Go, eat that chocolate!

Sweet Facts

Chocolate has been credited with numerous health benefits. For one, it is packed with anti-oxidants. Then there are claims that it can bring down blood pressure. As it is rich in milk and cream, it is handy nourishment.

As if to balance the good things, chocolate is the despair of those trying to keep their weight down. The Journal of Experimental Biology carries a report by Lee Fruson, Sarah Dalesman and Ken Lukowiak of the University of Calgary, Canada, to pin down the benefits of one anti-oxidant found in chocolate.

Anti-oxidants neutralise harmful agents created in living cells. The business of living needs substances to get ‘oxidised’ and this results in the release of ‘free radicals’ or charged particles that can overcome defenses and enter cells, where they cause the cell to break down and die. Anti-oxidants can block free radicals and limit the harm they do. Living things generate many anti-oxidants to maintain free-radical balance, vitamins C and E being examples.

An important category of anti-oxidants in human diet are flavonoids, responsible for the colours of petals and leaves of plants and serve many functions of plant life. Sources rich in flavonoids are fruits, yellow vegetables, tea and the extract of grape skin, which gives red wine its colour. A very rich source is the cacao seed, used to make chocolate.

Areas of health where flavonoids get credit are affected by so many factors that it has not been possible to clearly associate the benefits with the flavonoids themselves. The hard evidence is that flavonoids may not be as effective in living organisms as they are in the laboratory. In laboratory conditions, the anti-oxidant effect of flavonoids is found to be more powerful than Vitamins C or E, a strong case for their inclusion in diet. But in the living body, it is found that flavonoids are not able to gather in high concentration, as they are poorly absorbed and what is absorbed is rapidly metabolised or excreted.

Flavonoids & memory

The difficulty in studying the effect of flavonoids on memory is that there are too many factors that affect memory formation for one component of chocolate to be isolated. The shortest lived memory, called sensory memory, is the kind that is formed after an exposure to an item for less than half a second.

There is a limit to how much can be perceived in such a short glimpse and what is perceived can be retained for barely a second, leaving one with an impression, rather than detail. A slightly better form of memory is the short-term memory, which can last for several seconds or a minute.

This kind of memory arises when the subject has some spatial or acoustic form; for example, when one hears a telephone number and remembers it till one completes dialing the number. Increasing the level of order by repeating the number as two halves, or as an area code followed by the remaining digits as two groups, can improve retention.

Long-term memory can last a long time — addresses, names and faces of people we know, or events that we have experienced. Even telephone numbers that we use often can be remembered for a long time, even life-long. The difference is that while short-term memory is encoded acoustically or visually, long-term memory is encoded semantically, or by association with other events or memories.

At the physical level, short-term memory depends on transient patterns of communication between nerves and is limited to activity of certain specific parts of the brain. Long-term memory, on the other hand, arises through the formation of more long lived nerve connections, through repeated experience or the effect of other processes that act to consolidate and permanently fix memories. The hippocampus is found in humans and vertebrates deep within the brain and is associated with consolidating memory.

Simplify the problem

Given such complexity of memory mechanisms, not to mention individual history and associations that would help or hinder remembering, there is scarcely a possibility of assessing the effect of a single component like flavonoids on how well memories are formed. Ken Lukowiak and colleagues at Calgary decided to examine the effect of just one flavonoid, in the case of a simple instance of memory formation where there were minimal factors to complicate the act of remembering — the case of a pond snail trained to remember a particular response to a change in the environment.

The researchers limited the study to the effect of one flavonoid called epicatechin (epi), an important component of chocolate. The memory event that was studied was a learning related to a particular behaviour of lymnaea stagnalis, the pond snail, when the oxygen level of the pond water reduces. The snail usually breathes, or takes in oxygen, through the pores of its skin and this works well when the water in the pond has adequate dissolved oxygen.

But if the oxygen level falls, the snail extends its breathing tubes above the surface of the water. This behaviour, however, can be discouraged, or stopped, by gently poking the breathing tubes when they are extended, and the snail then keeps the tubes closed, more often than not, despite the lower oxygen level in the water.

This lesson, however, can usually last about three hours, which is the extent of short-term memory of the snails. Three hours after the snail has been taught to keep its breathing tubes closed, the snails forget and they revert to the normal behaviour of opening the breathing tubes whenever oxygen levels fall.

A concentration of 15 mg of epi per litre of pond water was identified as not affecting the breathing behaviour of the snails. The test then was whether this exposure to epi affected the duration of memory. Episodes of lowering of the oxygen level, followed by training to keep the breathing tube closed, were created with the epi exposed snails, in the same way as the snails in normal water.

The results were startling, the epi-exposed snails were able to retain the learning for a whole 24 hours, which amounts to long-term memory, with the same training schedule which otherwise resulted in retention for only three hours. Repeating the training session even resulted in the learning persisting for three days!

A further study was whether epi resulted in the memories being robust in the presence of memory extinction training. This is where a memory is over-written by a contrary memory. In this case, the snails were exposed to falling oxygen levels and not poked if they opened their breathing tubes.

The lack of adverse stimulus leads to new learning, that breathing tubes may be opened and this would extinguish the earlier training.  But in the case of epi-exposed snails, it was seen that they even resisted such extinction training – the lower frequency of tube opening instances persisted, even if there was no adverse consequence.

The Journal of Experimental Biology paper also describes other results which indicate that such long-term memory formation in snails did not arise from association with events, like predator activity, but through an effect at the level of nerve cells. And then, there are observations that the cacao derived flavonoids actually improve the blood flows to the central nervous system, which helps generation of nerve cells and memory formation.