What if your skin could make food from sunlight?

Plants don't eat lunch. They make food from sunlight!

What if your skin could make food from sunlight too?

A tree never eats a sandwich. Its leaves capture sunlight and use that energy to build sugars from water and carbon dioxide. This is photosynthesis, and it happens inside tiny green parts of plant cells called chloroplasts.

So what if your skin had chloroplasts too? Could you stand in the sunshine and skip lunch? To answer that, we need to compare two numbers: how much energy your body uses, and how much energy sunlight could really give you.

Photosynthesis is how most energy enters food chains: OpenStax describes it as the conversion of light energy into chemical energy, and Energy Education notes it is indirectly how animals and humans get energy from the Sun, as food. It takes place in chloroplasts.

This thought experiment gives human skin working chloroplasts and asks whether they could matter. It follows three threads: how much light skin could catch, how much energy a person needs, and the real case of sea slugs that keep chloroplasts from the algae they eat.

We get our energy from food. Plants catch the sunlight first.

When we eat plants, or animals that ate plants, we get some of that sunshine energy.

Right now, people get all their energy from food. The U.S. Food and Drug Administration uses 2,000 Calories a day as a general guide for nutrition advice, though real needs vary with age, size and activity.

That sounds like a lot, so let's turn it into power. A food Calorie is about 4,186 joules, so 2,000 Calories is about 8.4 million joules. Spread over a whole day, that's about 97 watts, around the power of an old-style 100-watt light bulb shining day and night.

The FDA uses 2,000 Calories a day as a general guide on nutrition labels, noting that needs vary with age, sex, height, weight and activity. OpenStax explains that a food Calorie is a kilocalorie, about 4,186 J.

So 2,000 Cal ≈ 8.37 MJ a day. Divided by 86,400 seconds, that is an average power of roughly 97 W, continuous, day and night. Any solar supply would have to deliver that average, not just a peak at noon.

Now pretend your skin can make food from sunlight.

It would help a little. But you would still need to eat! Sunshine on skin is not enough.

Strong, clear midday sunshine delivers about 1,000 watts on each square meter it hits. That sounds like plenty. But plants turn only a small share of light into food: even in the best conditions, scientists estimate the most is about 4.6 to 6%.

Suppose a square meter of your skin faced strong sun for 8 hours and worked as well as the best plants. That would make roughly 410 Calories, about a fifth of 2,000, on a perfect day. And skin is not all facing the sun, days are cloudy, and nights are dark. So green skin might help a little, but you would still need meals. That estimate is an idea, not a measurement.

PVEducation notes that the standard solar spectrum used for testing is normalized to 1 kW/m² in strong, clear conditions. Zhu, Long and Ort (2008) estimated the maximum conversion efficiency of solar energy to biomass at 4.6% for C3 plants and 6% for C4 plants.

An optimistic estimate: 1 m² of skin facing 1 kW/m² for 8 hours at 6% yields 480 Wh, about 410 Cal, roughly a fifth of the 2,000 Cal guide; at 4.6%, about 320 Cal. Real skin would face the Sun at varying angles, partly shaded, and for fewer hours; real crops typically fall well short of these ceilings. The conclusion that people could not live on skin photosynthesis is reasoned, not tested.

Plants have lots of flat green leaves to catch the sun. And they don't run around!

Your body uses lots of energy every day. That's why you need food.

Plants have a few tricks that help. A tree spreads huge numbers of flat leaves to catch sunlight, and plants don't run and play the way people do.

People use energy all day and all night, even while asleep: to keep the heart beating, to breathe, to think and to move. A body like ours needs a steady supply of energy, about 97 watts on average for 2,000 Calories a day. Sunlight is strong but spread out, and photosynthesis is not very efficient, so a person-sized patch of skin couldn't collect enough.

A reasonable reading is that the mismatch is one of area and demand: plants spread large leaf areas to intercept light, while a person offers a small surface and has a high, continuous energy demand. OpenStax's efficiency table also shows how much food energy activity costs: the human body converts it to mechanical work at only about 20% when cycling and climbing.

Photosynthetic efficiency is capped by losses at each step from light capture to carbohydrate synthesis, the steps Zhu, Long and Ort examined. Even at that ceiling, the arithmetic says sunlight on one person's sunward skin could supply only a fraction of a day's needs.

Some green sea slugs eat algae and keep the algae's green parts working inside them!

The green parts help the slugs when food is hard to find.

Nature does have an animal that comes close. Sea slugs in the genus Elysia, including the lettuce sea slug, eat algae and keep the algae's chloroplasts alive inside their bodies, which turns them bright green.

In 2025, a team at Harvard University reported that the slugs store the chloroplasts in special compartments they named kleptosomes, from a Greek word meaning 'to steal'. Without food, these slugs survived for up to four months, while another slug without kleptosomes lasted three to four weeks. After four weeks, though, the green slugs turned orange and stopped photosynthesizing.

Scientific American (reproducing a Nature news story, June 2025) reported a study in Cell by Nicholas Bellono's team at Harvard. They found most proteins in the slugs' stored chloroplasts were made by the slug, and that the chloroplasts sit in membrane-bound compartments they named kleptosomes; blocking a channel called P2X4 in those compartments reduced photosynthetic oxygen production.

In starvation tests, Elysia crispata survived up to four months, against three to four weeks for Aplysia californica, which lacks kleptosomes. After four weeks the Elysia slugs turned orange and stopped photosynthesizing, and the kleptosomes began fusing with lysosomes: in lean times the slugs can digest their stored chloroplasts, which Bellono likened to a moving refrigerator.