Teeth are made up of different layers — enamel, dentin, pulp, and cementum. Enamel, which is the hardest substance in the body, is on the outside of the tooth. The second layer is dentin, which is softer than enamel, and the deepest layer inside the tooth is pulp, which consists of nerves and blood vessels. Cementum is on the root of the tooth and is beneath the gums.

The number and types of teeth a person has changes as they age. Typically, people have two sets of teeth during their life — primary, or baby teeth, and permanent, or adult teeth. In this article, we look at the teeth that children and adults have, as well as their functions.

Humans have the following types of teeth:

Incisors

Incisors are the sharp teeth at the front of the mouth that bite into food and cut it into smaller pieces. They are flat with a thin edge. They are also called anterior teeth.

Both children and adults have eight incisors — four central incisors at the front of the mouth, two on each row, with one lateral incisor positioned on either side of them.

Canines

Canines are the sharp, pointed teeth that sit next to the incisors and look like fangs. Dentists also call them cuspids or eyeteeth. Canines are the longest of all the teeth, and people use them to tear food.

Both children and adults have four canines. Children usually get their first permanent canines between the ages of 9 and 12. The lower canines tend to come through slightly before those in the upper jaw.

Premolars

Premolars, or bicuspids, are bigger than the incisors and canines. They have many ridges and help chew and grind up food. Adults have eight premolars. The first and second premolars are the molars that sit next to the canines.

Young children do not have premolar teeth. These first appear as permanent teeth when children are 10–12 years old.

Molars

Molars are the biggest of all the teeth. They have a large, flat surface with ridges that allow them to chew food and grind it up. Adults have 12 permanent molars — six on the bottom and top jaw, and children have eight primary molars.

The last molars to erupt are wisdom teeth, or third molars, which usually come through between the ages of 17–21. These sit at the end of the row of teeth, in the far corners of the jaw. Some people do not have all four wisdom teeth, or the teeth may stay unerupted in the bone and never appear in the mouth.

Sometimes wisdom teeth can become impacted, which means they can become trapped under the gum and are unable to come through properly.

Wisdom teeth that only come through halfway or are in the wrong position can increase the risk for infection or damage in surrounding areas. It is essential to see a dentist if people have any issues with their wisdom teeth.

People may experience mild discomfort when their wisdom teeth start pushing through the gums, but anyone feeling a lot of pain or has swelling should see a dentist.

A dentist may need to remove wisdom teeth if a person has tooth decay, pain, or an infection. People do not need these teeth for chewing, and they are difficult to keep clean because of their position far back in the mouth.

Number of teeth

Children have 20 primary, or baby, teeth. Primary teeth first start to appear when babies are around 6 months old. Children usually get all their primary teeth by the age of 3.

These teeth gradually fall out, and 28 permanent teeth replace them. Sometimes, permanent teeth push the baby teeth out, but typically, permanent teeth come through the gums at the back of the mouth behind the last baby tooth in the jaw.

The first permanent teeth to erupt through the gums are four first, or ‘6-year’ molars, so-called because they usually come through when a child is about 6 years old.

The first baby teeth to fall out are the lower central incisors. The adult central incisors tend to erupt around the same time as the first permanent molars around age 6-7.

Usually, people have lost all of their baby teeth by around the age of 14.

If we cut our skin or break a bone, these tissues will repair themselves; our bodies are excellent at recovering from injury.

Tooth enamel, however, cannot regenerate, and the oral cavity is a hostile environment.

Every mealtime, enamel is put under incredible stress; it also weathers extreme changes in both pH and temperature.

Despite this adversity, the tooth enamel that we develop as a child remains with us throughout our days.

Researchers have long been interested in how enamel manages to stay functional and intact for a lifetime.

As one of the authors of the latest study, Prof. Pupa Gilbert from the University of Wisconsin–Madison puts it, “How does it prevent catastrophic failure?”

