Dietary fat intake – a complex issue

In recent times much confusion has arisen in the literature on optimal dietary fat intakes, particularly around the evidence for healthy eating advice to reduce saturated fat. The confusing findings emerging from long-term follow-up studies can be explained – but not simply. 

Dietary guidelines for population health 

On a global basis, dietary guidelines for human health in developed and developing countries are established by the World Health Organization (WHO). Different regions of the world adapt these global guidelines to suit their population needs. In Europe, the European Food Safety Authority (EFSA) adjusts the WHO guidelines into regional standards appropriate for the needs of people living in the EU. Similarly, the Institute of Medicine in North America develops the WHO guidelines into reference standards appropriate for the North American population. 

In Ireland, national food-based dietary guidelines are developed to meet the specific needs of the Irish population and are based mainly on EFSA standards with Institute of Medicine (IOM) guidelines used where there are gaps.

Food-based dietary guidelines focus on protecting populations against nutrition-related diseases which contribute to mortality and morbidity. 

In Ireland, the main contributors to ill health are cardiovascular disease (CVD) and cancer. Over the past three decades, the spiralling increases in obesity affecting all ages represents the most critical nutritional issue implicated in exacerbating several CVD risk factors (including type 2 diabetes) as well as directly increasing both CVD and cancer. 

Overweight and obesity in Ireland are more evident in men and increase with age in both genders, indicating high-risk of metabolic syndrome, which is a clustering of risk factors that substantially increases CVD risk. 

The metabolic syndrome is characterised by the presence of three out of five risk factors (abdominal distribution of body fat, elevated blood pressure, elevated fasting glucose, low HDL-C, high triglycerides).  

Diet-lipid-heart disease hypothesis 

Present-day knowledge on how dietary fat affects blood lipids has evolved over several decades. In the 1960s and 70s, after Dr Ancel Keys’ pioneering work linked dietary intake of saturated fat with blood cholesterol and CVD, blood lipid measurements mostly just involved total blood cholesterol. 

In these early days, dietary advice on fat was simple – saturated fat was identified as ‘the villain’ and PUFAn-6 as ‘the hero’. The benefits of other dietary fats (such as PUFAn-3 [eg. fish oils] and MUFA [eg. olive oil]) and detrimental effects of trans unsaturated fats were not appreciated until the 1980s/1990s, when blood lipid measurements included far more detail. 

Then, in addition to learning about what dietary factors reduce harmful LDL blood cholesterol and maintain/raise the beneficial HDL fraction, knowledge on how diet affects other harmful blood lipid combinations (eg. low HDL and high triglycerides) revealed the importance of these other dietary fats, as well as many other dietary factors. 

By the late 1980s, it was becoming clear that trans unsaturated fats were the real villains having much more detrimental effects on blood lipids compared with saturated fat. Trans unsaturated fat are mostly monounsaturated, but, for a time in Europe, hydrogenated marine oils were used, resulting in trans unsaturated fat including many polyunsaturated fatty acids also. 

In general, dietary advice rarely distinguishes different fatty acids in the main classes of saturated, PUFAn-3, PUFAn-6 and MUFA. Distinguishing particular PUFAn-3 fatty acids from fish oil sources (eg. EPA and DHA) represent an exception. 

Recently, claims that this is a significant oversight, especially in relation to saturated fat, have appeared in the literature. This view does not consider the distribution of the various saturated fatty acids in usual diets. The four saturated fatty acids that are most common in the Irish diet are all of longer chain length which, with one exception (stearic acid), have a blood cholesterol raising effect that extends to both LDL and HDL cholesterol fractions. 

While stearic acid does not raise LDL cholesterol, it has been associated with increased tendency towards clotting (thrombogenesis), although the evidence is mixed. Other practical barriers to dietary advice at the individual fatty acid level include:

• The limitations of food composition tables regarding processed foods which constantly vary in fatty acid composition depending on fat source used (this is influenced by availability and cost)

• The impracticality of reflecting such complex and changeable information on food labels

• The legal requirements for nutrition labelling do not facilitate such additional information.

In summary, while it has been known for a long time that different saturated fatty acids have different effects on blood lipids, there is justification for dietary advice treating saturated fatty acids similarly, with the exception of stearic fat. 

Stearic fat naturally occurs in cocoa and beef fat, so making some exception for dark chocolate (with a high cocoa content) may be warranted and this is always likely to be popular. However, due to the high energy density of dark chocolate, intakes need to be limited, even if the fat content is not particularly troublesome.

