We are often asked in our clinical practice what is the role of lipids, cholesterol, and diet? There seems to be a lot of confusion regarding this issue. So, let’s analyze the problem. This discussion should help providers and patients alike.

Basic biochemistry:

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The first thing to remember is that lipids are composed of many different types of fats but the two most relevant to our discussion are, fatty acids and cholesterol. Fatty acids can be good or bad. The good kind is the monounsaturated and polyunsaturated fats that are usually found in fish oil and fatty fish. The bad kind is saturated fatty acids and trans fats. Cholesterol, on the other hand, is a ubiquitous molecule (distinct from fatty acids) that is an integral part of all the body cells (especially cell membranes). It is the substance that can also clog up your arteries and cause heart attacks and strokes when the concentrations are too high in the blood.

Basic physiology:

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Most of the cholesterol inside the body is made endogenously from other raw materials like fatty acids but especially saturated fats and trans-fats in the diet. Therefore, a diet rich in saturated fats and trans-fats will raise blood cholesterol more than anything else. This endogenous cholesterol production is controlled by fatty acid content available to the liver mainly. Therefore, more fatty acids in the blood and liver will inhibit further endogenous cholesterol production and vice versa.

The rest of the cholesterol inside the body is exogenous cholesterol from food.

Interestingly, exogenous Cholesterol intake will raise the cholesterol levels in the blood but not as much, usually not more than a factor or +/- 15% (Guyton, 2011). This is because when more cholesterol is ingested exogenously the body slows down on endogenous cholesterol production. At the cellular level, this is achieved through inhibition of the rate-limiting enzyme called HMG Co-A reductase.

On the other hand, a diet high in saturated fat will increase the cholesterol level by a factor of 15 to 25%, especially when this is associated with excess weight gain and obesity. This is achieved by increased fat deposition in the liver, which then provides increased quantities of Acetyl Co-A in the liver cells for the production of cholesterol (Guyton, 2011).

Pathology:

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For an excellent discussion the topic please refer to this article. (Br J Nutr)

Dietary recommendations:

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Total cholesterol 300 mg for most people (FDA), 200 mg for people with heart disease.

Foods:

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EGGS
The content of cholesterol in one egg yolk is 184 mg.
Although some studies have found a link between eating eggs and heart disease, there may be other reasons for these findings. The foods people typically eat with eggs, such as bacon, sausage, and ham may do more to boost heart disease risk than eggs do. Plus, the way eggs and other foods are cooked — especially if fried in oil or butter — may play more of a role in the increased risk of heart disease than eggs themselves do.

Most healthy people can eat up to seven eggs a week without increasing their risk of heart disease. Some studies have shown that this level of egg consumption may even help prevent certain types of stroke and a serious eye condition called macular degeneration that can lead to blindness (Francisco Lopez-Jimenez, n.d.).

See below for full discussion.

Egg, yolk, raw

Amount Per 1 large (17 g)
Calories 55
% Daily Value*
Total Fat 4.5 g	6%
Saturated fat 1.6 g	8%
Polyunsaturated fat 0.7 g
Monounsaturated fat 2 g
Cholesterol 184 mg	61%

 

BEEF (STEAKS)

Content of cholesterol in 85 milligrams of steak.

Steak

Amount Per 1 serving (3 oz)
Calories 213
% Daily Value*
Total Fat 13 g	20%
Saturated fat 5 g	25%
Polyunsaturated fat 0.4 g
Monounsaturated fat 6 g
Trans fat 0.9 g
Cholesterol 77 mg	25%

SALMON

Salmon, Atlantic, raw

Amount Per 0.5 fillet (198 g)
Calories 412
% Daily Value*
Total Fat 27 g	41%
Saturated fat 6 g	30%
Polyunsaturated fat 8 g
Monounsaturated fat 7 g
Cholesterol 109 mg	36%

 

Inferences:

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• Diets high in saturated fatty acids and trans-fatty acids will increase cholesterol levels more than diets that only contain cholesterol.
• Foods that are rich in cholesterol only without many saturated fats and trans fats may not have ill health effects with the following caveats:  that the food is rich in cholesterol only (notably eggs) without trans fats and/or saturated fats being consumed. Also, the person consuming this food is not genetically predisposed to high cholesterol levels after ingestion of cholesterol in the food and is not a diabetic.
• Mostly when people consume diets rich in Cholesterol (e.g., eggs) they consume diets rich in saturated fats at the same time (e.g., bacon). This makes it difficult to study the effects of Cholesterol alone on heart disease (a phenomenon called confounding in studies).
• for a full discussion on eggs please keep reading.

 

Special case: eggs

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One way of circumventing the potentially confounding effects of cholesterol and other lipids is to focus one’s analysis on a particular food, which is rich in cholesterol intake (e.g. eggs), rather than cholesterol itself. Of course, eggs contain lipids other than cholesterol (and which could change the risk of CVD), but in view of its very high cholesterol content (see above), one can accept eggs as a special case study of cardiac risk modification from cholesterol alone.

What do the few studies on eggs and the risk of CVD tell us?

The Framingham study (with fewer than 1000 subjects) concluded as to the absence of the relationship between egg consumption and the risk of CVD (Am J Clin Nutr)

The Harvard study on a cohort of 38 000 men and 80 000 women has shown that eating up to one egg/d (or seven eggs/week) did not increase the risk of CVD (JAMA)

Similarly, in the prospective cohort study of 21 327 men from the Physicians’ Health Study during an average follow-up of 20 years, egg consumption was not associated with incident myocardial infarction or stroke (Am J Clin Nutr).

The only exception concerned diabetic women, with a slight increase in risk for those who ate more than one egg/d relative to those who ate less than one egg/week. In the Physicians’ Health Study, all-cause mortality was increased for the consumption of ≥ seven eggs/week, and this was stronger among diabetic subjects. In a study concerning a representative cohort of 9734 adults aged 25–74 years over a 20-year follow-up, consumption of >six eggs/week does not increase the risk of stroke and ischaemic stroke or coronary artery disease. However, in subgroup analysis among diabetics, consumption of >six eggs/week was associated with a significantly increased risk of coronary artery disease (Med Sci Monit)

 

Bibliography

Br J Nutr. 2011 Jul;106(1):6-14. doi: 10.1017/S0007114511000237. Epub 2011 Mar 9.

Dawber, TR, Nickerson, RJ, Brand, FN, et al. (1982) Eggs, serum cholesterol, and coronary heart disease. Am J Clin Nutr 36, 617–625

Djousse, L & Gaziano, JM (2008) Egg consumption in relation to cardiovascular disease and mortality: the Physicians’ Health Study. Am J Clin Nutr 87, 964–969

FDA.gov https://www.accessdata.fda.gov/scripts/interactivenutritionfactslabel/cholesterol.html

Francisco Lopez-Jimenez, M. (n.d.). Mayo Clinic. Retrieved from Mayo Clinic: https://www.mayoclinic.org/diseases-conditions/high-blood-cholesterol/expert-answers/cholesterol/faq-20058468

Hu, FB, Stampfer, MJ, Rimm, EB, et al. (1999) A prospective study of egg consumption and risk of cardiovascular disease in men and women. JAMA 281, 1387–1394

Guyton, A. C. (2011). Medical Physiology.

Qureshi, AI, Suri, FK, Ahmed, S, et al. (2007) Regular egg consumption does not increase the risk of stroke and cardiovascular diseases. Med Sci Monit 13, CR1–CR8.