What is Lactose Intolerance?
Lactose is a healthy carbohydrate commonly available in milk and milk products. It has two simpler sugar units, namely glucose and galactose. Lactose improves the absorption of calcium, which is essential for your bone health. Additionally, lactose is the only source of galactose, a simple sugar that is necessary for good brain function and a healthy immune system.
Lactose intolerance occurs when lactose is not digested well by our digestive system.
When we say that the food is digested in our stomach, it is to be understood that the food that we take in is broken down into smaller pieces so that the body can absorb them. In the mouth, the breakdown occurs as we chew the food, but in the stomach, the food is broken down through chemical reactions. This process is aided by certain proteins known as enzymes.
One such enzyme is lactase. This is responsible for breaking down lactose molecules. Any deficiency in this enzyme would ultimately prevent lactose from being digested, thereby causing lactose intolerance.
When lactose intolerant people consume dairy products, they’re usually faced with unpleasant gastrointestinal (stomach and intestines) symptoms. Some of the symptoms that occur immediately include:
- Cramps in the abdominal region,
- Bloating,
- Diarrhea
- Indigestion
- Gas
How Did Humans Develop Lactose Intolerance?
It is believed that a few thousand ago, only infants were able to digest lactose, and as they grew, they began losing the lactase enzyme. In other words, all adults were lactose intolerant.
But around 8,000 years ago in what's now Turkey — just when humans were starting to milk newly domesticated cows, goats, and sheep — changes in the gene responsible for producing lactase appeared more frequently. Around the same time, adult lactose tolerance developed. But of course, not the entire population was lactose tolerant. There were still human adults who were lactose intolerant.
But how did lactose tolerance continued to prevail?
An evolutionary geneticist, Mark Thomas, seems to think that it is the combination of famine and 'deadly diarrhea.' In times of famine, where the crops failed, the consumption of milk probably increased. While it may just cause some gastrointestinal disturbances in the intolerant, it can be deadly for the malnourished. So, the lactose tolerant people had a better survival advantage at this time and were more likely to pass on the gene that ensured the ability to consume dairy.
The Genetics Behind Lactose Intolerance
The LCT gene is responsible for producing the lactase enzyme, which degrades the lactose molecules. This LCT gene is controlled by another gene, the MCM6.
Located inside the long arm of chromosome 2, the MCM6 gene plays a governing role in our ability to digest milk products. A specific portion of the MCM6 gene - known as a regulatory element - helps control the activity (expression) of the LCT gene, thus influencing how much lactase is produced.
rs4988235
Also known as "C/T(-13910)" or just 13910T, rs4988235 is located in the MCM6 gene. However, it exerts its influence on the lactase-producing-LCT gene. The T allele of rs4988235 is the more common allele and enables the lactase enzyme production. So people who have the T allele are likely to be lactose persistent or lactose tolerant. The lactase activity is more in people with two T alleles than with one.
Secondary Lactose Intolerance
Lactose intolerance due to a deficiency of lactase enzyme in certain LCT gene types (genetic) is referred to as primary lactose intolerance. Secondary lactose intolerance, on the other hand, is caused by any illness or injuries. Any such conditions might affect your small intestine and lead to a reduction in lactase secretion. Celiac disease and Crohn’s disease are the two most common intestinal diseases linked to low lactase secretion.
Can You Be Lactose Tolerant and Suddenly Become Intolerant?
Answering this question also partially solves the mystery of why, for some people, the genetic tests indicate lactose intolerance, despite them being able to consume dairy products without any issues.
Let’s take an example of a person who has been following a vegan lifestyle for a while now. Before taking up veganism, he could comfortably eat and digest dairy products. However, after a few years on a vegan diet, re-introducing dairy products in his diet is suddenly giving him digestive problems. So, what could have happened?
The cells that produce the enzyme to digest lactose (lactase) are present in the intestines and are few in number. When a person goes off-dairy for a while, there’s basically no work for these cells, and they temporarily disappear. When dairy is introduced back into the diet, the person may exhibit lactose intolerance for a while till these cells reappear.
Another factor that could explain this is our gut flora - the millions of bacteria present in our digestive tract. Some of these bacteria may help in breaking down lactose molecules. A diet that includes dairy products will offer a survival advantage to the bacteria that can digest lactose and will let them flourish better. On the other hand, these bacteria will not thrive for long when dairy is removed from the diet. When re-introducing dairy products, due to the lack of lactose-digesting-bacteria, the person may experience temporary lactose intolerance.
