Dietary modification for lipid management

Changing a person’s dietary fatty acid profile is not always intuitive, and can be difficult to achieve in isolation, writes Dr Michael Skilton.

Dietary modification is a frontline therapy for lipid management (here). Implementation is particularly challenging and onerous when compared to pharmaceutical prescription. This is in part due to the complexities of diet, which has multilevel nutritional characteristics, foods and food groups, and synergies between these that occur at all levels. The need to account for food intolerances and preferences, while being bounded by ethnic and cultural requirements, complicates dietary modification further. In real-world applications, compliance with dietary advice in people with cardiometabolic diseases is mixed (here); however, the potential for safe and efficacious lipid management ensures that dietary modification remains a treatment priority.

In a heavily controlled environment, changes in dietary fatty acid profile account for over 85% of the variance in low-density lipoprotein (LDL-cholesterol levels (here). Saturated fat is the primary nutritional driver of LDL-cholesterol levels (here). Consumption of saturated fat increases circulating LDL-cholesterol levels predominantly by downregulating hepatic LDL receptor activity (here). Trans fats also increase LDL-cholesterol levels (here), albeit via distinctly different mechanisms. While the increase in LDL-cholesterol with trans fats is greater than that for saturated fats on a gram-to-gram basis (here), population intake of trans fats is in general much lower than that of saturated fat and in Australia is very low (here).

In contrast, consumption of monounsaturated and polyunsaturated fats both reduce LDL-cholesterol levels relative to saturated fat intake (here). Of these unsaturated fats, the LDL-cholesterol lowering effect size is greatest for polyunsaturated fats. This hierarchy of fatty acids with regards to their effects on LDL-cholesterol levels is mirrored by their associations with all-cause mortality in cohort studies (here).

From a practical perspective, changing dietary fatty acid profile is not always intuitive, and can be difficult to achieve in isolation. How does someone effectively lower their saturated fat intake? Or perhaps more relevant is the question, how does someone replace their intake of saturated fat with that of polyunsaturated or monounsaturated fats? The answers are enacted in the supermarket aisles, where food choices are made despite the obvious disconnect from advice given at a nutrient level. Some examples of choices that can be made to replace dietary saturated fat with unsaturated fats includes the purchase (and consumption) of oils and spreads that have lower saturated fat and higher polyunsaturated or monounsaturated fats, or skinless chicken instead of red meat.

This leads to obvious questions regarding other nutrients affected by these food substitutions. For example, dietary cholesterol is another nutrient that has a long history within lipid management and dietary guidelines (here). The evidence base no longer supports dietary cholesterol itself being an important modifier of LDL-cholesterol levels, and thus recommendations to limit dietary cholesterol intake have been removed from some dietary guidelines. Nonetheless, foods that are rich in dietary cholesterol also tend to be rich in dietary saturated fats (here), and, as such, recommendations to limit dietary cholesterol intake may have the reciprocated benefit of reducing dietary saturated fat intake and thus indirectly lowering LDL-cholesterol levels.

In theory, the net effect of all nutrients in a food could be modelled. In practice, however, using the nutrient profile in isolation is not the best indication of the overall effect of a food on lipid levels or other disease outcomes. This is likely due to incomplete evidence for the physiological effects of all components of the food and the ways in which those components may augment or decrease the effects of each other. Quantifying the effect of the food itself, or food group, on lipids or other disease outcomes overcomes these inherent limitations of the reductionist approach. A very modern example comes in the form of plant-based meat replacements, for which there is now some evidence to suggest that they lower LDL-cholesterol relative to traditional animal-derived beef (here).

Furthermore, dietary advice based on food consumption is more practicable to individuals when they are at the point of purchase in the supermarket or restaurant. Evidence at the food level provides clear insights into optimal food swaps to maximise health outcomes (here), with foods such as fruit and vegetables, legumes, wholegrain cereals, nuts, and fatty fish (such as salmon) being most beneficial, particularly when replacing red meat and processed meat. The changes in nutrient profile resulting from these food swaps is also generally consistent with lowering LDL-cholesterol levels.

Two modern challenges to dietary modification are worthwhile noting. First, food production has an outsized impact on environmental sustainability, and thus the carbon footprint and overall environmental impact of individual foods and food groups should be considered as part of dietary modification (here). Fortunately, the healthiest foods, as listed above, also tend to be those with the smallest environmental footprint (here), and, as such, evidence-based dietary modification for lipid management will have unintended, yet welcomed, environmental benefits. Second, there is continual background noise on social media that spreads and amplifies messages and advice that is not consistent with the dietary guidelines and best practice for dietary lipid management. This is largely not disinformation, but rather is often not unreasonable interpretations of select parts of the literature. This is enabled by the relatively low quality of evidence from which our dietary guidelines are derived, and specifically the lack of large high quality randomised trials of hard clinical outcomes.

Although the evidence linking dietary management with short term lipid management is robust and built on a foundation of well controlled short term trials, most of the best evidence for dietary prevention of heart disease and mortality comes from observational studies. Large trials are particularly difficult and problematic for dietary interventions given inherent issues with participant blinding, long term compliance and lack of industry funding partners. Of the limited number that have been undertaken, they demonstrate the healthiness of a Mediterranean diet characterised by low red meat consumption, while being rich in nuts, vegetables, legumes, fish and olive oil (here). For dietary fatty acid replacement, a meta-analysis of eight randomised trials shows a 10% reduction in risk of heart disease for every 5% increase in dietary energy from polyunsaturated fats when replacing saturated fats (here). Among these trials, the effect size was greatest in longer trials, emphasising the importance of long term adherence.  

This complements evidence from a trial comparing multiple diets — all with previous evidence of weight loss benefits — in which it was the participants with the highest dietary adherence who had the best outcomes (here). This was irrespective of what diet they were randomly allocated to follow. In other words, while there are many healthy diets that are consistent with the evidence above for lipid management, guiding patients to the right diet for them, at this time in their life, will give them the best opportunity to achieve the healthiest outcomes.

Dr Michael Skilton is a Deputy Medical Editor at the Medical Journal of Australia.

The statements or opinions expressed in this article reflect the views of the authors and do not necessarily represent the official policy of the AMA, the MJA or InSight+ unless so stated. 

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