What type of milk do you prefer? Most people will give you their answers quickly without much hesitation. Besides taste and flavor, whether to choose whole milk (~3.5% fat), reduced-fat milk (2% fat) or skim milk (0% fat) depends mostly on how much fat do you prefer in your diet. Reduced fat milk and skim milk have become the poster children for heart beneficial diets in the past decades. The long-held belief that fat is bad for your heart originates from a famous epidemiology study conducted by Ancel Keys and colleagues1.

Diet, Fat, and Healthy Heart

Ancel Keys' Seven Countries Studies influenced dietary recommendation on fat for decades. Keys believed that fatty foods such as dairy products and red meat are the culprit for coronary heart disease. He studied diet, lifestyle, and incidence of coronary heart diseases in about 13,000 adult men in Finland, Greece, Italy, Japan, the Netherlands, the United States, and Yugoslavia1, and found that countries with diets high in saturated fat including the United States have the highest blood cholesterol levels and heart-attack death rates. Based on Keys' studies and other similar findings, the United States and the United Kingdom introduced dietary guidelines which recommend reducing consumption of saturated fat to about 10% of total energy intake, to lower cholesterol in the blood and therefore decrease the risks of a heart attack. A low-fat diet has been associated with good health practices ever since. Here is a twist to this story, Keys didn't include France, where the nation's high-fat diet doesn't correlate with the occurrence of heart diseases. It turns out to be the opposite.

Not all fat is created equal. Let's take milk fat for example. Milk fat contains a variety of fats such as saturated fat, unsaturated fat, and trans-fat. Generally, trans-fat is considered as "bad" fat in processed foods and fried foods, however, naturally found trans-fat in milk is beneficial. Another example is cholesterol. It's taken for granted to associate dysregulated blood cholesterol levels with dietary cholesterol intake. In fact, it's not cholesterol itself that causes high blood cholesterol levels, but rather the lipoproteins that move cholesterol in and out of cells. Broadly, there are the "good" cholesterol– high-density lipoprotein (HDL) and the "bad" cholesterol­­­– low-density and very-low-density lipoproteins (LDL and VLDL). Seventy percent of milk fat is saturated fat, and saturated fats in milk raise both HDL (good) and LDL (bad) cholesterol. The net effect of milk fat might be neutral. Processed foods, fried foods and stick margarine have lots of trans-fats from production and are known for raising LDL cholesterol and lowering HDL cholesterol.

The "good" and "bad" cholesterol levels are considered as the golden standard for cardiovascular risk prediction. However, recent research shows that high HDL levels in some cases associated with higher risks in heart disease2. The plot is thickened. It turns out that some people with a genetic mutation in HDL receptor gene fail to transport cholesterol outside of blood, therefore results in higher level of fats in the body despite having high levels of HDL cholesterols in the circulation. In conclusion, blindly relying on fat content in the Nutrition label is simply not enough.

Now, let's go back to the milk choice question one more time. Not only we need to consider what type of fat in cow milk, but we also need to look at other factors too. Sugar is often ignored when it comes to buying milk. Reduced fat and skim milk contain slightly more carbohydrates than whole fat milk does. If your goal is to lose weight by reducing fat content in your milk, you might get disappointed. The relationship between milk fat and weight management is still not clear. An epidemiology study shows that women who consumed more than 1 serving of whole fat milk per day were 15% less likely to gain weight compared to those who drink low fat milk3. Also recent research show that consuming saturated fat does not directly cause heart disease4. Therefore, how much you eat doesn't necessarily translate to how much will end up in your body. It depends on how you body metabolizes it, what's your genetic makeup and what else in your diet potentially positively or negatively contribute to the net outcomes. Last not the least, even not all fat creates equal, trans-fat from fried foods and processed foods are still universally considered bad for your health. Try to avoid those if it's possible.

REFERENCE

  1. Keys A. Seven Countries. Harvard University Press; 2013.
  2. Zanoni P, Khetarpal SA, Larach DB, Hancock-Cerutti WF, Millar JS, Cuchel M, DerOhannessian S, Kontush A, Surendran P, Saleheen D, Trompet S, Jukema JW, De Craen A, Deloukas P, Sattar N, et al. Rare variant in scavenger receptor BI raises HDL cholesterol and increases risk of coronary heart disease. Science. 2016;351(6278):1166 LP – 1171.
  3. Rosell M, Håkansson NN, Wolk A. Association between dairy food consumption and weight change over 9 y in 19 352 perimenopausal women. The American Journal of Clinical Nutrition. 2006;84(6):1481–1488.
  4. Weinberg SL. The diet–heart hypothesis: a critique. Journal of the American College of Cardiology. 2004;43(5):731–733.
  5. "The facts on fats infographic" [Image] (2017). American Heart Association. https://www.heart.org/en/healthy-living/healthy-eating/eat-smart/fats/the-facts-on-fats

"The views, opinions and positions expressed within this blog are those of the author(s) alone and do not represent those of the American Heart Association. The accuracy, completeness and validity of any statements made within this article are not guaranteed. We accept no liability for any errors, omissions or representations. The copyright of this content belongs to the author and any liability with regards to infringement of intellectual property rights remains with them. The Early Career Voice blog is not intended to provide medical advice or treatment. Only your healthcare provider can provide that. The American Heart Association recommends that you consult your healthcare provider regarding your personal health matters. If you think you are having a heart attack, stroke or another emergency, please call 911 immediately."