling) history of how the low-fat idea became embedded in official advice despite the continual appearance of evidence to contradict it, wrote a detailed response on PubPeer, which is a post-publication peer-review website.
Iterating a claim made in their initial Comment, the authors again assert that there is a “substantial body of observational, clinical trail, and experimental evidence…[to] support the recommendation to reduce total saturated fatty acids and that they might be replaced with unsaturated vegetable oils.” However, this body of evidence does not exist. There is now a total of at least 17 systematic reviews and meta-analyses looking at the totality of the evidence on saturated fats (1) which have largely concluded that saturated fats have no association with nor any effect on cardiovascular or total mortality.
Furthermore, the authors did not, as they state, summarize the above evidence in their original Comment. Instead, they chose a small selection of the evidence, which they then misrepresented to support their claims about saturated fat. I wrote about this when their Comment was first published (2).
The authors write that this body of evidence “does not negate advice to reduce total saturated fat,” but if a large body of rigorous, government-funded, randomized controlled trials testing saturated fats on more than 50,000 people have found no effect of saturated fats on cardiovascular mortality, then this does indeed negate advice to reduce total saturated fat.
The authors further write that they are “unaware of any deleterious effects of minimally processed whole grains or fibre-rich intact vegetables (notably legumes and pulses) and fruits. If so, then the authors are unaware of the large body of clinical trial research demonstrating that reducing total carbohydrate intake is highly effective for managing or even reversing obesity and diabetes. Thus, for people who are struggling with weight or diabetes, the high-carbohydrate foods listed above might be enjoyed in small amounts, but taken together as a majority of one’s diet, these foods would constitute a high-carbohydrate diet–which has been shown to be entirely ineffective, if not actually detrimental, in fighting these diseases.
That the authors characterize “carbohydrate intolerance” as “lactose intolerance” suggests that they have not read the literature on the effect of glucose and fructose on insulin, fat deposition, fatty liver disease and adverse lipid effects. This large body of literature describes the body’s unique metabolic response to carbohydrates, compared to other macronutrients. It seems uncharitable for the authors to accuse their critics of being engaged in a “continuing attempt to pit one macronutrient against another.” Science is not politics–or at least shouldn’t be. No one is “pitting” macronutrients against each other, like Hillary vs. Trump. Rather, researchers who discuss the observations that the body has a differing metabolic response to different macronutrients are simply following the duty of any scientist: to respond and explain the observations. If their explanation can be countered by a more convincing one, then that’s where a good scientific exchange could take place. Debate over science should be allowed to happen. To accuse researchers who disagree with them of seeking only to “perpetuate confusion,” as the authors write, appears merely to be an attempt to shut down legitimate debate.
Finally, it is untrue that “existing population-based dietary guidelines permit a wide range of macronutrient intake.” In the US, the three suggested “Dietary Patterns” are all modeled at more than 50% carbohydrates (3), which, by any definition, cannot be considered a low-carbohydrate diet.
People who say that carbohydrate restriction means fibre restriction, that carbohydrate intolerance can only mean lactose intolerance, and that 50% carbohydrate diets are the high fat extreme of a wide range of macronutrient intakes, should not accuse others of perpetuating confusion.
I (GH) added a bit of detail below Nina’s PubPeer comment about the circumstances under which fruit and wholegrains appear beneficial for diabetes prevention in epidemiological studies – the amount of carbohydrate from these foods associated with greatest benefit is actually minimal and would fit in most low carb diets.
We wrote another letter in response to Mann et al’s author reply, but as it is unlikely that The Lancet will keep a correspondence going over such a long period, we will post it here.
Dietary Guidelines are already confused.
The position that Mann et al propose be taken towards full-fat dairy foods, to await the results of further research into their benefits, is the opposite of what a public health nutrition approach should be. Nutritious, popular, and traditional foods should never have been advised against until after such research was completed. Advice to use unsaturated oils instead has been based on population studies that did not differentiate adequately between oils and wholefoods as sources of unsaturated fat, except in the case of olive oil, a traditional fat, and the olive oil studies have not shown that the avoidance of full-fat dairy or meat is required for benefit.[1,2]
Semantic quibbles about carbohydrate intolerance are inappropriate – most readers of the Lancet are familiar with the uses of the oral glucose tolerance test, and many will also be familiar with the importance of the fasting TG/HDL ratio, fasting insulin, or two-hour insulin response in predicting the future risk of chronic disease. These are measurements which, if abnormal, will be more sensitive to the ingestion of carbohydrate than of other nutrients.[4, 5] Carbohydrate intolerance is thus a simple formula allowing the public to understand a concept of considerable importance in public health.
Advice to use low-fat or lean versions of traditional foods, in part because of the outdated notion that eating the whole-fat versions of these foods leads to excess energy intake, does not in practice allow a wide range of macronutrient intakes. Instead we propose that it would help to reverse the burden of chronic disease to acknowledge the benefit for some of replacing foods rich in starch or sugar with less carbohydrate-dense whole foods. When conditions such as obesity, type 2 diabetes, and the metabolic syndrome are as widespread as they are today, it is remiss not to include those simple instructions most likely to assist with their reversal in public health diet advice.
 Buckland G, Mayen AL, Agudo A, et al. Olive oil intake and mortality within the Spanish population (EPIC-Spain). Am J Clin Nutr. 2012; 96: 142-149.
 Guasch-Ferré M, Babio N, Martínez-González MA, Corella D et al. Dietary fat intake and risk of cardiovascular disease and all-cause mortality in a population at high risk of cardiovascular disease. Am J Clin Nutr. 2015; 102(6):1563-73. doi: 10.3945/ajcn.115.116046.
 Temelkova-Kurktschiev T, Henkel E, Schaper F et al. Prevalence and atherosclerosis risk in different types of non-diabetic hyperglycemia. Is mild hyperglycemia an underestimated evil? Exp Clin Endocrinol Diabetes 2000; Vol. 108(2): 93-99.
 Volek JS, Feinman RD. Carbohydrate restriction improves the features of Metabolic Syndrome. Metabolic Syndrome may be defined by the response to carbohydrate restriction. Nutrition & Metabolism. 2005; 2:31.
 McKenzie MR, Illingworth S. Should a Low Carbohydrate Diet be Recommended for Diabetes Management? Proceedings of the Nutrition Society. 2017; 76 (OCE1), E19