Do you burn more calories when thinking?

We put Tracey's question to nutrition expert Toni Steer...

Toni - That's an interesting one. Avocados have got things going for them so, for example, they are quite high in what we call a healthier fat, which is the monounsaturates, and the monounsaturated fats have been shown to modify your blood fat levels more favourably than things like saturated fat.

Chris - So when we have them, although you're getting a lot of 'fat' calories coming in, it's 'good for you fat' in the same way that olive oil is said to be 'good for you fat', this monounsaturated fat?

Toni - Yes. So overall you want to aim to reduce the amount of fat in your diet but, the fat that you do include, you want to modify; and avocados have got that kind of monounsaturated fat that is more desirable. One of the other things that avocados have got is that they've got some reasonable amounts of fibre, so around 3.5 grams of fibre per 100 grams, so that would an average avocado. They also contain potassium, like many fruits and vegetables.

Chris - So making them mildly radioactive?

Toni - Yes..

Chris - Aren't they Dave. Aren't bananas a little bit radioactive because they've got lots of potassium in them?

Dave - Yes it's the same isotopic form of potassium that they use in our hospitals and it's just floating around...

Chris - Potassium-40?

Toni - If you eat potassium then you're going to get irradiated slightly!

Chris - Because also Brazil nuts are radioactive, because of the soil they grow on and someone did tell me that you can equate a chest X-ray to the equivalent of a bag of brazil nuts. You get about the same dose of radiation equivalent to doing the two things. Anyway, I didn't mean to hijack your answer Toni, so do carry on and tell us about avocados. Is Tracey going to be okay?

Toni - The other thing that's quite interesting with them is there's some suggestion that adding avocados to other meals can enhance your carotenoid or vitamin A absorption. The evidence is kind of a little bit weak in that area. I think it's still really to be confirmed.

Chris - Is the rationale just because there's just loads of fat in the avocado?

Toni - Yes, so fat soluble...

Chris - And vitamin A's a fat soluble vitamin, so it helps?

Toni - Absolutely. What I would say though is on the downside, avocados are not really a great source of things like vitamin C, they're quite low in calcium, for example, not much iron so, actually, would you really want to survive just on a diet of avocados and, actually, you are going to be lacking.

Chris Smith debunked this commonly repeated myth... Chris - Well it's definitely not true because actually your brain cells - you are born with those brain cells - they're formed as you're developing and they have to last you a lifetime and that's why we get neurodegenerative diseases when you lose brain cells. Also, there are other cells in things like your retina, which is part of your brain, those photoreceptors have to last you a lifetime. Muscle cells are only very slow at replacing themselves, for instance, cardiac muscle in your heart is only very, very slowly turning over, if at all, so that muscle has to last a lifetime. But, some cells, including fat cells, do last about twelve years and we know this because someone wrote to us at the Naked Scientists a little while ago and they said "can I carbon date my gran?" And this is someone from South Africa who said their grandma didn't know how old she was because she'd never been told the date of her birth. They were intrigued to find out how old she really was because they said she really is very old indeed - can we find out? And, in fact, there's a lady Kirsty Spalding at the Karolinska Institute in Sweden and she did some studies where they have done carbon dating of fat cells and you can use DNA for this because the DNA incorporates atmospheric carbon, which has got a carbon 14 radioactive signature in it and we know what the radioactive signature is doping over time. So we can use that as an index of age and they found that fat cells last about twelve years and then they sort of disappear and you replace them with new ones.

Physicist Dave Ansell had a spin at answering David Oehl's question... Dave - Microgravity is really bad for you because all sorts of things stop working. Your bones get very, very weak, your muscles get very, very weak.

Chris - What does it actually mean?

Dave - Microgravity, - if you're in space, if you're in orbit you're just free falling which means that everything is falling with you which means that you're effectively weightless because everything is falling at exactly the same speed as you are. So you can push off and float around - really quite fun but quite unhealthy. So the suggestion and it's actually been suggested for a long time (it might have been Arthur C. Clarke who came up with it)...

Chris - 2001: A Space Odyssey had a spinning space station.

Dave - A spinning space station because if you get a bucket and spin it round your head, then you need to apply a force to pull it in to keep it going in a circle.

