Highlights and Discussion of Major Findings from:
"Organic Production Enhances Milk Nutritional Quality by Shifting Fatty Acid Composition: A United States-Wide, 18-Month Study."
by Charles M. Benbrook, Gillian Butler, Maged A. Latif, Carlo Leifert, and Donald R. Davis, published in Public Library of Science One (available free of charge after 5pm ET), December 2013.
This page is available as a pdf or you can print it. There is also a powerpoint of the figures with text as notes, and a zipped file that includes all of the figures in png format.
Measuring the Nutritional Quality of Milk
The nutritional quality of milk is typically measured by its protein content and amounts of different fatty acids. Commercial whole milk averages about 3.3% fat, nearly all of which (3.1%) consists of fatty acids. The other 0.2% consists mostly of glycerol, cholesterol, phosphate, and other hydroxyl-containing substances such as choline and inositol.
Differences in the levels, or concentration, of different types of fatty acids in organic and conventional milk are the major focus of this study. A basic understanding of the unique characteristics of the different fatty acids in milk and other foods is therefore helpful in interpreting the significance of the paper's core findings. Toward this end, please see the Primer on the Fatty Acid Content of Milk.
The main factor driving differences in the fatty acid composition of milk is the diet of lactating dairy cows, especially the portion of daily dry matter intake that comes from pasture and forage-based feeds, as opposed to grains, corn silage and other high-energy feedstuffs.
The team analyzed 384 monthly samples of organic and conventional whole milk from seven regions throughout the United States, from January 2011 through June 2012.
As expected, this study found that whole milk contains about 2% saturated fatty acids, with little difference between organic and conventional milk, as is evident in Figure 1. Similarly, there is little difference in the content of monounsaturated fatty acids.
However, large differences in the fatty acid composition of organic and conventional milk were found in another key category of milk fatty acids—those that are polyunsaturated.
Differences in Polyunsaturated Fatty Acids
There are two major types of polyunsaturated fatty acids in milk, called omega-6 (ω-6) and omega-3 (ω-3) fatty acids. Both are essential in a healthy diet. The primary omega-6 is linoleic acid (LA) and the primary omega-3 is alpha-linolenic acid (ALA).
Total polyunsaturated fatty acids were about 10% lower in organic compared to conventional milk, as shown in Figure 2, but there were major differences in the levels of ω-6 and ω-3 fatty acids. Organic milk was found to have much lower levels of ω-6 fatty acids, and higher levels of ω-3 fatty acids. These differences become even more significant in terms of the ω-6/ω-3 ratio, as is clear in Figure 3.
The Omega-6 to Omega-3 Ratio
One of the rarely discussed but most important indicators of a healthy diet is its ratio of ω-6 to ω-3 fatty acids. Hundreds of scientific studies have found links between the ratio of ω-6 to ω-3 fatty acids and various health outcomes. An ω-6/ω-3 ratio around 2.3 is cited as optimal for promoting heart health, while higher ratios are associated with a number of degenerative health problems, especially cardiovascular disease.
The ω-6/ω-3 ratio of typical American diets has dramatically increased over the last century from the relatively healthy level of about 5 to around 10 today. A typical couch potato diet high in fried and baked foods and vegetable-oil based condiments may have an ω-6/ω-3 ratio of 20 or higher!
Increasing consumption of soybean, corn, and safflower oils and fried foods in American diets has dramatically increased ω-6 intakes, while consumption of fish and other sources of ω-3 fatty acids has gradually declined. The shift of beef and dairy cattle from pasture feeding to diets high in corn and other grains has also increased ω-6 and reduced ω-3 intakes.
Key Fatty Acids in Milk
The primary ω-6 and ω-3 fatty acids in milk and U.S. diets are linoleic acid (LA, ω-6), and alpha-linolenic acid (ALA, ω-3). Together with conjugated linoleic acid (CLA), they are the three most important fatty acids in milk for heart health. There is increasing evidence that the saturated fatty acids in milk may be less important than the LA, ALA, and CLA content.
For most Americans, less LA and ω-6 are preferable, while higher intakes of ALA and ω-3 are associated with reduced risk of cardiovascular disease. CLA has proven benefits in animals and is believed to help prevent heart disease and cancer in humans.
In this study, organic milk contained 25% less LA (ω-6) and 60% more ALA (ω-3). In addition, organic milk contained 18% more CLA compared to conventional milk, averaged over a full year (more in summer, less in winter). These differences are highly significant in terms of statistical reliability (i.e., they cannot be due to chance).
The Major Finding
A key measure of the nutritional quality of milk and other dairy products is the ratio of ω-6/ω-3 fatty acids. Figure 5 presents the most important findings in this study—the dramatically lower LA/ALA and ω-6/ω-3 ratios in organic versus conventional milk.
In conventional milk these ratios are 2.4 and 2.5 times higher than in organic milk, differences that are both large and highly statistically significant.
The 143 samples of organic milk tested during the first 12 months of this study had a year-round average ω-6/ω-3 ratio of 2.28—just about perfect in terms of the heart-healthy goal of 2.3. The conventional milk average ratio of 5.8 is also good compared to the ratio in typical American diets (about 10).