Gut bacteria is also the reason why many lactose-intolerant individuals can still consume up to a cup of milk per day.
If lactose-digesting bacteria dominate your gut flora, you may even be able to stick to a normal diet (that includes dairy) even if your gene does not produce lactase!
In fact, introducing some ‘good bacteria’ (with the ability to digest lactose) has been studied to improve and provide relief from lactose intolerance symptoms.
Ethnicity and Lactose Intolerance
The incidence of lactose intolerance varies among different population groups. The population that’s the most affected with lactose intolerance is the East Asian descent, with 70-100% of the people affected.
On the other end is North European descent with 18-26% incidence due to their long-standing dependence on unfermented dairy products - the European ancestors may have nested in a place where dairy farming flourished and passed on the specific gene type that produces lactase.
Ethnicity also influences the age of onset of lactose intolerance.
Lactose intolerance in infancy is the most common in Finland - 1 in 60,000 newborns affected. In white people, intolerance develops in children older than five years. In African Americans, most children become intolerant by the age of two years.
Lactose-intolerance Friendly Diet
A lactose-free diet is the best way to combat lactose intolerance. But before eliminating lactose from your diet, it is important to test your extent of intolerance. Research suggests that many people can have a cup of milk every day (12 grams of lactose) without any or very mild symptoms. This is important because dairy is a common source of vitamin D and calcium. So if you can tolerate a cup of milk a day, it's better to use it to your advantage.
But even if a hint of lactose in your diet puts you in trouble, then it is advisable to cut out dairy and all the other sources of lactose from your diet.
What can you eat?
Some healthy food groups that are also lactose-free include:
- Fruits and Vegetables
- Seafood
- Lactose-free milk: Almond milk, soy milk
- Eggs
- Legumes
- Nuts and Seeds
- Whole grains
Meeting your Calcium and Vitamin D Requirements
Calcium and vitamin D are important for your bone health. Getting adequate levels of these nutrients when on a dairy-free diet can be challenging. Some food sources are naturally high in calcium and vitamin D. Finding a way to incorporate them into your diet can ensure good bone health.
- Dark green vegetables like kale, broccoli, collar beans, and bok choy are excellent sources of calcium.
- Almonds also supply you with a good amount of calcium.
- Fish with soft bones, such as canned salmon or sardines, are good sources of vitamin D.
- Soy foods like tofu or soy milk are great alternatives to dairy products.
- Orange juice fortified with vitamin D and calcium (ensure that there are no added sugars)
- Chia and poppy seeds are dense with calcium.
- Supplements: Lactase supplements, lactose-free vitamin D, and calcium supplements - only after consulting a doctor
Summary
- Lactose intolerance occurs due to an inability to digest lactose, a sugar usually found in dairy products. It is a very common condition, affecting up to 75% of the global population.
- Primary lactose intolerance prevents the body from producing the enzyme lactase, which degrades the lactose sugar. It is mostly genetic in nature. Secondary intolerance can occur due to illness/injuries that affect the intestines, resulting in reduced lactase production.
Ethnicity plays a major role in the differences in incidence and age of onset of lactose intolerance across populations. - The LCT gene is responsible for producing the lactase enzyme. The expression of the LCT gene is governed by another gene called MCM6. People with certain types of the MCM6 gene have reduced expression of the LCT gene, and hence, decreased levels of the lactase enzyme. Such individuals may experience lactose intolerance symptoms upon dairy consumption.
- The best way to deal with lactose intolerance is to adopt a lactose-free (dairy-free) diet - at the same time, it is important to enrich your diet with calcium and vitamin D from other food sources.
References
https://evolution.berkeley.edu/evolibrary/news/070401_lactose
https://www.independent.co.uk/life-style/health-and-families/dairy-cut-diet-lactose-intolerant-symptoms-development-skin-cheese-milk-yoghurt-a8302626.html
https://medlineplus.gov/genetics/gene/mcm6/
https://www.snpedia.com/index.php/Rs4988235
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3048992/
https://www.ncbi.nlm.nih.gov/pubmed/25625576?dopt=Abstract
https://link.springer.com/article/10.1007/s12602-018-9507-7
https://medlineplus.gov/genetics/condition/lactose-intolerance/#frequency
https://pubmed.ncbi.nlm.nih.gov/22643754/
