Dietitian Mary M. Flynn, PhD, RD, LDN, looks at this and several other misconceptions.
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|Mary M. Flynn, PhD, RD, LDN||Dr. Flynn|
Early studies testing the effect of fat on blood cholesterol used liquid formulas (drinks-based diets), and many showed that saturated fats would increase total cholesterol compared to unsaturated fats. Later studies examining lipoproteins showed that saturated fat would increase LDL-c, which was well publicized, but that saturated fat also increases HDL-c, which was less well publicized.
Some epidemiologic studies that looked at nutrient intake also showed that saturated fat intake could be related to heart disease rates. However, when you look at the main food sources of saturated fat-red meat and dairy fat-studies have shown red meat intake can increase heart disease risk (Arch Intern Med 2009; 169:562-71), but full-fat dairy fat intake does not (Adv Nutr 2012; 3:266-85).
The saturated fat content of red meat does not seem to be the issue; one could better explain the relationship of red meat to heart disease risk through other components in red meat. For example, red meat is the main source of arachidonic acid (the 20-carbon polyunsaturated fat in the omega 6 family), which would compete with EPA for the eicosanoid pathways and lead to the synthesis of pro-inflammatory eicosanoids that are made by omega-6 fatty acids (Oxid Med Cell Longev 2013; 610950); some amino acids in red meat cause insulin to be secreted (Diabetes Care 2003; 26:625-30) and high normal insulin increases coronary heart disease (CHD) risk (J Am Diet Assoc 2004; 104:176-9). Red meat is rich in methionine, which could increase homocysteine levels if not completely metabolized to cysteine, and homocysteine would increase oxidation, which increases CHD risk.
Myth debunked. While this myth should have been debunked by now, it still seems to be commonly considered true and somewhat ingrained in the health messages of U.S. health professionals. To decrease CHD risk, it would be better to suggest that men (and women) decrease their red meat intake, but besides the potential for excess calories, full-fat dairy is not a health concern.
Dietary treatment of hypertension began in the 1970s, and the original focus was sodium chloride, or table salt, as sodium is the main extracellular mineral. Excess amounts of sodium outside the cell would draw fluid out of the cell, increasing blood volume, which could increase blood pressure, so this dietary intervention made sense to many at the time. This led to many patients being told not to use salt on their food and the proliferation of low-salt or salt-free foods.
However, as many physicians and patients would report, not using salt at home does little to improve blood pressure. The main dietary source of sodium is processed food-anything in a box or jar, such as breakfast cereal, crackers, commercial salad dressing, sauces, etc., and most canned items, such as soup, vegetables, etc. The other main source of sodium is fast food and restaurant food. With the enormous increase in eating out that has occurred in the past 30 years, most of the dietary sodium in the U.S. diet is from eating out and convenience foods. Adding salt to one’s food adds little sodium to the diet.
An alternative approach was shown in the DASH (Dietary Approaches to Stop Hypertension) study. This study focused on increasing potassium, the major intracellular mineral, by increasing fruits and vegetables, which are all good-to-excellent sources of potassium. This study showed that a daily intake of approximately nine servings of fruits and vegetables (a serving being half-cup, or half a piece of most fruit) would decrease blood pressure (N Engl J Med 1997; 336:1117-24). This intake could easily be achieved by including fruit at breakfast and lunch and vegetables at lunch and dinner.
Myth debunked? Unfortunately, this myth seems to be somewhat ingrained in the health field and I am not sure when, or even if, this myth would be debunked. Part of the confusion comes from nutrition guidelines that do not make it clear that one would need to add a lot of salt to food to exceed the current recommended upper limit for sodium intake. I think it is helpful for publications to consistently present this data to professionals likely to treat patients with hypertension and that they include dietary treatment of blood pressure, stressing increased fruit and vegetable intake and a decrease in canned and packaged foods, fast food, and restaurant eating. Any information about adding salt to food should advise limiting table salt to one teaspoon per day.
This is one of the oldest nutrition myths. I think the myth started when it was initially thought that lowering blood cholesterol would help decrease heart disease, and instead of saying “eat a diet to lower your blood cholesterol,” the message went out as “eat a low-cholesterol diet.” The latter does little to change blood cholesterol, as for most people, eating dietary cholesterol results in the body producing less cholesterol.
This myth was also likely due to incorrectly focusing on nutrients in the diet versus considering food sources of the nutrient. If you simply look at nutrient intake and not the food sources, you will always have confusing and conflicting information.
Cholesterol is only found in animal products, and a typical large egg has 215 mg of cholesterol; this is the equivalent to the cholesterol in about 7 ounces of beef. So, while a vegan diet would reflect no cholesterol intake, a diet record showing a high intake of cholesterol could belong to someone eating eggs or someone eating meat. You would need to know the food source to examine the effect of the diet on heart disease, as eggs have never been related to increasing CHD risk, while a number of studies have related frequent red meat intake to increasing CHD risk.
Myth debunked. Dietary cholesterol minimally changes blood cholesterol, and one needs to know the food sources of the cholesterol for assessing health risk. While red meat has been related to increasing heart disease risk, egg consumption of any frequency has not. The yolk provides a rich source of nutrients, so not eating it means a meal with only egg whites is less nutrition that consuming the whole egg. In addition, eating only egg whites would likely mean consuming protein in excess of dietary need; protein consumed in excess of need is stored as fat, so a diet high in protein could increase fat storage. Not eating the yolk of the egg also means you are missing out on the rich source of nutrients found in the yolk, as the yolk has the nutrients needed to support the growth of a chicken.
|Section Editor Steven A. Kaplan, MD||Dr. Kaplan|
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