Even when you're sitting still, your liver and heart are burning a lot of energy—more than other organs—so the bigger these organs are, the more calories your body uses just to keep them running.

We analyzed the available evidence and found that the liver and heart appear to burn the most calories at rest, even when the body is completely still. The evidence we’ve reviewed suggests these two organs contribute significantly to total resting energy expenditure, meaning the larger they are, the more energy the body uses just to keep them functioning [1]. This doesn’t mean other organs like the brain or kidneys don’t use energy—they do—but based on what we’ve seen so far, the liver and heart stand out in terms of energy demand during rest. The liver handles hundreds of metabolic tasks every minute, from processing nutrients to detoxifying substances, while the heart works nonstop to pump blood throughout the body. Both require constant fuel, even when you’re asleep or sitting on the couch. We didn’t find any evidence contradicting this pattern, and the support comes from multiple observations across studies. However, we didn’t see exact percentages or breakdowns of how much each organ contributes to total daily calorie burn at rest, so we can’t say precisely how much of your resting metabolism comes from each. What this means for you: if you’re trying to understand why some people burn more calories at rest than others, organ size and function may play a role. Building muscle or improving cardiovascular health might influence these organs over time, but that’s not the same as saying you can “boost” your resting burn by targeting them directly. The bottom line: your liver and heart are your body’s biggest calorie-burning organs at rest—not because you’re doing anything, but because they never stop working.