In most comparisons of nutrients in organic and conventional foods, the differences one way or the other rarely exceed 30%, and often are inconsistent or uncertain. The ω-3 and ω-6 fatty acid differences found in this study are among the largest and most reliable ever documented in such studies. The health impact of these differences is potentially large, especially in infants and children who consume large amounts of milk and other dairy products. Also, numerous studies show that maternal diets with either low ω-3 intakes or high ω-6/ω-3 ratios increase the risk of a number of developmental problems in the neurological and immune systems of infants, as well as in the healthy development of eyes.
Long-Chain Omega-3 Fatty Acids
ALA, the major ω-3 fatty acid in milk and human diets, contains a chain of 18 carbon atoms. Humans have a limited ability to convert some ALA into two "long-chain" ω-3 fatty acids containing 20 carbon atoms--eicosapentaenoic acid (EPA) and docosapentaenoic acid (DPA).
EPA has special significance, because it is essential for making body regulators called eicosanoids. These regulators help control blood pressure, inflammation, blood clotting, and many other functions.
Partly because human conversion of ALA to EPA is very limited, dietary EPA in milk can be important, especially for those who eat little fish (another source of EPA). Dietary EPA and DPA also can be converted in small amounts to another long-chain ω-3 fatty acid containing 22 carbon atoms, docosahexaenoic acid (DHA).
DHA is prominent in fatty fish and is needed in our brains, nerves, and eyes. It is especially important for fetuses and infants during the rapid growth of pregnancy and infancy.
Unfortunately, human conversion of ALA to the long-chain forms EPA, DPA, and DHA is inhibited by the high ratios of LA/ALA in many American diets, thus increasing the value of dietary sources of these long-chain ω-3 fatty acids.
In this study organic production increased the average level of EPA and DPA in milk by 33% and 18% respectively.
Fatty Acid Intakes and Ratios from Recommended Servings of Dairy Foods and Fish
The study authors compared the amounts of key fatty acids in recommended servings of dairy products and fish. They used the recommended serving sizes in the Dietary Guidelines for Americans, 2010, and assumed that the milk servings are whole milk. The daily servings are:
The fish servings supply relatively little LA and ALA compared to either organic or conventional milk. The fish LA/ALA ratio is modestly higher than conventional dairy, and is almost three times higher than the ratio in organic milk.
Many people will be surprised to learn that the dairy servings supply far more ALA (ω-3) than the fish and that both conventional and organic milk have lower LA/ALA ratios than fish.
The milk servings supply about one-third as much EPA as the recommended fish servings, and slightly more DPA than the fish, with modest advantages for organic milk. Fish is, however, a superior source of DHA, because there is negligible DHA in cow’s milk (unlike human milk).
Minor Differences in Probably Beneficial Trans Fatty Acids
Trans fatty acids are unsaturated fatty acids with an unusual trans shape at an unsaturated bond. In natural foods, nearly all unsaturated fatty acids have a shape called cis. The cis shape is a bend in the molecule at the unsaturated bond, whereas the trans shape is not bent.
These two examples have the same atoms, bonds, and chemical formula (C18H34O2). They are called geometric isomers, because they differ only in their shape at the unsaturated bond in the middle of the fatty acid.
In natural foods, the unusual trans shape occurs in small amounts in the milk and meat of cows, goats, and other ruminant animals with multiple stomachs. But in vegetable oils and all other natural foods, unsaturated fatty acids all have the bent cis shape. The bend, or lack of bend, affects some functions of fatty acids, such as the properties they give to cell membranes and likely their role in building important regulatory molecules called eicosanoids (see Figure 6 introduction).
Trans fatty acids have received special attention in recent years because of evidence that some of the many forms produced artificially by partial hydrogenation can promote heart disease and possibly cancer. Before labeling of trans fatty acids began in the U.S. in 2006, the Food and Drug Administration (FDA) estimated that average daily consumption of trans fat was 5.8 grams per day, mostly from baked goods and margarine, with about 1.2 grams per day (20%) from animal products. Since then, “industrial” trans fatty acids have been gradually removed by food manufacturers.
Although some believe or assume that the trans fatty acids in milk have the same disadvantages as industrially produced forms, there is evidence for most of the four main trans fatty acids in milk that they are beneficial or neutral. The benefit is most clear for CLA (see Figure 4), which led the FDA in 2003 to exclude CLA from its definition of trans fatty acid for food labeling purposes. About 75% of CLA in U.S. foods comes from milk products. The major type of trans fatty acid in milk is “trans-18:1,” mostly vaccenic acid. It has little or no effect on blood lipids, and humans convert about 20% of it to beneficial CLA. The 2nd and 3rd most common trans fatty acids in milk are CLA and similar forms called “trans-18:2.” The 4th trans fatty acid in milk is “trans-16:1,” or trans-palmitoleic acid. A recent study found that human blood levels come mainly from milk and are associated with broad health benefits.
