Milk is a highly nutritious fluid formed in the udders of dairy cows, designed to sustain the newborn calf during its first months of life.
A huge variety of food products are made from cow's milk, such as cheese, cream, butter and yogurt.
These foods are referred to as dairy products, or milk products, and they are a major part of the modern diet.
The nutritional composition of milk is highly complex, and it contains almost every single nutrient that the human body needs.
The table below contains detailed information on nutrients in milk (1).
Keep in mind that many milk products are fortified with vitamins, including D and A.
It has approximately 1 g of protein in each fluid ounce (30.5 g), or 7.7 g in each cup (244 g).
Proteins in milk can be divided into two groups based on their solubility in water.
Insoluble milk proteins are called casein, whereas soluble proteins are known as whey proteins.
Both of these groups of milk proteins are considered to be of excellent quality, with a high proportion of essential amino acids and good digestibility.
Bottom Line: Milk is a very good source of high-quality protein, which can be divided into two categories, casein and whey proteins.
Casein forms the majority (80%) of proteins in milk.
Casein is actually a family of different proteins and the most abundant one is called alpha-casein.
One important property of casein is its ability to increase the absorption of minerals, such as calcium and phosphorus (2).
Bottom Line: Most of the proteins in milk are categorized as casein, which has a number of health benefits.
Whey is another family of proteins, accounting for 20% of the protein content in milk.
Whey is particularly rich in branched-chain amino acids (BCAAs), such as leucine, isoleucine, and valine.
It is composed of many types of soluble proteins with different properties.
Consumption of whey protein is excellent for the growth and maintenance of muscles. As a result, it is a popular supplement among athletes and bodybuilders.
Bottom Line: Whey proteins are one of the two main families of milk proteins. In addition to being good for muscle growth and maintenance, they may decrease blood pressure and improve mood.
Whole milk, straight from the cow, is around 4% fat.
In many countries, marketing of milk is mainly based on fat content. In the US, whole milk is 3.25% fat, whereas reduced-fat milk is 2% fat, and low-fat milk 1%.
Milk fat is one of the most complex of all natural fats, containing about 400 different types of fatty acids (7).
Whole milk is very high in saturated fats. About 70% of the fatty acids in milk are saturated.
Polyunsaturated fats are present in minimal amounts. They constitute around 2.3% of the total fat content.
Monounsaturated fats make up the rest, about 28% of the total fat content.
Bottom Line: Unprocessed milk is 4% fat, but the content in commercial milk varies, depending on the type. Milk fat is mainly composed of saturated fat.
Ruminant Trans Fats
Trans fats are naturally found in dairy products.
In contrast to trans fats found in processed foods, dairy trans fats, also called ruminant trans fats, are generally considered to have beneficial effects on health.
Bottom Line: Milk contains small amounts of ruminant trans fats. Conjugated linoleic acid (CLA) is the most studied and has been linked with several health benefits.
Carbs in milk are mainly in the form of a simple sugar called lactose, which constitutes around 5% of the weight of milk.
In the digestive system, lactose breaks down into glucose and galactose. These are absorbed into the bloodstream, and galactose is converted into glucose by the liver.
Some people lack the enzyme required to break down lactose. This condition is called lactose intolerance.
Bottom Line: Carbs constitute around 5% of milk, most of which is lactose (milk sugar), which many people are intolerant to.
Vitamins and Minerals
Milk contains all the vitamins and minerals necessary to sustain growth and development in the young calf during its first months of life.
It also contains almost every single nutrient needed by humans, making it one of the most nutritious foods on the planet.
The following vitamins and minerals are found in particularly large amounts in milk:
- Vitamin B12: This essential vitamin is only found in foods of animal origin (14), and milk is very high in B12 (1).
- Calcium: Milk is not only one of the best dietary sources of calcium, but the calcium found in milk is also easily absorbed (15).
- Riboflavin: One of the B-vitamins, also called vitamin B2. Dairy products are the biggest source of riboflavin in the Western diet (16).
- Phosphorus: Dairy products are a good source of phosphorus, a mineral that plays an essential role in many biological processes.
Milk Enrichment with Vitamin D
Enrichment, also called fortification, is the process of adding minerals or vitamins to food products.
As a public health strategy, enrichment of milk products with vitamin D is common and even mandatory in some countries (17).
In the US, one cup of vitamin D enriched milk (244 g) may contain 65% of the daily recommended allowance for vitamin D (18).
Bottom Line: Milk is an excellent source of many vitamins and minerals, including vitamin B12, calcium, riboflavin, and phosphorus. It is often enriched with other vitamins, especially vitamin D.
More than 50 different hormones are naturally present in cow's milk.
These hormones are important for the development of the newborn calf (19).
With the exception of insulin-like growth factor-1 (IGF-1), cow milk hormones have no known effects in humans.
IGF-1 is also found in human breast milk and is the only hormone known to be absorbed from cow's milk. It is involved in growth and regeneration (20).
Bovine growth hormone is another hormone naturally present in milk in small quantities. It is only biologically active in cows and has no effect in humans.
Bottom Line: Milk contains a wide variety of hormones that promote the development of the newborn calf. Only one of them, insulin-like growth-factor 1 (IGF-1), is active in humans.
Bone Health and Osteoporosis
Osteoporosis, a condition characterized by a decrease in bone density, is the main risk factor for bone fractures among elderly people.
One of the functions of cow's milk is to promote bone growth and development in the young calf.
