An in-depth review
This HEART UK Nutrition Academy webpage has been funded by Novartis Pharmaceuticals UK Ltd who have had no input into the content or development of this material. |
Welcome to the first of our four-part blog series, where we will explore essential dietary interventions for achieving optimal blood lipids.
Dietary advice is pivotal in lipid management for optimal cardiovascular health. The leading cardiology organisations, such as the American Heart Association1, European Society of Cardiology2, and NICE3, have aligned their primary and secondary prevention guidance with similar dietary recommendations.
A critical dietary aspect of lipid management hinges on the quality of fats consumed and is the focus of our first blog.
The balance of dietary fats consumed by an individual can significantly influence the quality of plasma lipids: low density lipoprotein (LDL) cholesterol, high density lipoprotein (HDL) cholesterol and triglycerides (TGs)2,4–6.
Plasma lipids and cardiovascular risk
It is well established that elevated plasma LDL cholesterol is atherogenic and both raised LDL cholesterol and TGs are directly associated with increased cardiovascular disease (CVD) risk2,6–9.
With regard to HDL cholesterol, studies have found an inverse association with atherosclerotic CVD as a consequence of HDL's role to remove cholesterol from peripheral tissue and the circulation and return it to the liver for removal. However, recent evidence indicates a U-shaped correlation, with very high levels potentially increasing risk2,10. Furthermore, intervention studies trialling medication to increase HDL cholesterol have failed to demonstrate a benefit in cardiovascular outcomes11,12. Protective effects of HDL cholesterol seam to peak at levels around 1.3-1.4 mmol/L with little additional benefits seen at levels beyond 1.4mmol/L13.
Table 1. Target blood lipid levels in primary care14. This table is a general guide for ideal cholesterol and triglyceride levels for healthy adults in the UK. Blood lipid targets for secondary prevention should be tailored to each patient i.e., for those with high cholesterol or at higher risk e.g., existing heart disease or diabetes, target levels may be lower.
Blood lipids | mmol/L |
Total cholesterol | <5 |
Non-HDL cholesterol | <4 |
LDL cholesterol | <3 |
HDL cholesterol |
>1 for men; >1.2 for women Ideally, around 1.4. Very high levels may not give extra protection. |
Fasting triglycerides | <1.7 |
Non-fasting triglycerides | <2.3 |
Non-HDL cholesterol includes all the atherogenic lipoprotein particles: LDL, very-low-density lipoprotein (VLDL) and intermediate-density lipoprotein (IDL).
The primary role of dietary and pharmaceutical interventions is to lower plasma LDL cholesterol and elevated TGs. In particular, elevated LDL cholesterol levels and the duration of exposure are now undisputed (through Mendelian, observational and randomised controlled trials) direct causal risk factors for the development of atherosclerotic CVD2,6,7. 1 mmol/L reduction in LDL cholesterol has been associated with6:
|
UK prevalence & impact of high cholesterol
Dietary fats: quality over quantity
It is the balance of the different types of dietary fats rather than the quantity of fat in the diet that will determine the impact on plasma lipids1,2,5,6,15.
Saturated fat
Saturated fats, especially those rich in lauric (C12:0), myristic (C14:0) or palmitic (C16:0) fatty acids significantly increase LDL cholesterol6,15,16.
The mechanism: high saturated fatty acids reduce liver LDL receptor numbers and expression, thus reducing the liver's capacity to take up cholesterol from the circulation for removal via bile acids.
What replaces saturated fat will determine the impact on lipids
The nutrient replacing saturated fat in the diet is crucial in determining whether plasma lipids improve, remain unchanged, or deteriorate6,15–17. Thus, when considering outcomes of clinical studies investigating the impact of dietary fats, it is important to critique the methodology and ensure the saturated fat replacement is identified.
Table 2. The impact on blood lipids of replacing 5% dietary energy from saturated fats with other fats and carbohydrates15
Nutrient replacing 5% energy from saturated fat | Change in blood lipids - mmol/L | ||
LDL cholesterol |
Triglycerides |
HDL cholesterol |
|
Polyunsaturated fatty acids (PUFA - in the main n-6) |
-0.28 | -0.05 | -0.02 |
Monounsaturated fatty acids (MUFA) | -0.21 | -0.02 | -0.01 |
Carbohydrates* | -0.17 | +0.55 | -0.05 |
*carbohydrate's impact on lipids will vary depending on whether refined or whole grain carbohydrates replace saturated fats.
MUFA and PUFA for optimal LDL cholesterol reductions
Replacing saturated fats with either monounsaturated fatty acids (MUFA) or polyunsaturated fatty acids (PUFA) significantly reduces LDL cholesterol with little impact on HDL cholesterol or TGs6,15. Additionally, PUFA lowers LDL cholesterol to a greater magnitude than MUFA.
Mechanism: unlike saturated fats, unsaturated fats increase hepatic LDL receptors and expression, enhancing cholesterol uptake and clearance from the bloodstream.
Plant vs animal sources of MUFA
Meat and animal fats as well as plant fats can be significant sources of MUFA. However, the World Health Organization (WHO) has undertaken an extensive review of the evidence and demonstrated that plant food sources of MUFA are superior to animal-sourced MUFA with regard to both coronary heart disease (CHD) incidence and mortality5. Large cohort studies have concurred that plant sources of MUFA are cardioprotective, while consuming animal sources of MUFA can be detrimental to overall health18.