Chris - But the evidence is if you fill it with water, the water doesn't come out.

Dave - Doesn't fall out. So basically you change the direction of gravity, and if you're in space the same thing happens and you'd create artificial gravity.

Chris - So if you had a big ring that was spinning, the person standing in one part of that ring in the same way as the fairground ride creates a sort of centrifugal effect, you feel thrown outwards and the ride pushes you back in to stop you flying off, the space station would, effectively, push up through the floor at you?

Dave - Yes. I think the reason why they haven't done it so far is that you either need to spin very, very fast or you need to be very, very big and that makes everything very heavy and basically, no-ones build a space station big enough or had people up there long enough to make it worthwhile doing it.

Chris - So if you had something very small that was turning very fast would that, nonetheless, whilst making some gravity make people feel extremely unwell whereas the rationale would be if you make it very big, the sense of rotation for a person would be smaller but they'd still, nonetheless, get the effect?

Dave - You probably would feel quite uncomfortable because, apart from anything else, everything would behave very, very strangely because you would get not just the centrifugal force, you get what's called the coriolis force so if you threw something into the middle of a spaceship it would actually spin round and end up going in a completely different direction from one you'd expect it to be. I don't know at what point it's actually worth doing this. Certainly there has been talk of doing it if you're sending someone to Mars over a few months if they're still in one piece to get there but certainly so far, no-one's tried.

We put Guido's question to zoologist Chris Basu... Ben - Well, spoiler for the question - they actually can get the bends, we think. The bends is decompression sickness so, as you say, when scuba divers go down underwater they're using equipment which matches the pressure of the air to the pressure of the water around them. So, as you go down underwater, the water exerts massive pressure, it's pushing down on you. When you're breathing in air under pressure, air is about 78% nitrogen, under pressure that means more of the nitrogen gets pushed into your blood and when divers want to come back to the surface, they come up to the surface, the pressure around them decreases and all that excess nitrogen gas comes out of their blood. So imagine if you've got a....

Chris - So when you say comes out of their blood you mean as in while it's still in the blood?

Ben - Exactly, if you imagine like a can or a bottle of fizzy drink, if you open the cap really quickly, if you open it too quickly you see all these bubbles suddenly magically appearing, whereas if you do that slowly, you don't see that happening as much and that's exactly the same as what's happening in the blood of scuba divers when they're coming up. Now.

Chris - So when they come up slowly, why doesn't it happen?

Ben - Exactly. When they come up slowly, it allows the blood to form an equilibrium with the air much more slowly.

Chris - In the lungs?

Ben - Exactly. If you do it too quickly, then all that gas comes out the blood as bubbles and that causes problems.

Chris - So what you're saying is then, if they got lots of this dissolved nitrogen in their blood as a scuba diver, as they surface slowly because the blood's going past their lungs, it can slowly surrender the nitrogen to the lungs, the lungs inflate, they breath out the nitrogen harmlessly?

Ben - Exactly. So, in effect, the divers are actually off gassing. They're breathing out that nitrogen gas slowly and safely.

Chris - So what do the whales do then?

Ben - The whales have one major, major advantage - they're not breathing compressed air. When whales come to the surface, they take a breath. A lot of whales actually take a breath and they exhale before they go down. So they've got environmental sea air, if you like, and as they go down that air actually gets compressed but it's not the same as a scuba diver breathing in massive lungs full of.

Chris - Compressed air?

Ben - Pressurised air, exactly. But they still have a little bit of air in their lungs. They can't empty their lungs and their trachea, their windpipe completely so they've got this little bit of air. It means they can actually get some pressurised nitrogen seeping into their blood but it's in really, really, small amounts and they've got a couple of things that helps them. They've got lots of special fat in their body and that helps to actually mop up the excess nitrogen and they can also store the air in funny places. Their trachea (windpipe) is actually distensible so when they go down their lungs get compressed but a lot of that air gets forced up into their trachea and it doesn't enter the blood at all. It's really amazing these beach whales, they can actually dive down to three kilometers, which is 300 times the atmospheric pressure of the air.

