NHD Issue 151 Saturated fat and health Friend or foe

COVER STORY

SATURATED FAT AND HEALTH: FRIEND OR FOE? The consumption of saturated fat (SFA) and its role in disease prevention has been hotly debated in the past few years, with some stakeholders questioning the relevance of this guideline for public health. So, where does the debate come from and what is the current available evidence actually saying?

In August this year, the latest report on SFA and health from the Scientific Advisory Committee on Nutrition (SACN) maintained the reference value for SFA unchanged, recommending that SFA should not contribute to more than 10% of total energy intakes (see Table 1).1 According to the latest National Diet and Nutrition Survey (NDNS), published in January 2019,2 British adults aged 19-64 years consumed in average 11.9% of their total dietary energy (including alcohol) from SFA. The proportion is higher in children aged 4-10 years (13.0%), in teenagers aged 11-18 years (12.4%) and in older adults aged 65-74 years (14.3%). In all age groups, the foods contributing the most to dietary SFA are mainly meat and meat products, dairy products and cereals or cereal products (pizza, biscuits, buns, cakes, pastries, fruit pies and puddings).

This leaves most Britons exceeding the current recommendation that SFA should not exceed 10% of total energy.

WHERE DOES THE DEBATE COME FROM?

In 2014, a group of researchers published a paper challenging the relevance of the current guidelines on SFA.3 Their conclusion that evidence does not support current guidelines of reducing SFA for the prevention of cardiovascular diseases, was heavily reported in the media, with sensational headlines stating that, ‘Butter is Back’.4

Headlines like this, along with other studies or opinion papers showing no associations between SFA consumption and cardiovascular disease risk,5,6 popularised the idea that high SFA consumption does not increase the risk of cardiovascular diseases. This debate has caused confusion amongst the public and a lack of trust in public health guidelines, whilst reinforcing the existing beliefs in certain communities of people who follow high-SFA diets like keto or paleo.

Laury Sellem Freelance Nutrition Consultant and Doctoral Researcher After pursuing a BSc and MSc in Nutrition in France, Laury is now a PhD candidate in the University of Reading. Her research focuses on dietary fat and cardiovascular health. www.lauryfrench nutritionist.com thefrenchnutritionist laurysllm

NUTRITION RESEARCH IS NOT BLACK AND WHITE

Unfortunately, the heavy media coverage of the aforementioned studies failed to critically assess the methods used by researchers to form their conclusions. As often in science, nutrition studies always have limitations, preventing researchers from formulating clear cut conclusions.

The 2014 paper from Chowdhury and colleagues, which re-initiated the debate on SFA and cardiovascular health, is a great example of the lack of nuance often seen in the media.3 In their study, the researchers compiled the results from several individual studies on SFA and cardiovascular diseases, in order to get a clearer image of the overall effect of SFA. This type of study, a meta-analysis, is often considered the most reliable evidence when it comes to nutrition research. However, meta-analyses need

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COVER STORY Table 1: UK government dietary recommendations for energy and macronutrients and salt for men and women in the UK Energy (adults 19-64 years old)

2500 kcal/day for men; 2000 kcal/day for women

Proteins

0.75g of proteins per kilogram of bodyweight

Total fats Of which - Saturated fats

Reduce to about 35% of dietary energy

MUFA

No specific recommendations for MUFA

n-6 PUFA - Linoleic acid - Long chain n-3 PUFA - Alpha linolenic acid

6.5% of dietary energy - Provide at least 1% of total energy - There is no specific recommendation for long chain n-3 PUFA in the UK. Most health organisations recommend a daily intake between 250-500mg/day - Provide at least 0.2% of total energy

Trans fats

Provide no more than about 2% of dietary energy

Carbohydrates Of which - Free sugars - Dietary fibre

Approximately 50% of total dietary energy

Salt

6g/day

to be conducted in a rigorous way to ensure that all the individual studies included in the overall analysis are comparable. The comparability of all studies is one of the main limitations of the analysis conducted by Chowdhury and colleagues. Indeed, they looked at the reduction of SFA from the diets, without assessing the overall dietary patterns of participants, or whether SFA was being replaced by carbohydrates, proteins, or unsaturated fat. However, this major caveat was not discussed in the articles published in the media aimed at the general public.

More importantly, Chowdhury and colleagues admitted in their paper that their analysis could not establish with certainty that reducing SFA would not lower the risk of cardiovascular diseases. Indeed, the studies they included in their analysis provide observational evidence. In this type of study, researchers follow a large number of people over time, observing their dietary habits and then looking at participants who will develop certain diseases. Thanks to their observations, they can can make conclusions on associations between exposures (here, the consumption of SFA) and health outcomes (here, cardiovascular diseases). However, the lack of association between the two is not sufficient to rule out SFA as a potential cause of disease.

