Creams of the crop: Milk is ubiquitous, used in everything from cereal to tea, but how good is it for us?

Splashed on our cornflakes, added to tea or coffee and warmed on the stove to help us sleep, milk is a central part of our everyday lives.

But how much do we really know about the white stuff? And, with changing tastes and diets, is milk still “milk” when we opt for oat, almond, soya and even pea in our morning latte?

In her new book, Drinkology: The Science of What We Drink and What It Does to Us, Dr Alexia Willett explores the science, myths and mysteries surrounding every sip that enters our bodies – including breast, plant, dairy, formula and all the other modern variations of a once-simple drink.

Here, we discover what really goes into each glass, mug, pint and baby’s bottle.

Breast milk

There are different types of breast milk depending on the stage of lactation and there are three main stages. In the first few days after birth colostrum is secreted (a yellowish, sticky milk), which contains more protein, less fat and a number of immunising factors for the newborn.

Transitional milk comes next and is produced from about day eight to 20; this is where the milk transitions to mature milk.

Mature milk finally comes in after about 20 days onwards and varies within, and between, individuals in terms of its content.

Breast milk comprises everything a new baby needs for healthy development, including water, proteins, fats, carbohydrates (mainly lactose), minerals (particularly sodium, potassium, calcium, magnesium and phosphorous), all the vitamins (except vitamin K), although some only in small amounts, and trace elements.

As well as essential nutrients, breast milk also contains thousands of distinct bioactive molecules that help protect against infection and inflammation and support the immune system, organ development and promote healthy microbes.

Animal milk

The colour, flavour and composition of milk is dependent on a range of factors, such as the animal species it comes from, variation of breed, its age and diet, stage of lactation, individual and herd variations, as well as farming practices and seasonal and environmental variations.

In general, animal milk is comprised of water, minute fat globules, proteins, milk sugar (lactose), minerals and trace amounts of other molecules, including vitamins (particularly B vitamins), enzymes, pigments and gases.

By far the largest constituent of milk is water. The water content of milk from different animal species ranges from 83% in yaks to 91% in donkeys. Cow milk contains about 87% water.

Full-fat, semi-skimmed and skimmed milk

In the UK the terms 1%, skimmed, semi-skimmed, whole and full-cream relate to the proportion of fat in the milk: 1% milk has a fat content of 1g per 100g, skimmed milk contains a maximum of 0.3g fat per 100g, semi-skimmed milk has a fat content between 1.5 and 1.8g per 100g and whole milk has a minimum fat content of 3.5g per 100g.

Other than less fat and fewer calories, there is little difference in nutrient content between skimmed, semi-skimmed and whole milk.

The exception is vitamin A content, which increases as the proportion of fat increases (e.g. whole milk contains around twice the amount of vitamin A in semi-skimmed milk but around 50 times the amount that’s in skimmed milk). This is because it is found within the fat in milk.

You often hear that milk is a good source of vitamin D. One reason for this is that in some countries, such as the US, vitamin D is added to the milk. However, milk is not typically fortified in the UK and the naturally occurring amount of vitamin D in milk is negligible.

Fermented milks

Controlled cultures, fermentation temperature and time are key to developing edible fermented milks. It’s not just a case of letting your pint of milk go off in the fridge.

Fermented milks are thought to be easier to digest than regular milk, so much so that in some cultures fermented products are used as weaning foods for infants. One reason for this is that fermented milks have lower lactose levels than regular milk as the fermentation process breaks down some of the lactose, making it more digestible for some who don’t tolerate milk well.

A number of fermented milks have shown promise for our health, one of which is the increasingly popular kefir. And benefits associated with kefir are emerging, from improved digestion and lactose tolerance, aiding the immune system, improving cholesterol metabolism and potentially having a role in alleviating allergies, as well as demonstrating anti-inflammatory and anti-cancer properties.

However, many of the findings are from animal or lab studies. Robust clinical trials are now needed to demonstrate measurable effects in humans, as well as research to unpick the mechanisms underlying the effects of kefir, to what extent it is useful and who it may benefit.

Plant milk

Plant milk is a booming industry. In the UK alone, plant milk sales are predicted to rise to $400 million by 2021. Almond milk, soya milk and coconut milk are the best sellers, although other plant milks are gaining traction in the marketplace. Once the preserve of vegans and the dairy-intolerant, plant-based milks have gained enormous popularity among the rest of the population.

Almond milk is the most popular plant milk on the market, having overtaken soya milk a few years ago.

Plant-based milk alternatives are often positioned as being healthier than regular dairy milks but while there has been considerable research into the effects of cow milk on health, there is a distinct lack of research into the direct and measurable effects of plant milks.

Night-time milk

We’ve all heard that drinking milk, particularly warm milk, at bedtime can help you drop off to sleep. The evidence to prove this actually works is pretty thin but did you know that there is actually something called ‘night-time milk’?

This isn’t referring to your usual milk but to milk that has been collected from cows milked at night. This is because they apparently have an increased level of melatonin in their milk. Melatonin is a hormone that plays a role in sleep and its levels increase and decrease depending on the time of day.

It increases in the evening and night-time (when it is dark), decreasing again when it gets light, and works in conjunction with other bodily mechanisms to help your body to prepare for sleep. You may feel calmer and less alert and, as such, melatonin sleep aids are very popular. While it is thought that melatonin-enriched milk aids sleep, and is marketed as having such an effect, the evidence so far from scientific studies about its effects is sparse and inconsistent.

Drinkology: The Science of What We Drink and What It Does to Us, from Milks to Martinis, by Dr Alexis Willett, Robinson, £13.99, is out now.