Chris - Incredible, isn't it? So you mentioned at the beginning you said well they actually can get this so why do they occasionally get the bends then?

Ben - The other thing that they do, they're very sensible. Like scuba divers they also don't come straight up to the surface, they come nice and slowly but when something disturbs that behaviour, they might actually be prone to getting the bends. A few years ago, people were noting certain associations between sonar activity of military vessels and whale strandings and they actually found in these stranded whales the telltale signs that they had the bends, they had decompression sickness. When people look at bones of whales, bones actually have little telltale signs as well when the animal's had chronic exposure to decompression sickness, they have these little spots on the bone. So we can actually see that whales do from time to time tend to suffer from it but it's something that they have learned to live with.

This week, NASA unveiled plans to set fire to an (unmanned) space station,  because we have relatively poor understanding of how fires behave in space. Dave Ansell gave Chris Smith further detail on the story...

Dave - Well the problem is that fire on earth we understand very well.  It works because air heats up, expands and then it floats upwards and that brings more air in underneath, and the flames go upwards and the air comes into the bottom and it burns away happily and we understand that really, really well.  In space basically, it's microgravity which means that everything's falling, so heavy things don't fall faster than light things which means light things don't float up above heavy things.  So fire in space behaves completely differently.  It's entirely dependent on the rate at which oxygen can diffuse into the fuel and that also means it gets cooled down less quickly because the air rushing past it also cools it down and, to be honest, they just don't know what works and they don't know how, especially, a big fire will happen because, luckily, there's never been one on a spaceship.

Chris - So just in summary then.  A candle on earth you light that, this will produce a chemical reaction that produces some heat which causes the hot gases, which are the combustion products (a lot of carbon dioxide), they're going to rise up out of the way pulling in nice fresh air with lots of oxygen, and that's because the candle "knows" where up and down is?

Dave - Yes.

Chris - Because of earth's gravity.  If you're in freefall in space, there's no net up and down so the gas blows up just like a bubble, the exhaust gases and this would choke off the flame, one would think.

Dave - Because it's not getting cooled down as quickly it still can burn.  It burns much more slowly and it's much less predictable because there's not up no down, you don't know what direction it's going to travel.

Chris - So what are NASA going to do?

Dave - So luckily, the cargo ships which they're sending up are one of the kind of ships which have an oxygen atmosphere because most spaceships don't have an oxygen atmosphere.  There's no people there: who cares? They go up to the space station, they unload a load of kit and then they just go down and get burned up in the atmosphere.  So someone thought ha ha, we can use this.  Let's put a big box inside with some stuff to burn on the way back down.  In a couple of months time on its way back down they're going to set fire to it and see what happens.  They don't think it's actually going to cause the spaceship to be damaged.

Chris - How are they going to monitor it?  Have they got cameras in there are something?

Dave - They've got cameras in there and lots of sensors (gas sensors), temperature sensors, as much as they can possibly get into a small box.  It's going to be a 4ft long by 2ft high box - it's actually quite big for sending out into space and hopefully that means that the next space station that they build, they'll understand fire a lot better, which means they can build it to cope with fires a lot better because at some point it's going to happen and it could be very, very nasty.

Chris - Sure.  Have they got various over-engineered solutions in the present space station just in case there were a fire and that's obviously adding a lot of weight, and if you could save a lot of weight because you engineer things slightly differently because you don't need it, it would obviously make space travel more predictable and viable ?

Dave - Definitely, when you've got an uncertainty, engineers will increase the margins and make everything heavier and more expensive and more high tech, and if we can get away without it, or we might find there's some great big holes in the defences and you could burn out a space station and we don't know that it's going to happen.

Chris Basu - How do people plan to put fires out on space anyway because you can't use sprinklers because the water would just float away?

Chris - The water would just be hanging on with the fire extinguisher wouldn't you?

Dave - I think you can, I guess, for small fires some kind of fire extinguisher will work.  Astronauts are trained to stand on things and as long as they're expecting it, they can deal with it. With extreme fires you can pump halon gas into the chamber which also will kill any people in there so you've got to make sure you evacuate first.  I guess, if it's really, really extreme you can just vent all the air out of it and there's no oxygen there to burn.