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- Reduce to no more than about 10% of dietary energy

- Should not exceed 5% of total dietary energy - 30g/day

LOW-SATURATED FAT DIETS ARE NOT ALL EQUAL

As most people are likely to maintain a constant energy intake over time, reducing SFA from the diet often means compensating with an increase in other nutrients. In particular, reducing SFA can be associated with an increase of carbohydrates or other type of fats like polyunsaturated or monounsaturated fat. In their 2019 report,1 SACN performed 47 meta-analyses on SFA and a number of health outcomes, including body composition, cardiovascular diseases, blood pressure, Type 2 diabetes, cancer and dementia. SACN concluded that dietary SFA should be mostly replaced with unsaturated fat, with more evidence showing benefits of replacement with polyunsaturated compared to monounsaturated fat.

In contrast, the few studies investigating the effects of replacing SFA with carbohydrates suggested that this eating pattern could be linked with increased risk of coronary heart disease7 and increased fasting insulin levels.8 Nonetheless, the research on carbohydrates as a replacement for SFA is still limited and the SACN report did not find clear evidence on overall cardiovascular diseases, cancer, Type 2 diabetes, body composition or blood pressure.

COVER STORY Finally, SACN reported no significant effect from replacing SFA with proteins, which reflects the contrasted evidence available on this topic so far. The effect of protein on heart health could depend on the source of protein, with some studies suggesting that replacing SFA with animal proteins could be associated with an increased risk of cardiovascular diseases,9 whilst plantbased protein may have the opposite effect.10 SFA AND HEALTH: SUMMARY FROM THE 2019 SACN REPORT

Amongst the numerous health outcomes investigated by SACN,1 the current evidence base was too limited or poorly designed to draw conclusions about the effect of reducing SFA on stroke, blood pressure, Type 2 diabetes, markers of glycaemic control (i.e. fasting glucose and insulin levels) and dementia.

Cardiovascular diseases The authors reported that reducing dietary SFA is likely to decrease the levels of circulating cholesterol (both HDL-cholesterol and LDLcholesterol), but may not impact other blood lipids like triacylglycerides. This is particularly relevant for cardiovascular health, since elevated LDL-cholesterol is an important risk factor in the development of cardiovascular diseases.11 This finding was more consistent when SFA was replaced by either polyunsaturated fat (which constituted most of the available studies) and monounsaturated fat, rather than carbohydrates.

Cancer The pooled analysis of observational studies on cancer revealed that a link between SFA and cancer is unlikely. However, SACN only investigated the risks of colorectal, pancreatic, lung, breast or prostate cancers. Moreover, the lack of well-controlled intervention studies, which are considered as the ‘gold standard’ to establish cause-effect relationships, prevented the authors from firmly concluding on SFA and cancer risk.

RESEARCH ON SFA: WHAT’S NEXT?

In order to tackle the ongoing debate on SFA in national guidelines, for the prevention

of cardiovascular diseases, research on personalised nutrition is making substantial progress. In particular, researchers have suggested that the lack of clear associations between SFA consumption and the risk of cardiovascular disease in meta-analyses could be due to the fact that the metabolic response to a high-SFA diet could vary amongst individuals: some people would have elevated LDL-cholesterol if they consume high amounts of SFA, whereas others could maintain stable LDL-cholesterol levels.12 Ongoing research in the Universities of Reading, Surrey and Imperial College London, is trying to investigate the mechanisms underlying this phenomenon.13 Findings could then contribute to the evidence base for more personalised recommendations on SFA consumption.

Moreover, emerging research suggests that the food sources of SFA could have different effects on health. In particular, the consumption of dairy foods is consistently associated with unchanged or decreased risk of cardiovascular diseases, despite being important sources of SFA in the British diet.14 Thus, studying SFA in the context of wholefoods and dietary patterns would provide a different perspective on their place within a healthy diet.

TAKE HOME MESSAGES

• The current national guidelines recommend that SFA should contribute to a maximum of 10% total energy to prevent the risk of cardiovascular diseases.

• In average, the British population still exceeds this recommendation. The biggest contributors to SFAs are dairy and meat products.

• The relevance of this recommendation has been hotly debated, mostly due to oversimplification of the evidence in the media.

• Nonetheless, current evidence consistently suggests that replacing SFA with unsaturated fat may reduce the risk of cardiovascular disease and may help lower LDL-cholesterol levels.

• In contrast, replacing SFA with carbohydrates may not have such beneficial effects on health. www.NHDmag.com February 2020 - Issue 151

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