Siri-Tarino PW, Sun Q, Hu FB, Krauss RM. American Journal of Clinical Nutrition. March 2010;91(3):535-46
Why the study was done: A reduction in dietary saturated fat has generally been thought to improve cardiovascular health, and public health campaigns in most developed countries now emphasize the importance of reducing intake of this type of fat.
How the study was carried out: Pooled data from twenty-one prospective studies on the impact of saturated fat on the development of cardiovascular disease or stroke were identified and included in this analysis.
What were the results: During 5–23 y of follow-up of 347,747 subjects, 11,006 of them developed cardiovascular disease (CVD), stroke, or coronary heart disease (CHD = combined CVD and stroke). Intake of saturated fat was not associated with an increased risk of CHD, stroke, or CVD. Consideration of age, sex, and study quality did not change the results.
What conclusion(s) did the researchers come to: The researchers concluded that there is no convincing evidence to suggest that dietary saturated fat is the bad guy we have all been led to believe. In their own words: “Our meta-analysis showed that there is insufficient evidence from prospective epidemiologic studies to conclude that dietary saturated fat is associated with an increased risk of coronary heart disease, stroke, or cardiovascular disease.”
AILEEN’S COMMENTARY
In a separate article in the same issue of the American Journal of Clinical Nutrition authors of this study speculate that the risk of heart disease and stroke may possibly be influenced by what we use to replace the fat we remove, in an effort to make food palatable.1 For example, fat free yogurt would be very unappealing without the starches and other food additives used to improve its texture.
One might suspect that butter is not so bad when added to cooked vegetables compared to spreading it on bread and topping with jam, or incorporating it with sugar and flour into muffins, cookies and desserts. A dollop of whipped cream on top of a fresh fruit salad might not carry the same risks to the heart as half a pint of ice cream. But it’s difficult (but not impossible) to capture this complexity and examine how good or bad a food is in relation to the total nutritional context in which it is consumed.
Putting eggs back on the breakfast table
Another concern that is also not captured by current research is the loss of other valuable nutrients we experience when we demonize certain foods – particularly those that have historically be part of human diets, both modern and ancient. A good example is the humble egg, which as humans we have eaten as long as we have walked this earth. As concerns about cholesterol came to the forefront of heart disease prevention in the 1950’s, their cholesterol content suddenly made them a “bad” food. The message went out: Do not eat eggs, or if you do, throw away the fatty yolk, and just eat protein-rich egg whites.
The unintentional consequences of the do-not-eat-eggs message was that it reduced our regular intake of some pretty important nutrients. Egg yolks are an excellent source of choline, a B-complex vitamin with multiple functions in the body, including fat transport and metabolism. It is also needed for the manufacture of cell membranes and as the precursor for the neurotransmitter acetylcholine. Eating choline in the diet raises blood levels of choline and subsequently brain levels of acetylcholine, popularly known as the brain’s “memory manager.”
In previous generations we got most of our daily supply of choline from egg yolks and liver, both foods we have learned to shun. It has been more than a quarter of a century since Richard Wurtman and his group at MIT demonstrated that the level of circulating choline and its availability as a substrate for acetylcholine synthesis depended on how much choline we got from our food.2
So, in avoiding traditional foods rich in choline we may unwittingly have set the stage for a rash of memory problems. This may have particular implications for pregnancy. The availability of choline in the mother’s diet is critical for optimal brain development of her baby, affecting its memory for the rest of its life. In animal studies giving extra choline to rat pups in utero results in life-long memory enhancement.3
Do you see what I see?
Egg yolks are also rich sources of two fatty yellow antioxidants of the carotenoid family – lutein and zeaxanthin. Both these compounds have come to prominence following studies in eye health and diet. The Age-Related Eye Disease Study (AREDS) is an ongoing study of the influence of diet on the health of the aging eye. The AREDS study has shown that macular degeneration, the leading cause of blindness in Canadians over 65 years of age, was prevented or its progression slowed with supplements of these two yellow pigments.
In supplements, the source of these important pigments is often marigold leaves. Although edible and sometimes found in salads in high-end restaurants, marigold leaves are not a major component of most diets. However, the daily consumption of eggs was common practice until concerns about their cholesterol content led to medical advice to exclude them. What a shame. Especially as studies have never shown that cutting egg consumption had any positive effects on blood cholesterol levels.4
But studies have shown that the more frequent consumption of eggs can impact the development of macular degeneration and indeed have shown that consumption of one egg per day for just 5 weeks can cause measurable improvements in the health of the macula without increasing cholesterol levels.5
The public health lesson to be learned from the egg debacle is simple: Advice that focuses on a single dietary component (cholesterol), and discourages the consumption of foods that are high in that component may lead to unintended consequences by removing other beneficial constituents. The cure may then turn out to be worse than the disease.6
References
1. Siri-Tarino PW, Sun Q, Hu FB, Krauss RM. Saturated fat, carbohydrate, and cardiovascular disease. American Journal of Clinical Nutrition. March 2010;91(3):502-9
2. Wurtman RJ. Food consumption, neurotransmitter synthesis, and human behaviour. Experientia Suppl. 1983;44:356-69
3. Zeisel SH. Nutritional importance of choline for brain development. J Am Coll Nutr. 2004 Dec;23(6 Suppl):621S-626S.
4. Constance C. The good and the bad: what researchers have learned about dietary cholesterol, lipid management and cardiovascular disease risk since the Harvard Egg Study. Int J Clin Pract Suppl. 2009 Oct;(163):9-14, 27-43.
5. Goodrow EF et al. Consumption of one egg per day increases serum lutein and zeaxanthin concentrations in older adults without altering serum lipid and lipoprotein cholesterol concentrations. J Nutr. 2006 Oct;136(10):2519-24.
6. Schmier JK, Barraj LM, Tran NL. Single food focus dietary guidance: lessons learned from an economic analysis of egg consumption. Cost Eff Resour Alloc. 2009 Apr 14;7:7.