Why do you eat cookies, cake, donuts, candy and drink soda pop?
Because they taste good – duh!
Of course, I’m talking nonsense. Of course, you eat sweet things because they’re sweet (and a little for the pink sprinkles) and if you magically couldn’t taste sweet anymore you’d stop eating sweets and lose bucketloads of fat. Right?
I’m serious. Sweetness may be why you started eating them a long time ago, but a really cool study I’m reviewing this week finds that you can prefer sweet even if you can’t taste it. Woah!
But before I go into the study, let’s understand two things:
- What is taste?
- What is sweet?
Back in elementary school you were taught that you had five senses, with the yummiest sense being the sense of taste. (I’ll let you figure out the rest of the senses on your own.)
Since you’ve been in elementary school, a lot of things have changed: cell phones fit in your pocket, there’s this thing called the Internet; we have indoor plumbing; and now we can detect five tastes.
For about a hundred years everybody thought there were four tastes, but after discovering a new taste we’re up to five distinct tastes. And everybody thought that each taste could only be tasted in one part of your tongue, but that’s not true either; you can taste different tastes all over your tongue.
Table 1: What you can taste, and how it works (1)
How do you taste?
When you put something on your tongue that is water soluble (dissolves in water), some of the chemicals on that thing dissolve and go into taste pores on your taste buds. Each taste bud has 50-150 taste receptor cells (cells that detect different tastes).
Right now, scientists aren’t sure whether each cell can only taste one taste, or if cells can taste multiple tastes. For now, let’s assume that each cell can only taste one taste; that’s a lot easier to understand and explain.
Each taste receptor cell has a specific protein (receptor) on its membrane. Each receptor “catches” a specific type of chemical, then through a bunch of neurons, tells the brain.
As an example, imagine drinking sugar water. The sugar (sucrose) goes into a taste pore, where it bumps into a taste receptor cell and sticks to its taste receptor. The receptor (T1r2, specific for sweet) tells the cell, “Hey I got something!” The cell tells its neighbouring nerve cell and so on until you get to the brain region that’s responsible for detecting sweetness. Really it’s just the telephone game with the non-specific message being “I got something” that comes from a sweet receptor.
What’s the point of tasting?
Taste lets you figure out the nutrient quality of the food and whether it’s likely to kill you. A long time ago taste governed what we should and shouldn’t eat, and it did a pretty good job of keeping us alive.
The rules were simple:
- Sweet foods are usually high in calories, therefore good; so go ahead — eat as much as you possibly can.
- Savory foods are high in protein, therefore good; eat as much as you possibly can.
- Salty foods help you maintain your electrolyte balance, therefore good; eat as much as you can.
- Sour or bitter tastes are likely toxic, like in poisons or when food has gone bad; thus you should avoid these foods.
The problem is that our environment changed but our taste preferences didn’t.
First, there are more sweet foods in our world today than you could possibly eat, which leads to obesity and a bunch of other diseases (cardiac disease, diabetes). We have more sweetness available, but no decrease in ability to taste — or want more of — sweetness. Instead of rarely available wild fruits or honey, we now have cotton candy and orange Crush.
On the other hand, we’ve acquired tastes for things we’ve been designed to avoid, like sourness and bitterness (lemon and coffee for example). Usually, we pair these sour and bitter things with sweetness to make them taste better and end up with lemonade and Frappucinos.
You’re probably wondering where fat fits in. As weird as this sounds you don’t taste fat, you feel fat.
Your tongue doesn’t have a cell that tastes fat (triglycerides) in the way there are specific cells to taste sweet, sour, savory, bitter or salty (although there is some research that you may be able to taste fatty acids (2)).
Some other tastes that aren’t technically tastes are dryness or astringency (for example tannins in red wine), spiciness/hotness (chili peppers) – actually pain — and coolness (menthol, spearmint).
This week I’m reviewing this cool study that looks at sugar addiction in the absence of tasting sweetness. Our research question: Can you be “addicted” to sugar consumption even if you can’t actually taste sweetness?
Ren X, Ferreira JG, Zhou L, Shammah-Lagnado SJ, Yeckel CW, de Araujo IE. Nutrient selection in the absence of taste receptor signaling. J Neurosci. 2010 Jun 9;30(23):8012-23.
There were two different mice in this study:
- Normal lab mice (C57BL/6) that can taste sweet, bitter, umami, salty and sour.
- Genetically modified lab mice with the Trpm5 gene removed (aka knock-out). Without the Trpm5 gene the mice can’t make the Trpm5 protein that is required to taste sweet, bitter, or umami (amino acids). They can only taste salty and sour tastes. By the way, Trpm5 stands for “transient receptor potential channel M5”. We’ll call these “non-taster” mice.
