Mechanisms to Explain Variation in Serum Low Density Lipoprotein Cholesterol Response to Dietary Saturated Fat

A raised level of blood cholesterol (also referred to as LDL or 'bad' cholesterol) is an important risk factor for developing heart disease. Lowering cholesterol levels by changing our diet and taking certain medication (e.g. statins) represents a major public health strategy to decrease the risk of developing and suffering from heart disease in the UK population. A type of fat eaten in our diet known as saturated fat (found predominately in animal products such as meat and dairy) is well recognised as the main dietary component responsible for raising blood cholesterol, and reducing its intake has been the mainstay of dietary guidelines for the prevention of heart disease for over 30 years. However, findings from large, powerful studies called meta-analyses show no evidence for a direct link between the intake of saturated fat and deaths from heart attack and stroke. One explanation for this outcome is that the link between saturated fat and heart disease is not a direct one, but relies on the ability of saturated fat to raise blood cholesterol (LDL-C) levels. This effect is complex, and highly variable between individuals because of important differences in metabolism and the way in which our body's absorb and digest cholesterol after eating meals high in dietary saturated fat. These individual differences make it difficult to study how dietary factors like saturated fat influence blood cholesterol in large numbers of people. However, these variations in metabolism can be measured relatively easily and used as biological markers to determine which people will respond well and those who will respond less well to diets which are lower in saturated fat.

The main aims of this proposal are to measure variation in blood LDL cholesterol in response to lowering the amount of saturated fat in the diet to the level recommended by the government for heart disease prevention (LDL Screening Study). From this initial study, we will select a group of high responders and a group of low responders in blood LDL cholesterol for a more detailed study to determine the metabolic processes responsible for the variation between these two groups (Metabolic Study). We predict that people who show a high blood LDL-cholesterol response to a diet lower in saturated fat will show a greater reduction in the absorption of dietary fat in their gut than low responders. We believe that this effect may be explained by changes in their gut bacteria, which can alter the composition of compounds called bile acids that promote the absorption of dietary fat. It may also be explained by a phenomenon known as gut permeability which may be increased by eating saturated fat. Gut permeability also differs between individuals and affects fat absorption and gut bacteria. In addition, we will measure the activity of specific receptors on the surface of cells (LDL receptors) that remove LDL from the blood and have been shown to be reduced by eating diets high in saturated fat (causing LDL cholesterol to rise). Finally, in the Metabolic Study we will also undertake a holistic measure of the metabolic state in high and low LDL-C responders using a technique known as metabonomics. This approach can measure thousands of metabolites simultaneously in samples of blood and urine, and detect subtle differences (metabolic signatures) between high and low responders that can be used as simple biomarkers for the sensitivity to dietary saturated fat. The results from this study will be used to overcome the problems of setting dietary guidelines for whole populations, which are frequently inappropriate for some subgroups of people in the population. This will be achieved by the tailoring of dietary advice to those at higher risk of developing heart disease and who therefore stand to gain the greatest benefit to their health.

The relationship between a high intake of dietary saturated fat and CVD is not direct, but largely mediated through the effects of certain saturated fatty acids (SFA) in raising serum LDL-cholesterol (LDL-C). A key outstanding question of fundamental importance to understanding the complexity of the relationship between dietary SFA, serum LDL-C and CVD risk is; what determines the biologically intrinsic, inter-individual differences in LDL-C response to dietary SFA? The overall aims of this proposal are to determine the metabolic and gut-related mechanisms that give rise to inter-individual variation in LDL-C to dietary SFA, and to identify biomarkers of these mechanisms that can be used to detect sensitivity to SFA.
The main objectives are; 1) to demonstrate variations in serum LDL-C in response to changing from a high to a low SFA diet; 2) to undertake a metabolic study in two groups of individuals, those in the top and bottom 10% of change in serum LDL-C, to demonstrate a relatively greater reduction in fat absorption in response to a reduction of dietary SFA, using stable isotope trace-labelling, in high versus low responders. This effect will be accompanied by relatively greater increases in the expression of LDL-receptors (in PBMCs by PCR), endogenous cholesterol synthesis (serum phytosterols), and serum deconjugated bile acids (targeted UPLC-MS) and/or short chain fatty acids (NMR & LCMS); 3) these effects will be related to changes in the gut microbiota (FISH, NGS & CLS genomics), and/or gut permeability (absorption & recovery of 51Cr-EDTA probe); 4) to identify serum and urinary biomarkers of these metabolic differences (1H-NMR & UPLC-MS metabonomics). Outputs from this study will provide new evidence to progress beyond dietary guidelines that are inappropriate for whole populations, towards the tailoring of advice to subgroups of individuals who are more or less responsive to dietary SFA, and who stand to gain more or less benefit to their CVD risk.