Based on strong evidence, the targets for optimal levels of blood lipids (LDL-C, HDL-C and triglycerides (TAGs)) have become much more stringent, especially for those at increased risk of CVD (presence of CVD, risk factors, diabetes etc). Healthy eating advice on fat focuses on achieving and maintaining optimal blood lipid levels in healthy and at-risk adults. However, the majority of those ‘at-risk’ require additional drug treatment (eg. statins) to achieve the very strict target blood lipid levels. 

Throughout life, healthy eating advice on fat intake aims to slow down the gradual atherosclerotic process. From middle age, such advice includes a focus on prevention of thrombotic events (myocardial infarction or stroke) through inclusion of nutrients such as PUFAn-3.

Nutritional goals

Diets providing a wide range of total fat can represent a healthy diet according to the recommendations of both EFSA and the IOM (20 – 35% energy).^1,2 This flexibility allows for tailoring of dietary advice to suit varying needs at the individual level. Depending on genetic make-up and health status, some individuals (eg. those at increased risk of gastrointestinal diseases such as colon cancer) will benefit more from a low-fat, high fibre-rich carbohydrate intake (ie. > 50% energy), while others (eg. those at high risk of type 2 diabetes) do better on more moderate levels of fat intake and lower intakes of carbohydrate (ie. 40 – 45% energy). It should be noted that this flexibility does not extend to extreme reductions in carbohydrate (ie. 10 to 20% of energy) which, among other nutritional disadvantages, provide grossly inadequate intakes of fibre.

Notwithstanding variable overall levels of fat in the diet, there is global consensus that saturated fat intakes need to be kept ‘as low as possible’.^2,3,4 Within this recommendation, it is recognised that some food sources of saturated fat are valuable sources of nutrients such as calcium and iron in the diet (eg. dairy and meat foods). Therefore, an upper limit of 10% of total energy intake is used for the development of food-based dietary guidelines that provide nutritionally adequate, low saturated fat diets.

Similar to overall fat intakes, there is corresponding flexibility inherent in the nutritional goals for overall carbohydrate intake levels because lower fat diets will be higher in carbohydrate and vice versa. However, just as there is consensus around the need to limit saturated fat, there is global consensus that added sugar in the diet needs to be limited as much as possible and that carbohydrates should primarily be complex and fibre rich in nature.

In most of the recent literature critically appraising healthy eating advice on fat and carbohydrate, there is awareness about the complexity of dietary fat, but not of dietary carbohydrate. Carbohydrate is made up of many different components and is equally, if not more, complex than dietary fat. In the simplest human health terms, some carbohydrates are beneficial (eg. components that are high in fibre and represent a complex mix of oligo-, and poly-saccharides) and some have adverse effects (refined carbohydrate low in fibre and high in sugar). The macronutrient goals for adults are outlined in Table 1 .

 (click to enlarge)

Fat intake and healthy eating goals

In this article three dietary intake studies are used to demonstrate how fat intake in Ireland compares with healthy eating goals and how this has changed over recent years. These studies include two national surveys of adult dietary intakes (2001 and 2011)⁶ and a very detailed study of adult fat intakes conducted in Ireland from 1996 to 1997.⁷

As food composition tables are not an accurate reflection of trans fat content of processed foods, Cantwell et al⁷ included fatty acid analysis of processed food intakes and validation of dietary fat intake assessment using adipose tissue biopsies.

Collectively these studies show that, on average, total fat intakes in Ireland, at 34% of energy, are at the high end of the recommended intake range. Average saturated fat intakes are between 30 and 40% higher than recommended (13-14% of energy vs recommended level of 10%) and average PUFA intakes are at recommended levels (between 6 and 7% energy). 

In terms of trans fatty acids, there are two types which are found in the diet. Firstly, naturally occurring trans fats which are present in dairy and meat products of ruminant animals and secondly, trans fatty acids synthetically produced by industry, referred to as industrial trans fatty acids. 

Cantwell et al⁷ showed trans unsaturated fat intakes were very high in the mid-to-late 1990s mainly due to industrial sources used in processed foods (such as margarines, cakes, pastries, biscuits and savoury snacks). These synthetic industrial sources made up 75% of trans fat intakes, but since this time, intakes have fallen considerably. 

Surveys involving fatty acid analysis of fat-containing processed foods carried out by the Food Safety Authority of Ireland (FSAI) in 2007⁸ and 2008⁹ indicate low levels of trans fat. This corroborates the findings of the national dietary intake surveys,⁶ which show that trans fat intakes from processed foods have fallen to very low levels.

Keeping intakes of naturally occurring trans fat that occur in dairy and meat fat from ruminant animals (cattle, sheep and goats) low is relatively easy for consumers who are advised to choose low fat dairy products and lean meat. However industrial trans fat in processed foods are not easy to avoid as they remain hidden among unsaturated fat and are not directly declared in food labelling. 

