Thursday, November 20, 2014

Taste buds - more than just about taste

A reader asked me the question “What happens to your taste buds as you age?” Well, I said (and assumed) that they decline with age. I remember my grandmother sucking on mints that were a bit too strong for some punk kid in his first decade. Anyway, I said I would find out more. And indeed I did. Come with me and I’ll guarantee you will learn something new.

Taste buds on the top of your tongue
My physiology text tells me that there are 10,000 taste buds in your mouth, with most concentrated on the top of your tongue. Each taste bud has a small opening called the taste pore through which liquids can come into contact with the taste receptor cells (see pic). That means you can taste soup or ice cream instantly, but a dry food will need the assistance of saliva. Most taste buds have a life span of just 8-12 days, so they are replaced three times a month.

You will have been taught that we have taste buds that sense the sweet, sour, salty and bitter tastes in food. You may also know that there is a 5th taste known as umami, the savoury flavour. There is even a suggestion of a 6th taste, one of a metallic sensation, partly because sometimes this is picked up in tasting labs for new processed foods and we don’t like anything that tastes ‘metallic’.

The chemistry of taste
In general, the sweet taste comes from sugars (obviously), alcohol and small proteins. The most famous example of the latter is the sweetener aspartame, which is a very small protein of two amino acids. Those two amino acids (aspartic acid and phenylalanine) are abundant in nature but still people consider them to be “artificial” and the downfall of western civilisation.

Salts, not just sodium chloride (table salt), are salty, and acids tend to give an sour flavour (eg citric acid in lemons, and compounds in spoiled foods). Many poisons in nature taste bitter, so it is speculated that the bitter sensation is to help us avoid becoming sick, as well as avoiding cabbage. Certain amino acids, like glutamate, provide the umami or savoury flavour. They are abundant in mushrooms, tomato paste and parmesan cheese.

Weird stuff
Taste receptors aren’t just in the taste buds in your mouth. They are all through your digestive system, your throat and lungs, and (if you have them) your testis. No, I’m not kidding. OK, what is going on here?

There are proteins in saliva that mediate the taste of bitter, sweet and umami. The compounds in food that trigger the bitter taste also seem to trigger the process of expelling things from the body. Give an infant a bitter food and it will be spat out. Even if as an adult you can handle some bitterness, the taste can evoke sneezing and coughing.

OK, that makes sense, but taste receptors (not taste buds) in the testis and sperm? Look, I can’t say I fully understand, although it seems that the receptors may sense a wholesome diet in Dad meaning there is abundant food, or sense toxic substances in Mum, suggesting she may not be ready for pregnancy, signalling sperm death. So, these receptors could be sensing the health of prospective parents. Weird, but seriously cool if that be the truth. (Here, I have taken artistic license; you read the paper and see if you can make better sense of it. Warning: It contains sentences like this: “Odorants directly activate CatSper without involving GPCRs or cAMP”).

Sweetness about bitter
The ability to taste certain bitter compounds is genetically based. Now there is a strong suggestion the bitter taste is also linked to longevity. The early evidence is that those of you who enjoy bitter tasting vegetables may live longer, no just due to the nutrition in a cabbage or Brussels sprouts. There are compounds in bitter fruits and veg that could be defensive against cancer and that puts the life odds in our favour. Some people have one specific gene for bitter tasting that is strongly associated with a long life.

Taste buds & aging
And speaking of a long life, let’s return to the original question: “What happens to taste buds as you age?” Certainly there is agreement that taste sensitivity declines as we age. A taste has to be stronger at age 75 than at age 25 before it is detected or recognised. Most studies agree that the salty and bitter tastes decline more than sour and sweet tastes over a lifetime.

As we age we seem to have fewer taste buds and fewer taste cells in each taste bud, with the biggest decline in the 74-85 year olds. Why this happens is not clear although there is suspicion that age might affect the stem cells involved in taste bud generation. 

What does it all mean?
Discover your taste buds. Eat a wide range of wholesome foods that take all your taste buds through a full range of experiences. Beats me why so many people rely on a hard-working teenager to cook them a predictable fare of fat, salt or sugar-laden easy-to-chew fast foods.

This was going to be a short article providing a simple answer to a simple question. But I learned so much more, and I hope that you didn’t mind me sharing it with you.

Sherwood L. Human Physiology, 6th edition p 221-225
Ross A et al. Modern Nutrition in Health & Disease, 11th edition p 577-579
Feng P et al 2014. Chemical Senses
Kinnamon SC 2012. Acta Physiology

Campa D et al 2012. PLoS ONE

Saliva - a very useful fluid

A quarter of a century ago saliva was described as:
“ …not one of the popular bodily fluids. It lacks the drama of blood, the sincerity of sweat and the emotional appeal of tears”.

Despite that, saliva is critical to the taste of foods and the health of your teeth. The salivary glands are located near the back of the mouth (at the sides) and under the tongue, producing a fluid that is 99.5% water. It is the other 0.5% that is really interesting as that comprises sodium, potassium, calcium, magnesium, bicarbonate, phosphates, immunoglobulins, enzymes, mucins, urea and ammonia.

Saliva production is very individual, with 500–1500 mL (17-50 fl oz) produced each day, the highest flow being when eating. What does saliva do?
1. Lubricates your food. Try eating a sandwich without it.
2. Clears food from the mouth.
3. Remineralises the enamel of your teeth after each meal, snack or drink.
4. Has immunoglobulins that help kill evil bacteria and support healthy bacteria in your mouth.
5. Provide amylase enzyme to start breaking down starch in your food.
6. Provides a liquid mix to food so that taste buds can sense the flavours in your meal

If your mouth gets dry, the brain will direct you to go and get a drink. If your mouth is frequently dry, which can happen with some medications, over-enjoying alcohol, regular dehydration (eg athletes who lose a lot of sweat each day), then you lose the protective effect of saliva. Tooth decay, mouth ulcers, bad breath and difficulty talking are common symptoms of too little saliva.

So next time you have lunch, just for a moment, think “how sensational is saliva”. Then blow a raspberry. Can’t do that without saliva. You can have other fun with saliva, but we best stop right there. This blog still has a G classification.


Benn & Thomson 2014. New Zealand Dental Journal.