Does drinking cold water really help you lose weight?
— Warming it up costs about as much energy as 10 grams of rice
When you drink cold water, your body warms it up to body temperature. Warming something takes heat, and heat is energy. That's the origin of the claim that "drinking cold water alone makes you lose weight." The logic is sound, but once you work out the actual numbers, the story changes completely.
A bottle of water fresh out of the fridge. Gulp it down and you feel a chill spread through your chest.
Then someone says: "Cold water makes your body burn calories to warm it up, so just drinking it helps you lose weight."
It's true that chilled water somehow ends up at body temperature inside you. Where does that heat come from? And how much of it is there, really?
There are only two reasons the weight-loss effect is nearly nothing
Warming 500 mL of chilled water to body temperature takes about 16 kilocalories — roughly 10 grams of rice, a small single bite.
Your body is always producing heat, and dumping most of it outside. Warming cold water simply redirects some of that "heat that was going to be discarded." It doesn't necessarily mean burning extra fuel.
Let's take these in order. The first is about "how much," the second is about "where that heat comes from."
Reason 1: Mixing up "calories" and "kilocalories"
The heat needed to raise 1 gram of water by 1°C is called 1 calorie. The calorie was, in fact, originally defined using the heat that warms water.
Fridge water is about 5°C, and body temperature is 37°C, so the difference is 32°C. 500 mL of water weighs 500 grams. The heat needed to warm it is 500 times 32, which comes to 16,000 calories.
"16,000 calories" sounds like a huge number. But food labels use kilocalories, not calories. One kilocalorie is 1,000 calories, so 16,000 calories is really just 16 kilocalories. This mix-up is one of the reasons the popular claim seemed so much bigger than it is.
Look at Figure 1. The top bar is the heat needed to warm 500 mL of cold water to 37°C. Turn the dial to lower the water's temperature and the bar barely grows — even ice-cold 0°C water tops out at 18.5 kilocalories. The middle bar is the energy in 10 grams of rice.
Reason 2: The body warms water with heat it was going to throw away anyway
The human body keeps producing heat simply by being alive. For an adult at rest, that's about 1,500 kilocalories a day, or roughly 60 kilocalories an hour. That's the bottom bar in Figure 1.
Most of this heat escapes through the skin and breath, because keeping body temperature at 37°C means constantly getting rid of the heat you make. The body is like a heater that's always carrying a surplus of heat.
When cold water comes in, the body can simply redirect some of that surplus heat to it. By narrowing the skin's blood vessels slightly to release less heat outward, body temperature stays steady. In that case, not a single extra gram of fuel needs to be burned. 16 kilocalories is only a quarter of the heat the body sheds in an hour — about 15 minutes' worth.
Some studies have actually measured this. A 2003 German study reported that drinking 500 mL of water raised metabolism by about 30%, and it made headlines. But later studies found much smaller increases. Some even reported increases smaller than the calculated heat needed just to warm the water.
Ice absorbs a large amount of heat as it melts. Melting 100 grams of ice and warming it to body temperature takes about 12 kilocalories — more than three times as much as the same weight of cold water. Still, chewing through 500 mL worth of ice is hard on both your teeth and your stomach. It's not something to recommend as a weight-loss method.
While the cold-water weight-loss effect is tiny, swapping sugary drinks for water does cut your actual intake. A 500 mL sugary soda, for example, is often around 200 kilocalories. What's doing the work here isn't "coldness" — it's "substitution."
Summary
Warming cold water to body temperature does take heat. But for 500 mL, that's only about 16 kilocalories — roughly 10 grams of rice. And in many cases, the body is thought to simply redirect heat it would normally dump outside anyway. To burn through the roughly 7,200 kilocalories in 1 kilogram of fat using cold water alone, you'd need to drink 450 bottles.
What cold water takes from your body isn't calories —
it's heat your body was going to throw away anyway.
For more on what the number behind "calories" actually measures, see What exactly does a food "calorie" measure?, and for the large heat involved when ice melts, see Why can ice cool things much better than 0°C water?
- Pour 200 mL of fridge-chilled water into a mug and measure its temperature with a thermometer.
- Heat it in the microwave (say, 500 watts) in 20-second bursts, stirring and measuring after each one. Record the total time until it reaches 37°C.
- Heating at 500 watts for 1 second supplies about 0.12 kilocalories. Multiply this by the total seconds for a rough estimate of the heat used to warm it. That's about the same amount of heat your body uses to warm cold water.
Some of the microwave's heat escapes into the cup and the oven itself, so it will take a bit longer than the calculation suggests. Be careful not to overheat the water and risk burns.
Want to go deeper? — terms, formulas, and how this connects to textbooksWe've marked which level each part belongs to, from middle-school science up to university-level specialized courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics" and "Basic Biology"
- High school+Advanced high-school content, or textbook sidebar material
- UniversityNot covered in high school — university-level specialized content (physiology, nutrition science)
- ResearchNot even settled as "established fact" at university — something researchers are actively investigating
Middle schoolTerms: this phenomenon has names
- Heat (quantity of heat): the amount of heat flowing into or out of an object. Measured in calories or joules.
- Specific heat: the heat needed to raise 1 gram of a substance by 1°C. Water's is 1 calorie per gram — remarkably large among everyday substances.
- Basal metabolic rate: the energy used just by being alive, even while completely at rest. Nearly all of it eventually leaves the body as heat.
Middle schoolHigh schoolCheck with a formula: how many kilocalories to warm 500 mL of cold water?
