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Dr Hypothesis (Issue 8)

Dear Dr Hypothesis,

When my sister was young she was often hyperactive. According to the adults, sweets were to blame. As my own sugar allowance was rationed, I wish to know if there is any evidence for this belief?

Feeling Sugar-High

Dear Feeling Sugar-High,

You’re right that parents are often adamant that sugar causes children to be unruly. However, in 2008 the British Medical Journal highlighted that at least twelve double-blind, randomised, controlled trials refute a causal link.

To test this effectively, researchers manipulated children’s diets to contain either sugar or an artificial sweetener such that neither child nor experimenter knew which was being consumed. Not only did they find no difference in the activity levels of ‘normal’ children, but there were also no consistent effects on those with attention deficit/hyperactivity disorder (ADHD). Rather, different studies showed conflicting effects on cognition and aggressive behaviour.

So how did this ‘myth’ arise? Its origins are unclear but scientists have demonstrated how it may have survived. In their study, mothers who were told that their children had consumed sugar were more likely to consider their behaviour to be hyperactive than those who thought their children had received aspartame, when all had received the latter. So although your sister may have been better off without all those adult expectations, reducing sugar in your diet was probably beneficial for your weight and your teeth.

Dear Dr Hypothesis,

My eyesight is really important to me so throughout my life I’ve stuck with the somewhat arbitrary advice to eat carrots and avoid reading books in low light. Is this just a way for parents to make their children eat vegetables and go to sleep?

Not Seeing Clearly

Dear Not Seeing Clearly,

I can’t say much for parental intentions but you’ve certainly hit upon two common myths. The best (or worst) aspect of the carrot myth is that it seems intuitive from a biological viewpoint; carrots provide a source of beta carotene, which is converted to vitamin A, and a deficiency in vitamin A can cause eyesight problems. There is, however, very little evidence to suggest that increasing vitamin A improves eyesight. In fact, the myth was perpetuated during the Second World War to deceive our enemies into thinking that our improbably high aircraft interception rate was due to exceptional night vision, instead of the newly invented radar.

Fortunately for all those children reading Harry Potter under the duvet, the link between poor eyesight and dim light is one of the top seven medical myths, according to an article published in the British Medical Journal in 2007. So where did it come from? When we read in dim light focusing feels difficult and our blinking rate is reduced, thereby causing some dryness in the eyes. It does cause temporary discomfort, but this should not be mistaken for permanent changes to the function or structure of the eye. However, this issue remains contentious and some researchers of myopia, a refractive defect of the eye, point to increased prevalence in academics who are said to read in low light, with the book too close to their eyes.

Dear Dr Hypothesis,

I’ve been to the seaside many times around the world and noticed a common theme that there are always two tides in a day, despite other differences. Why is this?

Just Curious

Dear Just Curious,

You are almost correct in that in most places there are indeed two tides a day; these are semidiurnal tides.

The presence of tides is a direct result of the gravitational force of the Moon, which effectively acts to pull the sea away from the Earth. There are two daily because the Earth rotates on its axis. It may not be immediately obvious but the force exerted by the Moon has an equivalent effect whether it is above you in the sky or underfoot on the other side of the Earth. High tide therefore occurs on opposite sides of the Earth at the same time. On the near side, the water is pulled away from the Earth, and on the far side the pull on the water is weakest so it bulges away from the Earth. Areas at 90 have low tide due to the Earth’s pull on the water. Although the Moon orbits the Earth and so changes its position in the sky from day to day, it orbits in the same direction as the Earth’s rotation. Thus, after every 24 hours and 50 minutes, the Moon finds itself in the same position in the sky having been overhead and underfoot once each; hence, two high tides a day.

This isn’t always true because in places such as the Gulf of Mexico there is only one tide a day (diurnal) due to its angle relative to the Sun and Moon. The Caribbean has a third type of tide, the mixed tide, where the pattern of high and low is highly varied.


Helen Ramsden is a PhD student in the School of Informatics


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