We put this brain teaser to nutritionist Toni Steer...

Toni - So, it’s absolutely right, the brain does require round about 20% of your daily energy intake. The brain relies on glucose; it has to have glucose for energy so unlike other parts of your body, for example, other organs or muscles which can use fatty acids, the brain has to use glucose and it needs around about 120 grams of glucose a day.  Most of the evidence indicates that, actually, you don’t see a significant increase in energy consumption with people doing mental tasks.  There have been some techniques where they can scan metabolic activity in different regions of the brain and it’s looking at how the glucose moves and is utilised in different regions, and there is some suggestion that when you’re given a specific task you see more metabolic activity and glucose utilisation in different regions of the brain.  But the overall impact of total energy requirement is very minimal and so the idea that if you’re going for a day’s lecture at university you need several chocolate bars, I’m afraid is not true.

Chris - Damn!

Toni - It’s not going to happen.  Maybe the tiredness could be many other factors and, actually, one of them is good hydration.  So tiredness and not being able to concentrate is often about making sure you’re well hydrated.

Dave Ansel had a go at answering Kevin's question... Dave - That's a really, really interesting question. So golf balls have dimples because if you had a perfectly smooth golf ball and you hit it really hard, what happens is you get really big swirls of air coming off the back of it, the air gets round the first half of the ball it detaches and you get these big swirls out the back of it and that causes a lot of drag. Now if you have the dimples and what it does is it creates small swirls, which actually tend to mean the air detaches from the ball later, and that means that you get less drag overall so the ball goes faster and further. Now, planes normally are in a very different regime. They've carefully designed their wings so you don't get very much turbulence off the top and, in fact, when you do get this kind of turbulence it's the detachment of the air flow (it's called a stall), that's normally something which you want to try and avoid and so mostly when you're flying dimples would just give you a bit more drag and make it worse. However, if you're about to stall, having dimples can be useful and, in fact, planes do have things like dimples (called vortex generators) and these are little fins attached to the front of the wing and they can delay the stall a bit so that the planes can fly slower, and land slower, and land on shorter airfields, and things like this. But most of the time you don't need them and all planes have them.

We put Denis' question to nutritionist Toni Steer... Toni - I think the short answer to Dennis' question is no. The human body is very efficient at extracting calories from food. There is some evidence to show, and I've looked at this actually with ripe and unripe bananas, is that with the riper banana you get a slightly quicker rise in blood glucose.

Chris - Did you eat an unripe banana?

Toni - This isn't me. This is a paper so.

Chris - Oh, so you make some other poor person eat it. That's disgusting!

Toni - It wasn't that bad. So with the unripe banana you see a slightly slower rise in blood glucose, so you could probably apply that same kind of theory as well to the pear in question but, nevertheless, the digestion of the starches and the sugars is very efficient. So we have the amylase in the mouth that breaks down the starch. You've got the stomach that mixes the food and breaks it down and actually, by the time you get to the small intestine, around about 50% of your starch is already started to be broken down. Then the breakdown really into the monosaccharides or the simple sugars from the fruit occurs then in the small intestine so, by the time time it gets to the small intestine it's very efficiently broken down and very efficiently absorbed. So I think the answer is; don't worry about the ripeness too much - you'll get the calories from it.

We ask Dave Ansel for the answer to Allan's question... Dave - You certainly can. In fact, it's absolutely tiny, but you will get pushed backwards very, very, slowly. In fact, they're planning to use the same principle to sail around the solar system. So, if you build yourself a very, very, very large mirror and then the sunlight shines on it, then the light is coming towards it in one direction, it's reflected back again and that means it's changed it's direction and your mirror will get pushed away. The force is a tiny micro-newtons (I mean mili-newtons), but because you don't need to carry fuel to do that you can keep your rocket engine going continuously as you fly round the solar system. It means you can actually travel round the solar system, especially if you're going a long way, far quicker than you could.

Chris - Because over time, it will build up, and build up, and build up with no losses because there's almost nothing to hit you and slow you down again.

Dave - It's been suggested it's one of the ways you could get to another solar system would be to have one of these solar sails and a very, very, very large laser. Point at the solar sail and then sail on it all the way out to another star.