Eggs and health

Eggs have been in our diet since we first started out as a species about 200 thousand years ago. Can a food that we've been eating for that long have bad effects on our health, or are eggs a nutritious, all-round food that will benefit our health if eaten on a daily basis?

What nutrients do eggs consist of?

Eggs consist of two fairly distinct parts, the egg white and egg yolk. For a typical hen's egg the nutrients that can be found in both are listed below:

Egg white.

An egg white is 90% water, but if we ignore that, protein consisting of albumins, globulins and mucoproteins makes up the bulk of egg whites  (>90%), with the rest mostly carbohydrate (c.10%). An egg white has virtually no fat content and a small amount of vitamins and minerals.

Egg yolk.

An egg yolk is 53% water. Ignoring this it consists of 56% fat, 34% protein, 7.5% carbohydrates and 2.5% cholesterol. There are also a large amount of vitamins and minerals in the yolk, much more so than in the egg white. In particular the fat soluble vitamins, A, D, E and K are present in good proportions. The fat content of an egg yolk is mostly made up of healthy monounsaturated fats (53%) with saturated fats at 28% and polyunsaturated fats at 18% (w6=16%, w3=2%). The ratios of these fats does depend on the feed given to the hens and so you will find some eggs touting their w3 content as being higher than others.

Does the cholesterol in an egg harm us?

In short, NO. The cholesterol in eggs is unlikely to be bad for health on two grounds. Firstly, it does not appear to raise blood cholesterol levels. Secondly, raised cholesterol levels are not as bad for health as most people assume. Lets investigate these two statements further.

Eggs don't raise blood cholesterol levels1. Most studies which have found a link have been based on short term effects. Over the longer term the evidence points to cholesterol levels being unchanged through a wide range of egg consumption. For instance eating 2 eggs a week or 14, leads to very little difference in total cholesterol levels.

Raised cholesterol is not as bad for your health as most people believe. This referenced article by Dr Briffa may give you  pause for thought2. In it he refers to a study of over 50,000 Norwegians for whom the most healthy cholesterol levels were between 5.0-5.9mmol/L, a level regarded as high by medical authorities throughout the world. Levels below 5.0 were associated with an increased risk of death through cancer and haemorrhagic stroke (strokes caused by uncontrolled bleeding through burst blood vessels).

There was a recommended daily cholesterol intake amount of 300mg (for a 2000 kcal diet) floating around at one time. This guideline is irrelevant in my opinion. One egg would normally contain about 200mg of cholesterol and so 2 would put you above the recommended daily amount. In reality restricting cholesterol intake is far more likely to damage your health rather than improve it, as many of the foods that contain it are very good for health (eggs, liver and prawns are examples). While your liver is able to create all the cholesterol you need, excess dietary cholesterol is metabolised by the body such that blood cholesterol levels remain at a steady level.

OK then, what about the saturated fat in eggs?

The evidence that saturated fat can raise blood cholesterol levels is by no means definitive. Many of the trials that linked saturated fat intake with high blood cholesterol were short term, and not well constructed. They often used polyunsaturated fats as a comparison fat. Polyunsaturated fats are known to lower cholesterol levels, and so if you replace polyunsaturated fat in the diet with saturated fat you would expect cholesterol levels to go up even if they had no effect on cholesterol. Many studies seem to bear this out, suggesting that saturated fat does not raise cholesterol at all3. Even studies that have associated saturated fat with higher cholesterol have also linked saturated fat intake with raised levels of the "good" HDL cholesterol and decreased levels of Lipoprotein A and triglycerides4. These three effects are all supposedly good for the heart!

So saturated fat may or may not raise cholesterol levels. It may well depend on whether you are male, female, your age or to what ethnic group you belong to. However even if your cholesterol levels are raised by saturated fats this does not mean that your health will be damaged as pointed out in the section on cholesterol above.

So my view is that the cholesterol and saturated fat in eggs is not a cause for concern for the vast majority of us.

What about the studies that associate egg intake with an increased risk of death?

There are studies, notably the US Physicians study that have related egg intake with an increased risk of death5. The correlation with heart disease was not as strong. What this epidemiological study shows to my mind, is that many US doctors, like their counterparts in the UK are a population of health conscious and not so health conscious people. I remember meeting plenty of the latter when I was at university. Doctors are atypical of society at large in that throughout the 80s and 90s a large proportion of them believed strongly that eggs were bad for health. What is likely in an epidemiological study like this is that the health concious doctors who didn't drink too much or smoke reduced their egg consumption. Those doctors that had a more laissez faire attitude to their own health would have been more likely to drink, smoke and eat eggs more liberally. The result, a study that linked early death with egg consumption.

Studies with other large population groups have shown absolutely no correlation with egg intake and heart attack and stroke6. Indeed women in this study had a significantly reduced risk of heart disease and stroke if they ate more than 1 egg per day compared to those who ate none. This study did however show an increased risk of cardiac events for people with diabetes who ate more eggs. However as I pointed out in the previous paragraph it is highly likely that diabetes patients share or most likely have stronger health concerns with cholesterol and saturated fat intake than doctors. This would lead to the more healthy conscious folk with diabetes eating fewer eggs along with a more healthy diet, less drinking and smoking, again resulting in a misleading association between eggs and bad health.

Are some eggs better than others?

Egg quality relates to the breed and health of the laying hen. The amount of space the hen has, the feed she receives and the hours of darkness are the most important factors that determine egg quality. 

Small scale producers.

Eggs produced locally may not be certified as organic, but they may well be superior to anything you would find in a supermarket. Check with your local vendor (butcher or greengrocer etc.) to see if they have visited the egg producer, and what they made of the conditions for the hens.

Medium scale producers.

Eggs produced on a larger scale can either be cage laid, barn laid, free-range, omega 3 and/or organic. They are all produced in different ways. The differences relate to the amount of space each hen has, the type of food they are given to eat. and hours of darkness. Genuine organic eggs are laid by hens with more space than eggs that are just labelled as free-range, meaning that your best bet for a healthy egg is likely to be an organic or omega 3 egg.

So the best eggs are likely to be organic, speciality breed, from a local small scale producer and possibly omega 3 eggs. These eggs are likely to contain more healthy omega 3 (w3) fats and more vitamins and minerals than those from other types of eggs. They invariably taste a lot better. I remember what must have been on of my worst eggs ever was in an airport departures lounge. I was given a breakfast of rather pallid scrambled eggs on toast, and it really tasted of absolutely nothing. I would happily bet that this was from a battery (cage fed) hen.

References:

1) http://content.karger.com/ProdukteDB/Katalogteile/isbn3_8055/_81/_79/Wld...  

2) http://www.drbriffa.com/2011/10/06/why-a-raised-cholesterol-level-may-be...

3) http://jn.nutrition.org/content/133/1/78.full 

4) http://www.ncbi.nlm.nih.gov/pubmed/21427738  

5) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2386667/  

6) http://www.ncbi.nlm.nih.gov/pubmed/10217054