A disclosure before we go further: I develop soup cleanse products, which is a reason to read the evidence sections closely rather than take my word for it. Where the research is thin, I’ve said so.
What people mean by slow
Metabolic rate is the energy your body burns, mostly at rest. When people say a diet slowed it, the claim bundles four separate things. A smaller body burns less, so resting rate falls as weight comes off. Less muscle burns less than more muscle. Digesting food costs energy, so eating less lowers that cost. And under some conditions the body spends less than its size and composition predict, which researchers call adaptive thermogenesis. Three days of soup touches each of these differently.
How metabolic rate is measured
Researchers measure resting expenditure by indirect calorimetry: the oxygen a person consumes and the carbon dioxide produced while lying still, in the morning, after an overnight fast. The result is sensitive to what you ate and how you slept the night before, which is one reason short fasting studies have disagreed with one another.
What the first days look like
In a 2000 study in the American Journal of Clinical Nutrition, a team at the University of Vienna measured resting energy expenditure in 11 healthy, lean volunteers across an 84-hour fast. It rose from 3.97 kilojoules a minute on day one to 4.53 on day three, an increase of about 14%. Norepinephrine, a hormone that raises heat production, more than doubled over the same days, and blood glucose fell from 4.9 to 3.5 millimoles per liter. The authors proposed that the falling glucose triggers the norepinephrine, which drives the extra burn.
Those were complete fasts in a small group, and the authors noted that earlier studies of short fasts had disagreed with one another. A soup cleanse leaves some food in the day, so it is a milder change than that, though how much milder depends on the pot. I haven’t found any study that measured resting metabolic rate during one. What the fasting data support is narrower: nothing in the first three days points to a slowdown, and the early signs point the other direction.
What 14 percent means in calories
Converted to calories, the Vienna figures go from about 1,370 a day on day one to about 1,560 on day three, roughly 190 more calories burned at rest. That is the size of the early change: about 190 calories a day, in the direction of more.
Where slowing starts
The slowdown belongs to longer, deeper restriction. In the Minnesota Starvation Experiment of the 1940s, 32 healthy men ate less than half their usual calories for 24 weeks. They lost about a quarter of their body weight, and their basal metabolic rate fell by roughly 40%. Most of that drop tracked their shrinking bodies. Researchers still argue over how much of it was adaptation, but re-analyses put the portion beyond what body size and composition explain at about 10% at week four, the first point at which it was measured, and about 25% by week 24. A 2015 study that revisited the Minnesota design, with a 50% energy deficit over three weeks, also saw resting expenditure fall. This is the measured version of what gets called starvation mode.
Those are weeks of eating at half of normal or less. The stretch between four days and four weeks is thinly measured, which is one more reason the 3-day guide holds the line at three days and the 5-day guide stops at five, even for experienced cleansers.
The weight that comes off in three days
A smaller body burns less, but the weight lost in three days is mostly water, as the post on why the weight comes back lays out. Water does no metabolic work of its own, so losing it should change resting rate very little. The muscle route is similarly small: the muscle post shows why three days doesn’t reach the range where muscle loss becomes a concern.
The cost of digesting
Digesting food costs energy: about 10% of the calories in a mixed diet, with protein costing more than carbohydrate and fat costing least. Reviews put the cost at 20 to 30 percent of protein calories, 5 to 10 percent of carbohydrate calories, and 0 to 3 percent of fat calories. If a day’s intake drops from 2,000 calories to 800, the cost of digesting falls by about 120 calories. Eat less and that cost falls in proportion, so a cleanse does lower this one for as long as intake stays low. It returns with the next ordinary meal. A soup built on legumes also keeps some protein in the day, and protein is the most expensive of the three to digest.
Movement outside exercise
About a fifth of daily energy goes to movement that isn’t exercise: standing, walking around, fidgeting, holding posture. Eating less can change that number as easily as it changes resting rate, and nobody has measured it during a soup cleanse.
After the cleanse
The other worry is a rebound: that a few days of restricted eating leaves metabolism lower once normal meals return. I haven’t found a study that measured resting metabolic rate after a short cleanse, so I can’t tell you it never happens. What the long studies show is a lag behind the lost weight. In Minnesota, the adaptive portion was still about 10% below prediction twelve weeks into refeeding. That followed 24 weeks of semistarvation, and it says little about three days.
Common questions
- Does a soup cleanse put you in starvation mode?
- Starvation mode is a popular name for adaptive thermogenesis, which was measured after weeks of eating less than half the usual amount. In a small study of the first days of a full fast, resting energy expenditure rose.
- Will a soup cleanse make it harder to lose weight afterward?
- I haven’t found a study that measured it. The weight lost in three days is mostly water, which returns with normal eating.
- How long does it take for metabolism to slow down?
- The Minnesota Starvation Experiment first measured adaptation at four weeks of eating less than half the usual amount. The stretch between four days and four weeks has little data.
- Does eating less change how many calories you burn moving around?
- It can. Movement outside exercise accounts for about a fifth of daily energy use, and nobody has measured it during a soup cleanse.