The secrets of enamel

With assistance from researchers at the Massachusetts Institute of Technology (MIT) in Cambridge and the University of Pittsburgh, PA, Prof. Gilbert took a detailed look at the structure of enamel.

The team of scientists has now published the results of its study in the journal Nature Communications.

Enamel is made up of so-called enamel rods, which consist of hydroxyapatite crystals. These long, thin enamel rods are around 50 nanometers wide and 10 micrometers long.

By using cutting edge imaging technology, the scientists could visualize how individual crystals in tooth enamel are aligned. The technique, which Prof. Gilbert designed, is called polarization-dependent imaging contrast (PIC) mapping.

Before the advent of PIC mapping, it was impossible to study enamel with this level of detail. “[Y]ou can measure and visualize, in color, the orientation of individual nanocrystals and see many millions of them at once,” explains Prof. Gilbert.

The architecture of complex biominerals, such as enamel, becomes immediately visible to the naked eye in a PIC map.

When they viewed the structure of enamel, the researchers uncovered patterns. “By and large, we saw that there was not a single orientation in each rod, but a gradual change in crystal orientations between adjacent nanocrystals,” explains Gilbert. “And then the question was, ‘Is this a useful observation?'”

The importance of crystal orientation

To test whether the change in crystal alignment influences the way that enamel responds to stress, the team recruited help from Prof. Markus Buehler of MIT. Using a computer model, they simulated the forces that hydroxyapatite crystals would experience when a person chews.

Within the model, they placed two blocks of crystals next to each other so that the blocks touched along one edge. The crystals within each of the two blocks were aligned, but where they came in contact with the other block, the crystals met at an angle.

To investigate, co-author Cayla Stifler returned to the original PIC mapping information and measured the angles between adjacent crystals. After generating millions of data points, Stifler found that 1 degree was the most common size of misorientation, and the maximum was 30 degrees.

This observation agreed with the simulation — smaller angles seem better able to deflect cracks.

Now we know that cracks are deflected at the nanoscale and, thus, can’t propagate very far. That’s the reason our teeth can last a lifetime without being replaced.

Prof. Pupa Gilbert

The bacteria in our mouths may hold the key to many facets of our health.

Researchers have found intriguing clues about pancreatic and esophageal cancer risk in mouth bacteria, and some studies have linked poor oral hygiene with respiratory problems.

Mounting evidence is also strengthening the link between oral health and cardiovascular health.

For instance, some studies have found oral bacteria in the blood clots of people receiving emergency treatment for stroke, and experts have linked severe gum disease with a significantly higher risk of hypertension.

Conversely, destroying “friendly” oral bacteria that help maintain a healthy and balanced oral microbiome could disrupt blood pressure levels and also lead to hypertension.

Maintaining good oral health, therefore, seems to be key to cardiovascular health.

Now, a new study that appears in the European Journal of Preventive Cardiology suggests that regular toothbrushing may keep heart failure and atrial fibrillation (A-fib) — a type of arrhythmia — at bay.

Dr. Tae-Jin Song of Ewha Womans University in Seoul, Korea, is the senior author of the new study.

In their paper, Dr. Song and team explain that the motivation for the study hinges on the mediating role of inflammation. They write, “Poor oral hygiene can provoke transient bacteremia and systemic inflammation, a mediator of atrial fibrillation and heart failure.”

Studying A-fib, heart failure, and oral hygiene

In their study, Dr. Song and team examined atrial fibrillation’s associations with both heart failure and poor oral hygiene. They used data from 161,286 people who were part of the Korean National Health Insurance System-Health Screening Cohort.

A-fib is a condition affecting at least 2.7 million people in the United States. In people with A-fib, the heart cannot efficiently pump blood to the rest of the body because it does not beat regularly.

The heart also does not pump blood as it should in people with heart failure. This inefficiency results in fatigue and, sometimes, breathing difficulties, as insufficient oxygen reaches the other organs in the body.