First, all mice were given a sugar drink (glucose) and an amino acid drink (serine) for 10 minutes to see if they preferred one or the other. Obviously researchers can’t ask mice what they liked, so instead, they counted how many times mice licked at the drink, then calculated a “preference ratio”. The preference ratio was a control that told researchers how much mice liked sugar or amino acid drinks in general.
Then, another set of mice (normal and non-taster) were fasted for 15 hours. After that, they got either a glucose or serine drink for 30 minutes. The idea is that after fasting, the mice would drink whatever they were given, regardless of whether they liked the taste, because they’d be thirsty.
This post-fast drink was used to condition the mice. Mice would get used to a certain type of drink.
After 8 days of this single-drink conditioning, the mice then got a choice of either serine or glucose drink. Researchers put the two types of drinks in different spots, so that non-taster mice would know where to find each type.
Initial taste preferences
Initially, the normal mice loved the glucose drink (over 600 licks in 10 minutes). The non-taster mice, on the other hand, responded with a rousing “meh” of indifference to glucose: only 165 licks in 10 minutes (Figure 2).
Maybe the normal mice were just thirstier?
Nope. If the normal mice were just thirstier they would’ve consumed more of the serine drink too, but they didn’t (about 75 licks). Meanwhile the non-taster mice had pretty much the same number of licks for serine drink as they did for the glucose drink — around 210 for the serine.
So, the normal mice couldn’t get enough glucose drink. Non-taster mice didn’t like the glucose drink any more or less than they liked the serine drink.
Post-conditioning: Learning to like sugar without tasting it
This is where things get really interesting. After 8 days of glucose drink conditioning, mice who couldn’t taste sweet (or bitter or umami) ended up preferring glucose (Figure 3).
Remember, they can’t taste a difference between the serine drink and the glucose drink, so they couldn’t tell them apart by the flavour. However, again, the mice could distinguish the drinks by location. The mice always knew where to get the glucose drink. Even though they didn’t have as much of a preference as the normals, non-taster mice still wanted more glucose.
How could the mice have a preference for something they can’t taste?
Damn good question.
Glucose addiction without taste
Even if you can’t taste glucose you can become addicted to it.
Dopamine is a brain chemical involved in our perceptions of reward, and it’s involved in behaviours that have a cyclical reward-seeking component, such as gambling. When we get a hit of dopamine, we want more of whatever it is — blackjack, cocaine, retail therapy, roller coasters — that inspired that dopamine in the first place.
The researchers looked at the dopamine concentrations in the brains of the mice after they got glucose or serine pumped into their stomachs for three 3-second glucose bursts. Researchers found glucose causes dopamine to over double in concentration in a particular part of the brain (ventral striatum – Figure 4). It’s like getting a bucket of dopamine sloshed right in the face. Whoo!
What if you bypass the entire digestive tract? Could glucose still be addictive?
Yup. Infusing glucose right into the mice also caused a huge increase in dopamine concentrations and increased the number of licks the mice took of water. Even though the rates weren’t getting any glucose from licking they licked more water when they were being intravenously infused with glucose, but not serine. Crazy.
Mice that couldn’t taste glucose initially weren’t particularly interested in drinking the glucose drink. Makes sense. However, after having only glucose available to them, they eventually preferred it to controls even though they couldn’t taste the sweetness.
And mice given glucose directly into their stomachs (bypassing the taste portion) also got hooked on it. Dopamine levels in mice given glucose directly into their stomachs were double of the L-serine controls, which is the beginning of an addiction. Yup, mice can become addicted to sugar just like humans, and they don’t even need to taste sweetness.
It would be like someone putting glucose or something similar (say high fructose corn syrup) in things you didn’t expect to have sugar in them or at least not very much — such as ketchup, salad dressing, baked beans, pickles, barbeque sauce, tomato paste, canned soup, cereals, stuffings, crackers and even cough medicine. Then you wonder why you want to eat these things all the time, or why you now have a craving for donuts or barbecue wings.
It doesn’t end with simple sugars (sucrose, fructose, maltose). Simple carbohydrates would do the same thing, because the sweet taste is only part of the equation. After the glucose gets into your system, it’s the dopamine in your brain you want.
The reason you crave that cookie, cake, or sweet goes beyond what it tastes like. Sugar has major brain effects.
Even though you joke about your “sugar addiction”, that is exactly what it is. If you want to get past it, you need to treat it like an addiction.
- Keep sugar out of your house.
- Try to stay away from places that have sugar.
- Stay away from fellow addicts and spend time with supportive people.
- If you can go cold turkey for a week or two, your cravings will decrease a lot. (That week or two will suck, though.) It doesn’t work as well to phase out sugar — you’re still getting that hit.
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