Why The Dose Really Matters

Coffee is the bloggers greatest friend, though being English I personally take tea. Both have the same effect, they are both good caffeine sources which is great for the late nights you have to spend looking for spelling errors and desperately trying to write a post that makes sense at least to yourself. The elusive reader has little idea of the pain and suffering that has gone into the blog that they quickly scan over for a few seconds before leaving in search something more interesting.


As I say, I drink tea. Like a lot of people I have given up sugar in it. But when I make one for someone else, I still ask if they take one lump or two. For people unfamiliar with English culture this refers to your preferred sugar dosage the lump being equally applicable to a sugar cube or a spoonful. One cube of sugar will give you a concentration in your tea of roughly 1.5%. Two would be about 3%. It is easy enough to tell the two dosages apart, and indeed to a one lump tea drinker, two lumps would be repulsive. Few people like 3 cubes and any more than that and the tea is simply too sweet for anyone to drink.

Dose Response Curve for a Nice Cup of Tea

So we have the ability to distinguish between one and two lumps, and less than and more than the right amount. But it is interesting that we rapidly lose the ability to quantify the sugar level outside the normal dosage range. We can’t tell the difference between one and two grains of sugar. In fact we wouldn’t even know they were there. And one very sweet cup of tea tastes much like another.

If you are a scientist you might well plot a graph of sugar concentration against perceived sweetness. You could call it a dose/response curve. This would look a bit like an elongated S. At low concentrations you get no response at all. At higher ones you get a response all right, but it doesn’t get any higher as the concentration increases.


The explanation is simple. What we register as sweetness is a signal from our taste buds to our brain. Exposed to low levels of sugar the taste bunds simply do nothing and we cannot taste the sweetness. Once they get to the critical level they start sending messages to the brain that they have found something sweet. Different taste buds have different trigger levels, so there is a level where some are firing and some are not. But once you reach a certain point all of them have sent their message and your sweetness detection system is saturated.

Dose Response Curves in Biology

In fact a lot of biological processes work like this. Anywhere where there is a signal and a receptor you get this pattern. We find it quite familiar in everyday life, not just with sugar. We can ignore the television if its volume is low. We might not notice a feather falling on our hand. And so on. And it is even more significant in a lot of the processes that happen in the body.

A good example would be cosmetic actives like retinol. This works by stimulating the production of collagen. But if the level is too low you won’t just get a poor effect. You’ll get no effect. Reading labels on cosmetic products in the hope of spotting the good stuff is only going to get you so far.

Dose Response Curves and Safety

The same principle applies to harmful effects. A very good example, and one that is topical right now, is an allergic reaction. Although it takes only a small amount of an allergen to trigger an attack, nonetheless it will still need to be enough to register with the body’s immune system in exactly the same way that sugar does with taste buds. Although it is hard to work this level out, careful assessment of the data can sometimes allow an estimate to be made of the quantity of allergen above which problems are likely. This is not of just academic interest. The fragrance industry funds a body called RIFM that investigates these issues and produces guidelines which virtually the whole industry follows.

You would imagine from their ingredients that fragrances would cause a very large number of allergic reactions. Such reactions aren’t unknown, but they are much lower than reading the labels would lead you to suggest. This is largely down to the work of RIFM and its IFRA guidelines.

Recent work on peanut allergies in children has used the fact that low levels don’t trigger an episode to good effect. By starting off at a low dose that has no effect and gradually increasing it the reaction rates could be considerably reduced. This sounds like a really promising area for future research. If allergies can be cured that is a huge benefit for large numbers of people.

Dose Response and Estrogen

Another category where this behaviour is relevant is the so-called estrogen mimicking compounds. A lot of chemicals can be shown to work in a similar way to estrogen in the laboratory. There are so many that a food scientist called Harry Eidenier coined the phrase ‘swimming in a sea of estrogen’ to describe how widespread they are. This sounds extremely scary. Estrogen controls a great many of the body’s processes so having things in our environment that can disrupt those processes could be very damaging.

Is there any evidence this is happening?

In fact even though the risk of estrogen mimics has been talked about since the fifties, actual cases of harm to humans arising from them have not been reported. The reason is probably that like the single grain of sugar in a cup of coffee, they simply don’t register. They might be present in your body. Molecules are really small and very mobile so they probably are. But they simply have no effect.

Cosmetic Safety

Most of the data which is used to demonstrate the safety of cosmetics is derived from short term studies. Critics sometimes seize on this. With few long term studies how do we know what effect continual exposure has? It is not a bad point and one that can only be fully answered by those long term trials. But it is a much weaker argument than most of the people who advance it realise. To do long term harm the ingredient needs to build up in the body over time, and to reach the critical level at which the body starts to react to it.

This is a tall order, but not totally impossible. But let’s see what obstacles the would be toxin needs to surmount. The molecule itself needs to be resistant to breakdown by the body’s enzymes – this rules out just about anything that resembles structures found in nature. It needs to find a tissue where it can persist. This effectively means the fat reserves: anything water soluble can’t build up. And having been stored it needs to get out again.

There has been one case where something like this might have happened with a pesticide. In Silent Spring Rachel Carson recounts the story of an agricultural worker who handled DDT for many years and suffered no ill effects from it. But he fell ill and lost a lot of weight very quickly. This released an unusually high level of DDT into his bloodstream which may have contributed to his death.

Of course nothing used in cosmetics is remotely as toxic as DDT, and even the most enthusiastic beauty addict won’t be exposed to as much product as a farm worker spraying fields is to a pesticide.

In any case running through the properties of widely used ingredients I can’t think of any that fit the bill. I will throw that out as a challenge to any chemists reading. The closest I could come up with is triclosan – though this has such a low toxicity I don’t think it would count. But hopefully this puts in perspective just how safe cosmetics really are.



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2 Responses to Why The Dose Really Matters

  1. Maria says:

    With reference to dosage, and in particular caffeine (which I think is present in both tea and coffee) is there any way to work out your daily caffeine intake?

    It must surely be different for (say) freshly brewed coffees/weak black teas or a cup of instant Nescafe?

  2. Colin says:

    Filter coffee makes me a lot jumpier than instant coffee or tea, so I think you are probably right. But I am afraid I don’t know. I am sure somebody somewhere knows, so maybe a bit of googling will reveal it.

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