We plug the fridge-water and body-temperature values into the formula for the heat needed to raise a temperature. Here, Q is the quantity of heat (calories), m is mass (grams), c is specific heat (calories per gram per °C), and ΔT is the temperature difference (°C).
| In symbols | Q = m × c × ΔT |
| In words | Heat required = mass of water × specific heat of water × temperature rise |
| Where it comes from | The very definition of specific heat, which in turn comes from conservation of energy (temperature rises exactly as much as the heat put in) |
| Mass of water m | 500 grams (500 mL) |
| Specific heat of water c | 1 calorie / (gram·°C) |
| Fridge water temperature | 5 °C |
| Body temperature | 37 °C |
| Energy in rice (white rice) | 156 kilocalories per 100 grams (per Japan's Standard Tables of Food Composition) |
| Daily resting energy use (basal metabolic rate estimate) | about 1,500 kilocalories |
| Energy in 1 kilogram of body fat | said to be about 7,200 kilocalories |
| Temperature rise ΔT | 37 − 5 = 32 °C |
| Heat required Q (c = 1, so m × ΔT) | 500 × 32 = 16,000 calories |
| Converted to kilocalories | 16,000 ÷ 1,000 = 16 kilocalories |
| Energy per gram of rice | 156 ÷ 100 = 1.56 kilocalories |
| Equivalent in grams of rice | 16 ÷ 1.56 ≈ 10.3 grams |
| Body heat output per hour | 1,500 ÷ 24 = 62.5 kilocalories |
| Equivalent in hours of that heat | 16 ÷ 62.5 = 0.256 hours |
| Converted to minutes | 0.256 × 60 ≈ 15.4 minutes |
| Bottles needed for 1 kg of fat | 7,200 ÷ 16 = 450 bottles |
The heat needed to warm 500 mL of cold water is about 16 kilocalories — about 15 minutes' worth of the heat your body sheds in an hour. Even drinking two bottles a day, every day, it would take 225 days to reach 1 kilogram of fat. And as Reason 2 explained, there's no guarantee that whole 16 kilocalories gets burned as extra fuel at all.
High schoolHigh school+How is body temperature kept constant?
High schoolBody temperature is held steady by a balance between the heat your body makes and the heat it releases. When it's hot, skin blood vessels widen and you sweat; when it's cold, they narrow to hold heat in. This is taught in "Basic Biology" as an example of homeostasis driven by the autonomic nervous system.
High school+At ordinary room temperatures, the body is running a heat surplus. Even if cold water pulls a little heat away from the inside, narrowing the blood vessels to cut outward heat loss is enough to restore the balance. Only when reducing heat loss alone isn't enough does the body start making extra heat, for example through shivering.
UniversityThe thermoneutral zone and non-shivering thermogenesis
Physiologists call the range of air temperature over which the body doesn't need to spend extra energy on temperature regulation the thermoneutral zone. Within this range, temperature control is handled mainly through skin blood flow. Outside it, heat production from muscle shivering and from non-shivering thermogenesis via brown adipose tissue kicks in. How much metabolism rises from drinking cold water likely depends on where the drinker sits within this range. This is considered a distinct mechanism from diet-induced thermogenesis, the metabolic rise that follows a meal.
📖 For the derivation of the formulas and further reading: Brown adipose tissue (Wikipedia, Japanese) / Basal metabolic rate (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- Does drinking water alone raise metabolism? The 2003 report showed a large increase, but later studies found smaller effects, or none at all — the matter isn't settled.
- How much does this vary between people? The amount of brown fat is said to vary by person and by season, and how far individual differences in cold-induced heat production go is still being studied.
- What about long-term effects? Whether the small differences seen in a single measurement show up as actual weight differences after months of continued practice isn't clear.
In other words, even this article's content reflects "the best explanation we have right now." That said, the fact that the ceiling on the warming heat is about 16 kilocalories can be stated with confidence, straight from the specific-heat formula.
How this connects to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science (heat and energy) | 1 calorie per gram per °C of water, the 16-kilocalorie calculation |
| High school | Basic Physics (specific heat, conservation of heat), Basic Biology (temperature regulation) | The formula Q = m × c × ΔT, the mechanism of narrowing blood vessels to cut heat loss |
| High school+ | Biology (advanced homeostasis) | How a heat surplus means no extra fuel is needed |
| University | Physiology / nutrition science | Thermoneutral zone, non-shivering thermogenesis, brown adipose tissue |
| Research | Energy metabolism research | How much drinking water actually raises metabolism |
| — | Connection to daily life | It's substitution, not coldness, that's doing the work when you swap sugary drinks for water |
- Boschmann M. et al., "Water-Induced Thermogenesis" (Journal of Clinical Endocrinology & Metabolism, 2003)
- Brown C. M., Dulloo A. G., Montani J. P., "Water-induced thermogenesis reconsidered: the effects of osmolality and water temperature on energy expenditure after drinking" (Journal of Clinical Endocrinology & Metabolism, 2006)
- Ministry of Education, Culture, Sports, Science and Technology, "Standard Tables of Food Composition in Japan, 2020 (Eighth Revised Edition)" (文部科学省「日本食品標準成分表2020年版（八訂）」)
- Specific heat capacity (Wikipedia, Japanese)
- Ministry of Health, Labour and Welfare, "Dietary Reference Intakes for Japanese" (厚生労働省「日本人の食事摂取基準」) (reference values for basal metabolic rate)
※This article is a general-audience science explainer. The figures given are rough estimates meant to help illustrate the underlying mechanism. Basal metabolic rate and body fat energy vary between individuals. If you have any concerns related to weight management or fluid intake due to a medical condition, please consult a doctor.