e usually a variety of choices. We get to choose the fat content we want in milk – whole (3.25%), 2%, 1%, or 0%. We may also be able to choose organic, grass-fed, or other specialty milk products with claims related to the types of fats they contain. Another option at the dairy case is a more concentrated form of dairy fat, which is butter.
Fats (nearly synonymous with “lipids” and “oils”) come in many different forms and are an important part of life systems. They are a concentrated source of energy, they help maintain proper body temperature, they protect body organs from physical harm, and they are essential in the human diet.
The fats we eat are primarily in the form of triglycerides. Triglycerides consist of three individual fatty acids, each linked to a glycerol backbone. Fatty acids (FA) are comprised of a string of carbon atoms, typically anywhere from 4 to 30 carbons long. In milk, the majority of FA are 14 to 18 carbons long. Chemically, the number of double bonds defines whether they are saturated (no double bonds), monounsaturated (one double bond), or polyunsaturated (multiple double bonds).
Each type of FA, depending on chain length, bonds, and structure, has unique physical properties and effects in the body. Saturated FA, the most prevalent FA in milk, are fairly stable molecules. Unsaturated FA may change form during the digestive process of the cow and are a significant part of the conversation when we look at impacts on human health.
Figure 1. Triglycerides are made up of a glycerol backbone and three individual fatty acids, which may differ from each other and are typically removed from the glycerol backbone during the digestion process in both cows and humans.
Figure 2. Alpha-linolenic acid is an example of an 18-carbon polyunsaturated fatty acid (FA). As with all FA, it is made up entirely of carbon, hydrogen, and oxygen atoms. Its core is a chain of carbon atoms, and unique characteristics arise from its three double bonds. It is also classified as an omega-3 FA.
Eighteen-carbon FA are abundant in milk fat, though in a variety of forms. Many originate in a cow’s diet, but there are also many that are modified in the rumen. Microbial activity in the rumen tends to move those FA that are unsaturated (double bonds in the chemical makeup; see Figure 2) toward a saturated state (no double bonds), a process called biohydrogenation. Some of the intermediate forms make their way into milk. This includes a family of conjugated linoleic acids (CLA) and trans fats.
Conjugated linoleic acid isomers make up a small percentage in the FA profile of milk, usually less than 1% of the total fat, but milk is a major source of them for the human diet. Milk and dairy products account for up to 67% of total dietary CLA intake, as it is only found in ruminant fat.
Small amounts of trans FA (approximately 2 to 5% of the total fat) are also naturally present in milk fat, arising in the stomach after hydrogenation of unsaturated FA during bacterial fermentation in the stomach.
Alpha-linolenic acid (ALA; diagrammed in Figure 2), an omega-3 FA found at relatively high levels in fresh pasture, is passed into cow milk and is classified as essential for the human diet.
Is milk fat healthy?
For many decades, there has been fear and debate about fat consumption and the impact on human health, but the cumulative research has begun to bring some clarity in recent years. In an international study, higher fat intake (not just dairy fat) was associated with lower risks of total mortality, stroke, and non-cardiovascular disease mortality (Dehghan et al., 2017). Multiple studies also suggest there may be a lowered heart disease risk with increased intake of polyunsaturated FA (Hugh and Park, 2012).
Nutritional guidelines still encourage low consumption of saturated fats, even though there is no clear association between the intake of saturated FA and coronary disease risk. Research and perspectives have shifted, as dairy fats are now associated with a beneficial or, at worst, a neutral effect on inflammation and cardiovascular disease (Chowdhury et al., 2014; Lordan and Zabetakis, 2017).
There has been limited research specifically comparing the consumption of high-fat versus lower fat dairy products. Though a small number of research publications comparing consumption of high-fat versus low-fat dairy products have suggested an association between whole milk intake and coronary heart disease or stroke, most report no association (Hugh and Park, 2012).
Consumption of whole milk increases total cholesterol and LDL cholesterol more than low-fat milk. However, saturated FA intake also increases HDL cholesterol (the “good” cholesterol), and it is less clear how HDL cholesterol levels are impacted relative to the known increase in LDL cholesterol and the resultant cholesterol ratio after dairy consumption (Hugh and Park, 2012).
Conjugated linoleic acids, which are present in dairy fat, are perceived to provide multiple health benefits, protecting against cardiovascular disease, cancer, and obesity. However, more research is necessary to know how much is applicable to humans, as much of the research has been done in other animals or in vitro.
Nutritional guidelines encourage avoidance of trans fats, particularly those from partially hydrogenated fat, to promote cardiovascular health, as there are positive associations between trans FA intake and coronary disease risk. However, the makeup of trans FA found in milk is different from those in vegetable sources, and there actually appear to be positive cardiovascular effects associated with trans FA from cattle (Lordan et al., 2018).