Cow's milk seems to have similar effects in adult humans and has been associated with a higher bone density (15).
The high calcium and high protein content of milk are the two main factors believed to be responsible for this effect (21).
Bottom Line: Being a rich source of calcium, milk may promote increased bone mineral density, cutting the risk of osteoporosis.
Other Health Benefits of Milk
Milk is one of the most nutritious foods you can find.
It has been widely studied, and seems to have several important health benefits.
Abnormally high blood pressure, also called hypertension, is a major risk factor for cardiovascular disease.
Bottom Line: Milk and milk products have been linked with reduced blood pressure.
Lactose, also called milk sugar, is the main carbohydrate found in milk.
In the digestive system, it is broken down into its subunits, glucose and galactose.
However, this doesn't happen in all people.
An enzyme called lactase is required for the breakdown of lactose. Some people lose the ability to digest lactose after childhood.
This inability to digest lactose is called lactose intolerance.
It has been estimated that about 75% of the world's population has lactose intolerance (26). However, the proportion of lactose intolerant people varies greatly depending on genetic makeup.
This photo shows the frequency of lactose intolerance in various parts of the world:Photo source.
In people with lactose intolerance, lactose is not fully absorbed and some (or most) of it passes down to the colon.
In the colon, the bacteria that reside there end up fermenting it. This fermentation process leads to the formation of short-chain fatty acids and gas, such as methane and carbon dioxide.
Lactose intolerance is associated with many unpleasant symptoms, such as gas, bloating, abdominal cramps, diarrhea, nausea, and vomiting.
Bottom Line: Many people are intolerant to milk sugar (lactose). The main symptoms are bloating, flatulence, and diarrhea.
Other Adverse Effects
The health effects of milk consumption are complex.
Some components of milk are quite beneficial, but others may have adverse effects.
Milk allergy is rare in adults, but more frequent in young children (27).
Most often, allergic symptoms are caused by whey proteins called alpha-lactoglobulin and beta-lactoglobulin, but they can also be due to caseins (28).
Acne is a common skin disease characterized by pimples, especially on the face, chest and back.
Milk and Cancer
Many observational studies have looked at the association between the consumption of dairy products and the risk of cancer.
Overall, the evidence is mixed and very few conclusions can be drawn from the data.
As a general recommendation, excessive consumption of milk should be avoided. Moderation is key.
Bottom Line: In addition to being allergenic to some people, milk has been linked with several adverse effects, such as increased risk for acne and prostate cancer.
Virtually all milk sold for human consumption is processed in some way.
This is done to increase the safety of milk consumption and the shelf life of milk products.
Pasteurization is the process of heating milk to destroy potentially harmful bacteria that are occasionally found in raw milk (40).
The heat eliminates beneficial as well as harmful bacteria, yeasts and molds.
However, pasteurization does not make milk sterile. Therefore it needs to be quickly cooled down after heating to keep any surviving bacteria from multiplying.
Pasteurization results in a slight loss of vitamins, due to their sensitivity to heat, but doesn't have a substantial effect on the nutritional value (41).
Milk fat is made up of countless globules of different sizes.
In raw milk, these fat globules have a tendency to stick together and float to the surface of milk.
Homogenization is the process of breaking these fat globules into smaller units.
This is done by heating the milk and pumping it through narrow pipes at high pressures.
The purpose of homogenization is to increase the shelf life of milk, and to give it a richer taste and whiter color.
Most milk products are produced from homogenized milk. An exception is cheese, which is usually produced from unhomogenized milk.
Homogenization does not have any adverse effects on the nutritional quality (42).
Bottom Line: To increase its shelf life and safety, commercial milk is pasteurized and homogenized.
Raw Milk vs. Pasteurized Milk
Raw milk is a term used for milk that has not been pasteurized or homogenized.
Pasteurization is the process of heating milk to increase shelf life and minimize the risk of illness from harmful microorganisms that may be present in raw milk.
Homogenization, which is the process of breaking the fat globules in milk into smaller units, has no known adverse health effects (42).
Consumption of raw milk has been associated with reduced risk of childhood asthma, eczema and allergy (46). However, the studies on this have been small and inconclusive.
Although raw milk is more "natural" than processed milk, its consumption is riskier.
In healthy cows, milk does not contain any bacteria. It is during the milking process, transport or storage that milk gets contaminated with bacteria, either from the cow itself or the environment.
Most of these bacteria are not harmful, and many of them even beneficial, but occasionally, milk gets contaminated with bacteria that have the potential to cause disease.
Although the risk of drinking raw milk is very small, a single milk-borne infection may have serious consequences.
Most people are quick to recover, but people with weak immune systems, such as the elderly or very young children, are more susceptible to severe illness.
Taken together, there is no strong evidence in support of raw milk consumption (47). Any potential health benefits are outweighed by possible health risks resulting from contamination with harmful bacteria.
Bottom Line: Raw milk has not been pasteurized or homogenized. Drinking raw milk is not recommended because it may be contaminated with harmful bacteria.
Milk is one of the most nutritious drinks in the world.
Not only is it rich in high-quality protein, it is also an excellent source of vitamins and minerals, such as calcium, vitamin B12, and riboflavin.
For this reason, it may cut the risk of osteoporosis and reduce blood pressure.
On the negative side, some people are allergic to milk proteins or intolerant to milk sugar (lactose).
It has also been linked with increased risk for prostate cancer, but the evidence is weak.
At the end of the day, moderate consumption of milk may be healthy, while excessive consumption should be avoided.