The participants of the current study were 40–79 years old and had no history of either A-fib or heart failure. During enrollment, which took place between 2003 and 2004, the team measured the height and weight of each of the participants and asked them questions about their lifestyle, oral health, and oral hygiene habits.

The participants also underwent some laboratory tests, which included blood tests, urine tests, and blood pressure readings.

Brushing lowers heart failure risk by 12%

Over a median follow-up period of 10.5 years, 4,911 participants received a diagnosis of A-fib, and 7,971 developed heart failure.

Brushing the teeth three times or more a day was linked with a 10% lower chance of developing A-fib and a 12% lower risk of heart failure.

Confounding factors — including age, sex, socioeconomic status, physical activity, alcohol intake, body mass index, and other coexisting conditions, such as hypertension — did not influence these results, as the researchers accounted for them in their analysis.

The authors conclude:

Improved oral hygiene care was associated with decreased risk of atrial fibrillation and heart failure. Healthier oral hygiene by frequent toothbrushing and professional dental cleanings may reduce risk of atrial fibrillation and heart failure.

Several scientific studies have examined the health benefits of baking soda and lemon juice separately, but there is not much research to support the combined effects of these two ingredients.

This article discusses some of the potential health benefits of consuming baking soda and lemon juice mixtures.

A note about pH

The idea of combining baking soda and lemon juice draws on basic principles of acidity and the pH scale.

Scientists use the pH scale to measure the acidity of a solution. A solution can have a pH level between 0 and 14.

The lower the pH, the more acidic the solution, so:

• pH levels below 7 indicate an acidic solution
• pH levels above 7 indicate an alkaline, or base solution
• neutral solutions, such as pure water, have a pH of 7

Baking soda, also known as sodium bicarbonate, is a base. This means that when people dissolve baking soda in water, it forms an alkaline solution. For example, a 0.1 molar solution of baking soda has a pH of around 8.3.

Lemon juice contains citric acid and has a pH of around 3. Adding baking soda to lemon juice will raise the pH to produce a more neutral solution.

Skin care

Usually, the skin has a weakly acidic pH of about 5.7. Bases, such as baking soda, will increase the pH of the skin. Higher pH levels can disrupt the barrier function of the skin, which may lead to dryness, excess oil production, and acne.

Lemon juice appears to have obvious skincare applications because it contains concentrations of vitamin C and citric acid, which both provide powerful skin benefits. Citric acid is an alpha-hydroxy acid (AHA) that manufacturers commonly use in chemical peels.

However, skin cells naturally repel water-soluble molecules, such as vitamin C. This means that very little vitamin C will actually penetrate the skin.

The high acid content of lemon juice can lower the pH level of the skin. Low pH levels may cause skin irritation, hyperpigmentation, and UV light sensitivity.

Alternatives

Using a homemade mixture of baking soda and lemon juice may potentially be harmful to the skin. Instead, a person can try using neutral cleansers or chemical peels that contain AHAs, such as glycolic acid.

Neutralizing stomach acid

Excess stomach acid can cause uncomfortable symptoms, such as heartburn, vomiting, and indigestion.

Many people with excess stomach acid take over-the-counter (OTC) antacids to relieve their symptoms. Consuming baking soda and lemon juice together may also neutralize stomach acid in a similar fashion as an antacid.

A 2017 study examined the antacid effects of various foods. The authors of this study created artificial stomach acid with a pH of 1.2. Although lemon juice by itself had almost no effect, sodium bicarbonate successfully neutralized the synthetic stomach acid.

Many OTC antacids contain sodium bicarbonate and citric acid. Lemons and other citrus fruits are rich sources of naturally-occurring citric acid.

When a person mixes lemon juice and baking soda, the citric acid reacts with the sodium bicarbonate to produce a buffer called sodium citrate. A buffer refers to a weak acid or base that prevents drastic pH changes. Although lemon juice does not neutralize stomach acid, it may help stabilize the pH level inside the stomach.

