Thursday, February 24, 2011

Feature Article: Eggs; Hero Or Villain?

The controversy surrounding the egg is a great example of the chasm between popular opinion and current research in the field of nutrition.  Eggs have been criticized for years because of their cholesterol content.  Many doctors and even nutritionists have people throw away their egg yolks, buy egg substitutes or avoid eggs altogether in an effort to reduce their risk of heart disease and improve their health.  Well designed research regarding eggs and our health has been conducted and it’s time to set the record straight. 

Why have eggs been vilified?
Eggs are considered an unhealthy food for one basic reason; they are high in cholesterol.  A single egg has 213 milligrams of cholesterol.  The daily recommended level of dietary cholesterol is 300 milligrams per day.  So consuming one egg comes close to the daily recommendation and consuming 2 eggs at a meal exceeds the daily recommendation for cholesterol.  The theory behind the advice to avoid eggs is that consuming a lot of dietary cholesterol will raise serum cholesterol and increase risk of heart disease and stroke.

Does the cholesterol you eat in your diet increase your risk of heart disease?
Cholesterol that you consume in your diet is only weakly associated with your serum cholesterol levels.  There is strong evidence that trans fat and saturated fat consumption have a negative impact on serum cholesterol, but this does not seem to be the case with dietary cholesterol.  In both the Nurses’ Health Study and the Framingham Heart Study (2 of the largest and most well designed cohort studies in existence) dietary cholesterol was not associated with an increased risk of coronary heart disease.  (See Reference 1, Reference 2)

Have eggs been shown to increase risk of heart disease?
Several studies have examined the association between egg consumption and risk of heart disease.  In 1999, researchers from Harvard published one such study in the Journal of the American Medical Association.  They looked at egg consumption and risk of coronary heart disease in 80,082 women from the Nurses’ Health Study and 37,851 men from the Health Professional Follow-up Study (See Reference 3). Subjects were followed for 8 years and there was no evidence of increased risk of heart disease in men or women who consumed 1 egg per day.   There was, however, an increased risk in diabetic men and women who consumed 1 egg per day.  In the Framingham Heart Study, there was also no association found between egg consumption and risk of heart disease (See Reference 2).

So what else is in an egg yolk besides cholesterol?
Egg yolks are like a multivitamin supplement.  They contain vitamin D, folate, vitamin E, vitamin A, monounsaturated fats, vitamin B12, vitamin B2, essential amino acids, linoleic acid, calcium and vitamin B1.  They are, in fact, one of the most nutrient dense foods you can eat.

So what’s the take home message on eggs?
I tell my healthy clients to enjoy up to 7 yolks a week.  This amount will not have any negative impact on cardiovascular health and will absolutely pack their breakfast with essential nutrients.  The increased risk with diabetics is definitely a cause for concern.  I advise my diabetic clients to limit themselves to 3 yolks per week.  

1) Hu FB, Stampfer MJ, Manson JE, et al. Dietary fat intake and risk of coronary heart disease in women. The New England Journal of Medicine 1997; 337:1491-1499.

2) Dawber TR, Nickerson RJ, Brand FN, et al. Eggs, serum cholesterol and coronary heart disease. American Journal of Clinical Nutrition 1982; 36:617-25.

3) Hu FB, Stampfer MJ, Rimm EB, et al. A prospective study of egg consumption and risk of cardiovascular disease in men and women. Journal of the American Medical Association. 1999; 281:1387-94.

Monday, February 14, 2011

Questions And Answers: Omega 3 Fatty Acids

I get a lot of questions about omega 3 fatty acids.  Many people don’t know much about what they do, but have heard they are healthy.   There is also a lot of confusion concerning the dangers of mercury consumption when eating fish.  I thought this topic would make a great post for my next Q and A, so here goes:

1) What are omega 3 fattys acids?
Omega 3 fattys acids are an essential polyunsaturated source of fat.  They are essential because our body can’t make them, we need to get them from our diet.  There are 3 subtypes of omega 3’s: 1) EPA 2) DHA 3) Alpha-linolenic acid.

2) Where are they found?
EPA and DHA are found in fish.  The fattier the fish, the higher the level of omega 3’s.  Alpha linolenic acid is found in vegetable oils like canola oil, nuts like walnuts, flaxseed, dark leafy vegetables and some animal fat, particularly if the animal was grass fed.

3) How do omega 3’s impact health?
Omega 3’s are believed to have an impact on the following: 1) The make up of cell membranes  2) Hormones that regulate clotting 3) Hormones that regulate contraction and relaxation of artery walls and 4) Inflammation.

Dietary intake of omega 3’s has been shown to decrease fatal heart arrhythmia, lower heart rate and blood pressure and improve the function of blood vessels.  At larger doses they have been shown to lower triglycerides and decrease inflammation. (See Reference 1)  These benefits appear to be strongest for EPA and DHA which are the marine sources of omega 3’s.

4) Do I need to supplement them?
Not at all.  If you work with me or have read my book, you’ll know I’m not a fan of supplementing in general.  There are several factors in whole foods that work synergistically to help nutrients work.  When you isolate a compound and put it in pill form, you lose this important aspect of nutrition.  If you get 2-3 servings of a variety of seafood such as fatty fish (tuna, salmon, sardines, mackerel) or shellfish each week you are doing great.  Vegetable sources of omega 3’s (oils, nuts, flaxseed) have generally not shown as much benefit, so try to focus on the marine sources.