The current UK dietary guideline to reduce total energy intake from dietary saturated fatty acids (SFA) in men from 12.7% to 10% of total energy or less, can be estimated to achieve a decrease in serum LDL-C of 0.13mmol/l. In terms of impact, this equates to a 6% reduction in coronary heart disease (CHD) risk or delaying or averting 3,000 deaths from CHD. In comparison, the inter-individual variation in serum LDL-C response to dietary SFA between the highest and lowest responders in two randomised controlled trials (DIVAS & RISCK) was in the order of 1.5 mmol/l. This represents a greater than 20-fold difference in CHD risk relative to the UK target, and provides a true perspective of the substantial impact of inter-individual variation in serum LDL-C response relative to our National dietary guideline for reducing CHD risk. This helps to explain why a dietary guideline for a population, with a diverse range of serum LDL-C responses, lacks impact in achieving its aim, and the urgent need to identify and intervene in responsive and unresponsive groups.

New knowledge and insight into the mechanisms that underlie variation in the serum LDL-C response to dietary SFA will exert multiple impacts by transforming the way in which dietary guidelines are formulated and applied, and in changing dietary practices to affect those most in need of dietary advice. These impacts will be achieved through academics, dieticians, health care professionals and medics at the interface of public health and medicine, who can inform and educate the end users of our research outputs. These end users include:

Policy makers - This includes academic and non-academic members of boards and panels who advise government and formulate policy on food and nutrition in relation to public health e.g. SACN, Public Health England and the Department of Health. Applicants on this proposal have a direct impact on these influential bodies as members, scientific advisers and consultants (see Pathways to Impact)

Charities, learned societies - bodies that exert significant impacts as active stakeholders of the role of nutrition in human health e.g. Nutrition Society, HEART UK, The British Nutrition Foundation, Association for Nutrition, British Heart Foundation. Applicants on this proposal also have a direct impact on these influential bodies as members, scientific advisers and consultants (see Pathways to Impact).

Industry and commerce - Our outputs will create opportunities in the food industry to reformulate foods and expand the portfolio diet for blood cholesterol reduction, by providing information on the mechanisms of action of functional foods. This also applies especially to the development of pre and pro-biotic products, with modes of action linked to the metabolic variables under study in this proposal. This translates into economic and societal impacts through the marketing and commercial sales of new products.

Research funders (e.g. BBSRC): our mechanistic outputs would inform future research by featuring in the frameworks and strategic priorities of research funding bodies in the UK and Europe, creating employment and new knowledge. This process will produce highly trained young researchers capable of tackling contemporary challenges in either the public or private sectors. All of which could make a substantial contribution to the economic competitiveness of the UK.

General public - subgroups of variable LDL-C responders may represent up to 20% of the UK population, who are either more or less in need of dietary advice to reduce CVD risk. The identification of both subgroups translates into economic and societal impact by reducing CVD risk in one group and the need for health care in the other. CVD has been described as a disease that begins in adolescence. Crucially, the impact of our research could exert impact in childhood, and thus be preventative through-out the life course, reducing the need for drugs, like statins, in later life.