Food for thought
Question: The brain is an energy-hungry organ, which presumably uses more fuel the harder it works.
Could thinking hard help a person lose weight? Is reading James Joyce鈥檚 Ulysses an alternative to dieting?
Answer: I work as a games designer, a largely sedentary occupation. Over the past few years, I鈥檝e noticed that when I get stuck into a particularly tortuous piece of design, I need to supplement my regular diet, usually with chocolate. My wife refers to Mars bars as my 鈥渂rain food鈥�, because they provide the extra energy my brain seems to need to cope with the difficult problems.
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However, it might take your correspondent more than James Joyce鈥檚 Ulysses to counteract the calories. The same effect doesn鈥檛 seem to apply when doing less brain-intensive work, such as dealing with e-mail, writing sales and marketing documents, reading other people鈥檚 game designs, or play-testing.
Matt Kelland
Cambridge
Answer: I have shown that the brain does indeed use more glucose (the fuel that the brain runs on almost exclusively) when faced with difficult cognitive tasks-and that this usage is localised, where it drains a large proportion of the available fuel. However, the amounts of energy involved are relatively small.
A rough calculation suggests that thinking hard about a task for 30 minutes might burn about 1/30th of a gram of fat. Thinking is not likely to turn an intellectual couch potato into a lean machine.
It does mean, though, that you can improve your performance in a test by taking glucose beforehand-if the test is sufficiently hard. For easy tasks, the brain doesn鈥檛 use much fuel so there鈥檚 no benefit from taking extra glucose.
Ewan McNay
Danbury, Connecticut
The above research may explain why Kelland needs his chocolate for tortuous design work. In the experiments mentioned (Proceedings of the National Academy of Sciences, vol 97, p 2881), McNay measured glucose levels in the hippocampal region (which is involved in memory) of the brains of rats as they learned to find their way around a T-shaped or a Y-shaped maze. Extracellular glucose levels in the hippocampal area decreased by 32 per cent while the rats were exploring the harder maze but by only 11 per cent in the less complex maze. Rats given extra glucose found their way around the mazes more effectively and their glucose levels stayed high-Ed
Answer: The brain consumes about 20 per cent of the body鈥檚 energy intake, despite constituting only about 2 per cent of its mass. It is indeed a very energy-hungry organ. However, cognitive processing is a relatively small part of the brain鈥檚 overall function, so concentrating on passages from James Joyce鈥檚 Ulysses, or any other strenuous mental activity such as revising hard, will not in itself make much difference. This is because most of the calories the brain uses support its more fundamental functions.
The brain plays a major role in homeostasis. It controls crucial systems such as cardiac output, blood pressure, respiratory function, body temperature and the like. Doing all this requires vast sensory input to the brain, complex processing and then an equally large output to produce the desired outcome. The control of arterial muscle tone alone, which is crucial for the maintenance of blood pressure, requires sending motor signals to innumerable blood vessels. Much of this activity occurs below the level of consciousness, and in the larger and evolutionarily older parts of the brain.
Another example of non-cognitive brain function is controlling body posture. It takes a veritable barrage of signals from the brain to control muscle tone, and body movement depends on sensory input from numerous systems, including the eyes, the semicircular canals of the ear, stretch receptors in the muscles, position sensors (proprioceptors) in the joints and pressure and pain receptors in the skin. The brain must integrate and interpret all of these signals and act accordingly to achieve the required physical act.
At the level of neurons, a great deal of energy is consumed in firing the nerve cells and then resetting them ready to fire again. Even when the nerves are not firing, energy is used to maintain the 鈥渞esting potential鈥�-the electrical gradient across the cell membranes of nerves. Because the membranes leak ions such as sodium and potassium, ion pumps in the membrane must work to maintain the gradient. The brain contains about 1012 nerve cells, so this activity accounts for a very significant part of brain metabolism and goes on all the time.
In addition, all brain function requires the synthesis of numerous chemicals such as neurotransmitters and the enzymes that break them down. These are responsible for passing signals across the synapses, the gaps between nerve cells. Finally, transporting the many biomolecules needed within the brain from the bloodstream to nerve cells is also an active process. All this adds to the energy burden.
So it appears that the thinking part of brain function, which is exercised when reading, consumes a relatively small amount of extra energy and is therefore of little use in weight loss-unless, of course, some of that thinking is aimed at producing exercise.
Roger Hicks
London
This week鈥檚 questions
Shock value: Could someone please tell me why and how fabric conditioners reduce the amount of static electricity in clothes?
Johanna
By e-mail, no address supplied
Distant hills: I once read that if you stand on the cliffs at Dover in England and look along a particular compass bearing, there will be nothing higher than the cliffs you鈥檙e standing on between yourself and the Urals. Is this true, and what would be the relevant compass bearing?
Kyle McKibben
Edinburgh
Carbon libraries: How much of the world鈥檚 carbon is locked up in paper in magazines, library collections, bureaucratic archives and the like?