Written by Ali Gangeh, TKS Alumni

People aren’t gonna go to Mars knowing that they will lose at least than 20% of their bones.

If a normal bone [on Earth] is like a block of cheddar cheese, a bone space would be more like Swiss cheese. (You know, the one with all the holes 😅).
An average astronaut loses 1-2% of their bone mass MONTHLY in zero or microgravity environments (i.e. space, the Moon, Mars, etc.)

Side note: that is only when they are eating intense calcium diets, exercising 2 hours a day, with bisphosphonates (a medicine for cases of osteoporosis on Earth); otherwise you would lose bone mass at an even faster rate!!

Bone loss makes creating colonies in space next to impossible… even if SpaceX Does reduce the cost of travel to Mars, it will 300 days to get there, which means that by the time they get to mars assuming they lose 1.5% monthly bone loss, the traveller will have only 85% of their bones left when they get to mars. And, they will continue to lose their bones while on Mars. Even if they live in mars for only one month (losing 1% of their bone mass) and go back to Earth (another 300 days later), they would only have 69% of their bone mass remaining.

That means an average person will lose 31% of their bone going to mars and back… that is if they only stay in mars for one month, take meds, exercise daily, and eat lots of calcium!!!

What are the effects of bone loss in space?

It is commonly argued that since we are in space, it doesn’t matter if you don’t have as much bone mass. There is no pressure on the bones, so they are hardly used anyway. This is only partially true.

When you don’t have 30% of your regular bone mass, even the slightest bump into anything can break your bones.

And, it doesn’t matter if there is gravity or not, bumping into stuff has the same effect in space and Earth.

Anyway, in most cases, astronauts are in a state of osteopenia (lack of bones) after returning from long-term spaceflight and, in extreme cases, osteoporosis (porous “Swiss cheese” bones). When you get osteoporosis on Earth, your bones like your hip bone can break without even hitting anything; even the smallest falls are deadly.

How does this happen?

Our bones are much more alive than most people think. They are constantly remodelling. Old bones are destroyed, and new bones are made.

This is done by two organisms, osteoclasts and osteoblasts (osteo meaning bone in Latin). The osteoclasts destroy bones, and the osteoblasts make it. Kind of like wreck it Ralph and the other guy which fixes stuff… I kind of forgot his name.

What controls bone growth in the body? 

1. Steroid hormones

Hormones like Estrogen and Testosterone are vital because they stimulate bone growth. The lack of these hormones in old age is the main reason for osteoporosis on Earth, which has 3 million US cases per year!

2. Thyroid and Parathyroid gland

This gland regulates calcium in the blood. If it is low, the parathyroid gland gives out a hormone called PTH, which causes three things. 1) Bone destruction, so calcium is released into the blood. 2) A higher intake of calcium by the intestines. 3) The kidneys filter out less calcium; if there is too much calcium, calcitonin is released, which does the exact opposite of what PTH does.

3. Osteocytes

These microorganisms live in our bone and command the osteoblasts/clasts in their tasks. They are also a mechanosensor meaning they sense the mechanical forces (cough *gravity* cough). When there is a lack of such forces it determines that the bone is being used less; therefore, it can be reduced.

Osteocytes are the reason we lose bones in space.

The Solution… not quite

There is no cure to osteoporosis or similar diseases like the bone loss in space (aka spaceflight osteopenia), though there are treatments. The main ways we fight bone loss in space is through:

  1. Exercise… lots of it
  2. Eating more calcium and vitamin D (which helps absorb the calcium)
  3. Bisphosphonates – a medication used to fight osteoporosis on Earth

None of these solutions that we have right now addressed the main problem with osteocytes except for exercise. Bisphosphonates kill osteoclasts (which destroy the bone). A surplus of calcium causes more calcitonin (which orders more bone to be made to absorb the calcium).

Consuming bisphosphonates is a temporary solution. Though it does reduce the osteocytes, it is a war between our body and an external source. 

Increased calcium intake can be helpful as it stimulates our thyroid gland to produce calcitonin. This orders our bone to absorb the calcium (through the growing bone) and excrete more calcium, which creates a negative calcium balance of -250mg/day (meaning our body has a net loss of calcium).

A negative calcium balance causes two things:

  1. The bones have less calcium to draw off making it harder to rebuild itself.
  2. The kidney is put under even harder pressure to filter the calcium making it even more susceptible to kidney stones.

This brings us to our last solution, exercise. Although it doesn’t look like it, this guy is doing a special type of exercise. Osteocytes don’t feel gravity; they feel the weight on the bone, and weight-bearing exercises stimulate the bone. Most weight-bearing exercises like running, playing soccer, standing, even weight lifting, require gravity. But, we can partially replace this by using special weight-lifting machines that use elasticity instead of actual weight. But even then, the bones are only experiencing a fraction of the weight they had on Earth. Technically if astronauts worked out 24/7 with those machines that equally stimulated all the bones, might solve the problem. But that, for obvious reasons, is not possible.

Even though exercise still deserves credit for at least partially solving the main problem of no gravity, in the end, there is still no full solution.


  1. Our bones are alive and constantly growing (and getting destroyed).
  2. Bone loss in space is a huge problem we will face soon with space travel.
  3. We have no complete solution. (Unless you want to exercise 24/7)