While trans fat content of food in Ireland is negligible (mostly less that 1%), some processed food in parts of eastern Europe remain high in trans fat. For these reasons the EU Commission has recently concluded that a legal limit on industrial trans of < 2% would be the most effective measure to reduce the amount of industrial trans fat in the diet.¹⁰ The EU Commission is currently carrying out an impact assessment across the EU to inform what action will be taken to control trans fats in the food supply.¹¹

Emerging evidence on dietary fat

Over the past seven years, several analyses of dietary fat intakes and CVD risk have been conducted. These mainly include studies that pool data from cohort studies and randomised control trials looking at CVD risk and saturated fat intake. Results that emerged from these studies are outlined in Table 2. 

 (click to enlarge)

The finding that stands out for dietitians from the research summarised above, is confirmation of the relevance of the food source of saturated fat in determining impact on CVD risk. Both EFSA and IOM^2,4 guidelines on minimising saturated fat in a healthy diet highlight the importance of some food sources of saturated fat because of other substances they contribute to the diet (eg. calcium and iron). 

This is fundamental to dietetic practice where a different approach is taken to saturated fat food sources that provide protective substances, compared with those that provide adverse substances, in terms of CVD risk. For example, the negative impact of saturated fat in dairy foods is ameliorated by the beneficial effects of dairy peptides and minerals on blood pressure. Alternatively, saturated fat from low-fibre snack foods are often associated with salt, which impacts negatively on blood pressure, or sugar which, in pre-type 2 diabetes impacts negatively on glycaemic control.

Considering how the findings summarised in Table 2 contribute to evidence for best practice, it raises the critical importance of study design, which determines the quality of this evidence. The recent Cochrane Library review²¹ has overcome the challenges involved in modelling different dietary fat intakes using cohort studies to determine effects on disease end points. 

This review²¹ used RCTs of at least two years duration and, as outlined in Table 2, found that reducing saturated fat intake led to a 17% reduction in CVD events, but not overall mortality. In agreement with other studies, this review also found that replacing saturated fat with PUFA has the most protective effect and there are no clear benefits when carbohydrate or protein was used as a replacement. 

Healthy eating advice and CVD risk

As outlined previously, average saturated fat intakes in Ireland,^{6, 7} are higher (by 30-40%) than the recommended upper limit of 10% energy.⁴ Therefore, reduction of saturated fat intakes is a key part of healthy eating advice. 

Reducing population saturated fat intakes by 30-40% is very challenging and warrants significant dietary changes because saturated fat is also present in many foods (such as fish, nuts and oils) which are used to replace foods primarily rich in saturated fat. 

Ongoing evaluation of the outcomes of such dietary interventions is critical in maintaining the evidence base for population-based healthy eating advice. 

Although the evidence base was not clearly outlined, it was recently suggested that replacing 5% of saturated fat energy with polyunsaturated fat would result in a 10% reduction in CVD.²² However this view limits the impact of dietary changes for healthy eating to modification of fat intakes for the sole purpose of improving blood lipid levels (diet-lipid hypothesis) and ignores all the other benefits of diet on CVD risk. 

Healthy eating advice in Ireland impacts on a multitude of risk factors for CVD, in addition to the diet-lipid hypothesis yielding greater benefits because of the many other CVD risk factors that are addressed. This includes food guidance that impacts positively on inflammation, blood clotting (thrombogenesis), blood pressure, blood lipids (cholesterol) and blood sugar levels.

In addition, critical issues in overall healthy eating advice have to be factored in, eg. guides on weight control and adequate nutrition to support optimal growth and development through critical life stages, including breastfeeding. 

To date, the divergent findings in the literature on the association of saturated fat and CVD can be largely explained by a number of factors including the failure to account for differences in carbohydrate used to replace saturated fat (high-fibre, low-sugar vs low-fibre, high-sugar), the high presence of trans unsaturated fat hidden in mono- and polyunsaturated fat during the time most of the large cohort studies were conducted and the research design of studies, where modelling different dietary fat intakes using cohort studies is flawed compared with RCTs.

The future

Ongoing developments in this area include the need to gain more understanding on how dietary factors impact population health (or health of individuals). For example, the interaction of diet and genetics, how diet during critical stages of growth and development impacts long-term health and how diet interacts beneficially with other risk factors such as physical activity etc.

Many foods needed to fulfil the requirements of a healthy diet contain saturated fat – that’s why choosing saturated fat containing foods that provide the most health benefits overall is the cornerstone of healthy eating advice in Ireland.

This article is a summary of two presentations made at INDI’s 2016 Annual Conference and Study Day, which took place in October

References available on request