The major trans fatty acid in milk, trans-18:1, is slightly lower in organic compared to conventional milk. Its health effects are probably about neutral, except that humans convert some to beneficial CLA. Little is known about the health effects of trans-18:2 in milk. A recent report suggests that trans-16:1 in milk is likely beneficial; it is 12% higher in organic milk. CLA is 18% higher in organic milk (year-round average). It has proven benefits in animals and is believed to help prevent heart disease and cancer in humans.
Modelling Fatty Acid Intakes
This study documents substantial differences in the polyunsaturated fatty acid content of organic and conventional milk. But how important are these differences relative to a person’s overall intakes of omega-3 and omega-6 fatty acids?
The study authors used model diet scenarios for an adult woman to calculate the impact of three interventions on LA and ALA intakes and the resulting LA/ALA ratio. The three interventions included switching from conventional to organic dairy products, increasing the level of dairy consumption, and avoiding some foods that contain high levels of LA.
The authors quantified the impact of each of these interventions, alone and in combination, on the woman’s overall LA/ALA ratio. Their goal was to determine the degree to which each intervention could reduce her LA/ALA ratio from 11.3 down toward the heart-healthy target ratio of 2.3.
Figure 9 provides an overview of the fatty acid intakes of a woman with a representative diet in which fat accounts for 33% of calories. The woman is assumed to ingest the recommended daily amounts of conventional dairy products (3 cups fluid milk equivalent, plus ½-cup of ice cream, as in Figure 7).
In the representative woman's diet with recommended amounts of conventional dairy products, such products account for only a small part of total dietary LA and ALA (respectively 8.5% and 18% in this scenario). Her LA/ALA ratio of 11.3 is determined mainly by non-dairy foods, and is well above the heart-healthy target of 2.3.
Single Dietary Interventions Can Improve Dietary LA/ALA Ratios
Figure 10 displays the effects that three dietary interventions have on a representative woman's projected LA/ALA ratio of 11.3, assuming each intervention is implemented alone. Each intervention is a single change from the representative diet discussed above Figure 9. The interventions are:
In the representative diet, the woman consumes a moderate amount of conventional dairy, contributing to a diet in which fat accounts for 33% of calories. By making the dietary changes shown in Figure 10, the LA/ALA ratio can be reduced.
Figure 10 shows the effects of three different dietary interventions on reducing the LA/ALA ratio of 11.3 in Figure 9. The interventions are a 50% increase in dairy product consumption, a switch from conventional to organic dairy products, and switching to some lower-LA foods: substituting pita chips for corn chips, canola-oil margarine for regular margarine, and canola oil for soy oil in salads and cooking.
Substituting relatively low-LA foods for some high-LA foods clearly has the most dramatic impact of the three interventions and would lower the woman’s LA/ALA ratio from 11.3 to 5.1. This drop in LA/ALA ratio brought about by a single intervention is 69% of the reduction needed to reach the heart-healthy target ratio of 2.3.
As single interventions, both the switch to organic milk and the increased consumption of dairy products achieve about 15% of the needed reduction in overall dietary LA/ALA ratio.
Two Dietary Interventions Yield Increased Improvements in LA/ALA Ratios
By combining two of the interventions shown in Figure 10, the LA/ALA ratio can be reduced more substantially. Figure 11 shows the impact of the three possible combinations of two interventions:
The dual interventions of switching to a high dairy consumption and consuming organic dairy products lowers the woman’s LA/ALA ratio to 7.8, thereby achieving 39% of the drop needed to reach the heart-healthy 2.3 target.
Combining a switch to reduced-LA foods with either increased consumption of dairy or switching to organic dairy yields even larger decreases, with LA/ALA ratios of 5.2 and 4.6, yielding progress toward the 2.3 target level of 68% and 75% respectively.
Combining All Three Dietary Interventions Comes Closer to Reaching the Heart-Healthy Goal
Making all three dietary interventions (switching from moderate to high dairy consumption, conventional to organic dairy, and substituting reduced LA foods) yields the largest decrease in the LA/ALA ratio. This ratio is substantially closer to the heart-healthy ratio of 2.3.
Implementing all three dietary interventions results in a LA/ALA ratio of 4.1, eliminating 80% of the difference between the ratio of 11.3 in the baseline diet and the heart-healthy ratio of 2.3.
Comparing All Combinations of Dietary Interventions
Figure 13 summarizes the LA/ALA ratios for each of the preceeding interventions and their combinations. For ease of reading, the interventions have been labeled A, B, and C. Intervention A is switching from moderate to high dairy consumption. Intervention B is switching from conventional dairy consumption to organic dairy consumption. Intervention C is substituting three reduced-LA foods for higher-LA foods.
Both of the two relatively simple interventions involving only dairy products can lower the LA/ALA ratio by about 15% of the total reduction needed to reach the target level. The combined switch to a high level of organic dairy products achieves 39% of the reduction needed to reach the 2.3 heart-healthy goal.
Coupling these two changes in dairy product consumption with substituting reduced-LA alternatives for about one-half of the high-LA foods in a person’s diet can dramatically improve the balance of LA and ALA (and thus ω-6 and ω-3) intakes in a typical daily diet.