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.

References
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.

5 comments:

  1. Love your blog and all the great info you send our way!

    Can you give us more info on how someone follows a vegetarian low-carb diet? And how that can include beans when they're running 20 grams carbs per 1/2 cup. Tofu seems to work with 5 grams carbs.....

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  2. Hey Jill,
    Thanks for the kind words. I'm going to post a feature article on vegetarian diets shortly.

    In this study, the women following the vegetable based low carb diet focused on beans, tofu, nuts and whole grains for their protein sources. While these sources do contain a fair amount of carbohyrates, their carb intake was significantly lower than the women who were focusing on low fat, high carb eating patterns. This was mostly because the vegetable low carb dieters ate way more healthy fat like vegetable oils, avocado's etc.

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  3. This is great news, Tom! I imagine we have long believed this, but it's always good to have hard data to back it up!

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  4. The Paleo adherents hide in the cave when it comes to this information. It's a very popular approach for young athletic males. Generally no grains - cavemen didn't eat them or at least some say so. No junk food for sure. Now perhaps cutting out a lot of the junkfood and trans fats, will counteract the apparent ill effects of the high animal protein.

    I say apparent, because I'm wondering if the vegan group perhaps ate much more healty in general - limited junk food, trans fat intake, ect, and if this could account for some of the all cause mortality differences, or was the study normalized for this?

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  5. Thanks for the comment Roseman,
    There really wasn't a vegan group in this study. Even participants that scored high on the vegetable based low carb score still consumed a good amount of animal protein, they just also consumed a lot of vegetable protein as well. This study really shows you that a low carb diet can be healthy if you pick lean protein sources (like beans, nuts, whole grains, and even lean animal sources like chicken, turkey and fish) and vegetable sources of fat (like oils, nuts, seeds, etc). Conversely, a low carb diet can be harmful if you are choosing saturated fat sources and red and processed meats for proteins. To answer your question, the researchers controlled for a healthy lifestye in the modelling (physical activity, body mass index, multivitamin use, alcohol consumption, etc), although you can never be sure there isn't some residual confounding on these factors. Trans fat levels were quite similar across all scoring groups.

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