Omega-3 FA, intake of which is encouraged in nutritional guidelines, have been associated with lower risk of coronary disease, though supplementation with them has not shown any significant reduction in the risk for coronary disease (Chowdhury et al., 2014).
In young children, consumption of full-fat dairy as opposed to low-fat dairy products has been associated with higher vitamin D stores and lower body mass index scores (Lordan et al., 2018).
A recent study (Vanderhout et al., 2016) in Ontario, Canada, was designed to explore the relationship between milk fat consumption and the risk of childhood obesity. In an urban population where 49% of the children drank whole milk, they found a negative relationship between the milk fat percentage that was consumed and body mass index score, and they found a positive association between milk fat and serum 25-hydroxyvitamin D, the chief circulating form of vitamin D. Participants in the study (2,745 children) who drank whole milk had a 2.43-fold lower odds of being overweight and 3.21-fold lower odds of obesity than participants who drank 1% milk.
What differences exist between different milk products?
Feeding dairy cows different diets results in differences in the FA composition of their milk.
Researchers have clearly shown significant differences between conventional and organic products. However most, if not all, of the differences can be attributed to differences in the types of feed the cows are consuming, not the organic-labeled feeds or practices. Fresh pasture has a different balance of FA than corn silage or grain, and differences carry through to the milk that is produced.
There are higher concentrations of CLA and ALA in organic milk than in conventional milk (see Table 1), regardless of country, sampling season, or year (Benbrook et al., 2013; Srednicka-Tober et al., 2016). However, the difference in ALA disappears when comparing to conventional farms that rely more heavily on fresh forage (Benbrook et al., 2013; Schwendel et al., 2015).
Table 1. Percentage of total fatty acid content of different fluid milk products.
|Conventional and organic management systems feed lactating cows a mixture of feed ingredients that may include corn silage, other silage, hay, grain, and other supplements. Organic management systems, however, have specific requirements for pasture access and forage inclusion in the diet (Source: Benbrook et al., 2018).|
|Total conjugated linoleic acid (CLA)||0.62%||0.73% (↑18%)||1.39% (↑124%)|
|a-linolenic acid (ALA)||0.51%||0.82% (↑60%)||1.23% (↑141%)|
|Total omega-3 fatty acids||0.64%||1.03% (↑62%)||1.58% (↑147%)|
Conjugated linoleic acid is at its peak in organic milk from May through October, which is during the time of year cows are most commonly on pasture (Benbrook et al., 2013).
In an international study, it was calculated that consumption of half of a liter of whole milk would provide approximately 11% of what the European Food Safety Authority recommends for omega-3 FA intake if it was conventionally-produced milk. In comparison, organic milk would provide 16% of the recommendation (Srednicka-Tober et al., 2016). The consumer can decide whether this difference is significant enough to factor into purchasing decisions.
There are other differences between conventional and organic milk, but the differences in CLA and omega-3 FA are the ones that currently stand out and may have the most impact on human health.
Traditional organic feeding systems allow for a variety of different types of feed. Other systems focus almost entirely on forage intake and prohibit the feeding of grain.
In a study where researchers were looking at the FA content of milk from cows only receiving perennial ryegrass pasture, omega-3 FA were 37% higher than in milk from cows fed a more traditional mixed ration with grass silage, corn silage, and concentrates (O’Callaghan et al., 2016b). In a related study, butter derived from cows on pasture was higher in omega-3 FA than from cows fed the traditional mixed ration (O’Callaghan et al., 2016a).
Conjugated linoleic acid concentrations were twice as high in milk from cows on pasture, as compared to cows eating a mixed ration, though there was a lot of variability from cow to cow (Kelly et al., 1998). Concentrations of CLA are impacted by diet factors such as forage to concentrate ratio, level of intake, and intake of unsaturated FA. For example, concentrations were more than double in butter from pasture-fed cows than from cows fed a mixed ration (O’Callaghan et al., 2016a).
The positive increases in ALA and CLA found in organic milk were even greater when looking at organic milk from cows fed nearly 100% forage (referred to as “grassmilk”; see Table 1; Benbrook et al., 2018).
Fat in milk: Conclusion
Fat is an important part of the human diet that, in milk, provides several essential FA and vitamins that contribute to healthy living for people of all ages. Though it may initially seem counterintuitive, youth who drink whole milk show better signs of health and weight control, possibly in part because drinking whole milk results in more satiation and less desire for excess food intake.
Looking at different dairy options in the food case, there are some proven differences in the FA profiles. Milk or butter that is labeled as organic or from cows being fed grass or forage only diets typically has higher CLA and omega-3 FA content. The differences can be attributed to the diet the cows are consuming, as they typically have a higher percentage of fresh pasture intake in those management systems. The impacts this may have on the human diet, however, may be negligible. A single serving may only provide a few percentage points more of the recommended daily allowance for omega-3 FA, which may also be met with other food products or more dairy.
What amount and types of fats do you prefer in your milk and dairy products? You decide.
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