Furthermore, there have been some reports of high levels of mercury in some of omega 3 supplements.  Since supplements are not regulated, you can never really be sure of the source.  This is another reason to get your omega 3’s from your food.

5) What about mercury in our fish- should I be concerned?
Yes and no.  Mercury is a compound that when ingested in large doses can damage nerves in adults and cause severe problems with the developing nervous systems of young children and fetuses. 

In general, the benefits of omega 3 consumption outweigh the risks.  I tell my client to strictly limit consumption of the 4 biggest sources of mercury in our fish: swordfish, shark, tilefish and king mackerel.   Outside of that, for adults, it appears that eating a variety of seafood will pose little problem, with the potential for serious benefit.

However, if you are pregnant or trying to become pregnant, you need to talk to your doctor about how to deal with the issue of mercury.  On the one hand, EPA and DHA from fish sources are extremely important to the neurological development of the fetus.  On the other hand, high amounts of mercury have the potential to have the opposite effect.  Your doctor will help you strike the proper balance of minimizing the risk and maximizing the benefit of fish.  Generally, the recommendation for pregnant women is to consume no more than 2 servings of seafood a week and only one of these servings should be albacore tuna.   Pregnant women are also told to avoid the 4 big sources of mercury: swordfish, shark, tilefish and king mackerel.

6) So what do you recommend?
I tell my clients to eat 2-3 servings of seafood each week and to vary the sources.  Good low mercury sources of omega 3’s are shrimp, canned light tuna, salmon, pollack and catfish.  I also have them limit the 4 big sources of mercury: swordfish, shark, tilefish and king mackerel.

I don’t have them supplement with fish oil and if they are pregnant or trying to get pregnant, I defer to the advice of their Ob/Gyn doctor.

Leaf A.  Prevention of sudden cardiac death by n-3 polyunsaturated fatty acids. Journal of Cardiovascular Medicine. 2007; 8 Suppl 1:S27-29.

Friday, February 4, 2011

Research Update: Low Carb Diets And All Cause Mortality

Low Carbohydrate Diets And All Cause Mortality And Cause Specific Mortality
Fung, et al.
Archives of Internal Medicine 153:289-98 2010.

Objective: To examine the relationship between a low carbohydrate diet and risk of all cause mortality over a study period of 26 years in women and 20 years in men.

Study Population: Participants in this study included 85,168 women from Harvard’s Nurses’ Health Study and 44,548 men from Harvard’s Health Professional Study. 

Results: During the period of follow-up there were 12,555 deaths in men and 8,678 deaths in women.  The researchers looked at the affect of 3 different types of low carbohydrate diets.  A general low carbohydrate diet, one based largely on animal sources of fat and protein and one based on vegetable sources of fat and protein.  The results were striking.  Those following a general low carb diet had a 12% increased risk of dying from any cause.  Those eating an animal based low carb diet had a 23% increased risk of dying from any cause.  Conversely, those following a vegetable low carb diet had a 20% lower rate of dying from any cause.

Comment:  This study is near and dear to my heart.  #1) My research group while I was at Harvard conducted it, so many of these people are my friends and colleagues. #2) They used the scoring system I helped develop for my 2006 New England Journal of Medicine article on low carb diets.  3) In many ways, this is an extension of my previous work.

These results are important for several reasons.
1) There have been a number of short term studies that have looked at the health effects of low carb diets.  The results of these studies have had mixed results.  There is some evidence that a low carb diet has a nice effect on HDL cholesterol, triglycerides and insulin sensitivity but there has also been some evidence of an increase in LDL cholesterol which may increase risk of cardiovascular disease.  This study helps to answer the question of the long term effects of these diets on health.

2) Many proponents of low carb diets theorize that red meat and saturated fats are OK in the context of a low carb diet because LDL particles become larger, more buoyant and therefore less damaging with regards to risk of heart disease.  This may not be the case.  Those who consumed animal sources of fat and protein (IE bacon, steak, cream, butter, etc) had a 23% increased risk of dying from any cause while those who consumed vegetable sources of fat and protein (legumes, nuts, olive oil, etc) had a 20% decreased rate of dying from any cause.

3) These results are consistent with some earlier research.   Among Greek participants of the European Prospective Investigation Cancer and Nutrition (EPIC) study, a low carbohydrate diet was associated with a statistically significant increased risk of death (Ref 1).

Take Home Message
The authors hypothesized that the vegetable based low carb diet was protective because it was high in unsaturated fats, dietary fiber and micronutrients such as magnesium and potassium.  The red and processed meats of the low carb diet were hypothesized to be the most harmful aspect of that dietary pattern.  Those of you who work with me privately or read my book know that I’m not an advocate of low carb diets.  However, if you plan to do one, make sure you choose predominantly vegetable sources of fats (like olive oil, nuts, avocados, etc) and lean sources of protein (fish, chicken, turkey, beans, etc.)  The idea that red meat, bacon, cheese, butter and cream are harmless in the context of a low carb diet appears to be unsubstantiated.

1) Trichopolou A et al. Low carbohydrate, high protein diet and long term survival in a general population cohort.  European Journal of Clinical Nutrition. 2007 61:575-81.