Alternatives

Using baking soda and lemon juice to combat excess stomach acid may be a good home remedy, as effective OTC antacids contain similar ingredients.

However, mixing the correct proportions of baking soda and lemon juice can be difficult.

Consuming a mixture with too much baking soda may cause diarrhea and gas, whereas too much lemon juice could trigger acid reflux and make symptoms worse. Purchasing an antacid at the drug store is often much safer.

Other home remedies for reducing excess stomach acid include:

• avoiding or reducing acidic foods and beverages
• limiting caffeine intake
• limiting alcohol consumption
• eating smaller meals
• drinking more water
• getting enough sleep

People with severe or persistent acid reflux or heartburn should speak to a doctor or gastroenterologist.

Xylitol is a sugar alcohol, which is a type of carbohydrate and does not actually contain alcohol. Xylitol occurs naturally in small amounts in fibrous fruits and vegetables, trees, corncobs, and even the human body.

Manufacturers use xylitol as a sugar substitute because its sweetness is comparable with that of table sugar but with fewer calories.

Xylitol is a common ingredient in many products, from sugar-free chewing gum to toothpaste. People also use xylitol as a table-top sweetener and in baking.

In this article, we look at the uses and potential health benefits of xylitol. We also cover its side effects, drug interactions, dosage, and alternatives.

Uses

Xylitol has a similar level of sweetness to sugar but with a fraction of the calories. It is a popular ingredient in a variety of products, including sugar-free gum and toothpaste.

Manufacturers add xylitol to a range of foods, including:

• sugar-free candies, such as gum, mints, and gummies
• jams and jellies
• honey
• nut butters, including peanut butter
• yogurt

Xylitol is also an ingredient in some dental care products, including:

• toothpaste
• mouthwash
• other fluoride products

Xylitol sweeteners are available to purchase online.

Potential benefits

Xylitol has several potential health benefits, including:

Low glycemic index

Xylitol has a low glycemic index (GI). This means that consuming it does not cause spikes in blood glucose or insulin levels in the body. For this reason, xylitol is a good sugar substitute for people with diabetes.

Due to its low GI, xylitol is also a weight loss-friendly sugar substitute.

Also, a 2015 study revealed that xylitol had significant blood glucose-lowering effects in rats that ate high-fat diets.

Dental health

Xylitol is an ingredient in many dental hygiene products, including toothpaste and mouthwash. This is due to the fact that xylitol is non-fermentable, which means that the bacteria in the mouth cannot convert it into the harmful acid that causes tooth decay.

The oral bacterium Streptococcus mutans is largely responsible for plaque, which is the sticky, white substance that can accumulate on the outside of a person’s teeth.

Plaque binds lactic acid against the surface of the tooth. This acid breaks down the enamel and leads to tooth decay.

While it is normal for people to have some plaque on their teeth, excess amounts can lead to tooth decay, cavities, and gum disease.

A 2017 systematic review suggests that xylitol reduces the amount of S. mutans bacteria in the mouth, which reduces the amount of plaque and may help prevent tooth decay.

A 2014 study examined the effects of xylitol on Porphyromonas gingivalis, which is the bacterium responsible for gingivitis, or gum disease. If left untreated, excess amounts of P. gingivalis can move into the bloodstream and lead to systemic inflammation.

In the study, scientists grew samples of P. gingivalis in a laboratory and added them to human cell cultures pretreated with xylitol. They saw that xylitol increased the production of immune system proteins and inhibited the growth of the bacteria.

Side effects and safety

The United States Food and Drug Administration (FDA) have approved xylitol as a food additive. Xylitol is generally safe, but like other sugar alcohols, it can cause digestive issues such as bloating and diarrhea in some people.

It is worth noting that xylitol can be very toxic to dogs. It is vital to store products containing xylitol in a safe place that pets cannot reach. Anyone who thinks that their dog has consumed xylitol should call their veterinarian or the Animal Poison Control Center immediately.