This should not be surprising as animal foods providing MUFA will also be a source of saturated fat, while plant food sources of MUFA will predominantly provide other unsaturated fatty acids.
What about omega-3?
Omega-3 unsaturated fatty acids have long been associated with reduced CVD and CHD risk. However, the evidence is not clear about the direct association19,20.
What is known is that replacing saturated fats with unsaturated fats (which includes omega-3), will lower LDL cholesterol which is a well-established causal factor for atherogenic CVD. Additionally, diets rich in oily fish, which is a major source of long-chain omega-3, such as the Mediterranean diet, have also been associated with lower CVD risk21.
Marine long-chain omega-3 fats, eicosapentaenoic acid (EPA; C20:5) and docosahexaenoic acid (DHA; C22:6) at pharmacological doses (2-4g daily), may be used therapeutically to lower elevated TGs by 20-30%8,20,22–24. In the updated Cochrane systematic review and meta-analysis, increasing intakes of EPA and DHA were associated with small improvement in CHD events and reduction in mortality20.
Short-chain plant omega-3 fats (alpha-linolenic acid; ALA C18:3). An extensive review of the studies investigating ALA supplements on cardiovascular health outcomes, suggests that ALA may reduce risk of CVD events and arrythmias, but more evidence is needed20.
Carbohydrates
The type of carbohydrate that replaces saturated fats in the diet will also play a critical role on plasma lipids25. Refined carbohydrates have been shown to increase TGs significantly and have little impact on LDL cholesterol, whereas whole grains (particularly those rich in viscous fibre) have been demonstrated to reduce both LDL cholesterol and triglycerides.
Dietary cholesterol
The focus of dietary intervention for lipid management seldom focuses on dietary cholesterol due to a number of reasons including:
- Typical intakes are low and below recommended maximum daily intakes of 300mg
- Saturated fats, due to the higher quantity they are consumed in, are far more potent at elevating serum LDL cholesterol levels
- Foods high in saturated fats are also often key sources of dietary cholesterol. Thus, reducing intakes of foods high in saturated fat, will automatically lower cholesterol intakes.
Individuals with familial hypercholesterolemia (FH) are the exception to this rule26. Due to their genetic predisposition, FH patients have exceptionally high levels of serum cholesterol. Dietary advice for FH thus focuses on dietary cholesterol with stricter maximum allowance (no more than 200mg per day). However, saturated fat reductions are still prioritised.
Trans fatty acids
Although naturally occurring trans fatty acids (TFA) can be found in red meat and dairy products, the concern lies with industrially produced TFAs. Industrially produced TFAs have been shown to increase LDL cholesterol and reduce HDL cholesterol, significantly raising CVD risk far more than saturated fats6,15,27,28. These TFAs are a result of the partial hydrogenation process used with vegetable oils to make them more solid and improve their technical qualities. This process was common in the 1970s, but as the harmful effects of TFAs became known, food regulations and manufacturing practices changed28,29. Consequently, TFAs are no longer a concern in most developed countries. In the UK, average intakes remain well below the 5g daily limit set30.
The impact of dietary fats on CHD outcomes
Studies have consistently demonstrated a similar pattern when comparing the impact of dietary saturated fat substitution with other macronutrients on CHD risk. Replacing saturated fat with TFAs increases CHD risk whilst replacement with MUFA, PUFA and (to a lower degree) whole grains reduces risk CHD27.
UK intakes vs recommendations
Current recommendations
- UK public health recommendation31: no more than 10% of energy from saturated fat.
- For adults aged 19-64 years, this equates to no more than 22g for women and 28g for men
- UK NICE guidelines for individuals with elevated cholesterol levels and/or high risk of CVD3: no more than 7% of energy from saturated fat
- For adults aged 19-64 years, this equates to no more than 16g for women and 19g for men
Current intakes
Intakes have persistently exceeded recommended daily limits over the last decade30. Current intakes: 12.3% of energy from saturated fat (adults 19-64 years)
Key food sources
62% of the saturated fat in the UK diet comes from three main sources30:
- Meat and meat products
- Cereal products (mainly ready meals, buns, cakes, pastries, and biscuits)
- Milk and dairy.
Good food sources of unsaturated fats
Food source | Type of unsaturated fat |
Olive oil and avocados | Mono-unsaturated fat |
Nuts and seeds and their oils, most vegetable oils and their spreads except coconut and palm | Polyunsaturated fat - Omega-6 |
Rapeseed oil (standard vegetable oil), walnuts and walnut oil, hemp and flaxseeds and their oils | Polyunsaturated fat - Alpha linolenic acid (ALA) - omega-3 |
Oil rich fish and algae supplements | Polyunsaturated fats - eicosapentaenoic acid (EPA) and docosahexaenoic (DHA) - omega-3 |
References
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On this page
Plasma lipids & cardiovascular risk
Prevalence & impact of hypercholesterolamia
Dietary fats: quality over quantity
MUFA & PUFA for optimal LDL cholesterol
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Published: 26th June 2024