Why Do Two People on the Same Diet Lose Weight at Different Rates?
Two people can follow the exact same calorie deficit and lose weight at noticeably different rates, and it’s rarely about who tried harder. Trials that carefully test known explanations, genotype, insulin response, and more, still find large, largely unexplained differences between people on identical plans. The same pattern shows up in reverse for people eating a surplus to gain weight.
In brief
- A 609-person, 12-month trial assigning everyone to the same diet type found individual weight change ranged from about a 30kg loss to a 10kg gain, with no significant link to genotype or insulin response.
- A small, decades-old twin study (7 pairs, young men only) found genetically identical twins responded more similarly to each other than to unrelated participants under an identical exercise-based deficit.
- A separate study found some people unconsciously eat more after starting an exercise programme, largely erasing an energy deficit that was working exactly as intended on paper.
- The same kind of individual variability shows up in reverse, people deliberately overfed by the same fixed amount stored dramatically different amounts of fat, an effect tied to how much their body's unconscious daily movement changed in response.
Why don’t two people on the same deficit lose weight at the same rate?
One of the clearest demonstrations of this comes from the DIETFITS trial, which randomly assigned 609 adults to either a healthy low-fat or healthy low-carbohydrate diet for 12 months. Average weight loss was similar between the two diets, about 5.3kg and 6.0kg. But that average hid something much larger, individual weight change within each diet group ranged across roughly 40kg (from about a 30kg loss to a 10kg gain).
The researchers had specifically set out to test whether genotype or a person’s own insulin response could explain who lost more, since both had been proposed as the missing piece. Neither one did. People who were genetically “matched” to their assigned diet (based on a specific 3-gene pattern) lost no more weight than those who weren’t, and baseline insulin secretion didn’t predict success either. The variation was real, and it wasn’t explained by the two most obvious candidate explanations the study was built to test.
It’s worth being clear about what this trial can and can’t tell us. People reported their own food intake rather than having it measured directly, and one small clinical study found a group of obese patients who’d repeatedly failed to lose weight underestimated their intake by nearly half without realizing it. That’s a specific, atypical group, not proof everyone underreports by that much, but it’s a real reason to think some of DIETFITS’s spread reflects how closely different people actually stuck to the diet, not biology alone. Some of the variation may also just be measurement noise and normal fluctuation rather than a true, stable difference between people, something a single before-and-after design like this can’t fully rule out. This study rules out the two specific factors it was designed to test, not every possible explanation.
Does genetics really play a role?
DIETFITS specifically tested one narrow genetic pattern, a set of 3 genes thought to predict who does better on low-fat versus low-carb. A different, older study points to something broader: overall genetic heritability, not tied to any specific gene. Researchers put seven pairs of identical twins through a tightly controlled negative energy balance protocol, exercising twice a day for 93 days while their food intake was held constant, so every twin pair faced an identical, precisely measured energy deficit, this time with no self-reporting involved.
Even so, there were large differences in how much fat each person lost. But twins were more similar to their own sibling than to unrelated participants in body fat, body energy, and abdominal fat changes, despite facing the exact same protocol. That pattern, twins resembling each other more than they resemble a stranger on the same plan, is a suggestive sign that some of this comes down to biology you didn’t choose, though with only seven twin pairs, it’s a small enough sample that the finding should be read as a lead worth taking seriously rather than a precise, settled estimate of how much genetics matters. It’s a different, broader kind of genetic evidence than the specific gene pattern DIETFITS tested and found nothing from, so the two studies aren’t actually in direct conflict.
This was a small, decades-old study of seven twin pairs, all young men, and the deficit came from exercise while food intake was held constant, not from cutting calories through diet alone. Whether the same genetic pattern applies equally to a purely dietary deficit isn’t something this study can tell us. It’s a good data point to consider, but not the full explanation.
Is it about appetite and behaviour, not willpower?
A third study considered how your appetite responds. Researchers put 35 overweight and obese adults through 12 weeks of supervised exercise, prescribed identically for everyone. On average, weight loss matched what was predicted. But underneath that average, individual results ranged from a 14.7kg loss to a 1.7kg gain.
The researchers split people into two groups based on how their body actually responded, people whose eating unconsciously increased to offset the exercise (“compensators”) lost only 1.5kg on average, while those whose eating didn’t change (“non-compensators”) lost 6.3kg. The difference wasn’t that compensators exercised less. It was that their eating and hunger increased in response to the exercise, adding back a meaningful chunk of the energy they’d burned, mostly without them consciously deciding to eat more.
This isn’t a willpower story so much as an appetite-regulation one, though again, this study used exercise to create the deficit rather than eating less, and splitting a group into “compensators” and “non-compensators” after the fact can partly reflect normal day-to-day fluctuation rather than a fixed trait, so it’s a real pattern worth knowing about rather than a precise label for any one person.
Does this apply if you’re trying to gain weight too?
Yes, this isn’t a losing-weight-only phenomenon. A classic overfeeding study gave 16 healthy adults the exact same fixed calorie surplus, about 1000 extra calories a day for eight weeks, and measured what happened to their non-exercise daily movement, the fidgeting, posture shifts, and general restlessness known as NEAT. Individual NEAT changes ranged enormously, from a 98-calorie-a-day decrease to a 692-calorie-a-day increase, and that change was strongly linked to how much fat someone stored on the identical surplus.
In other words, two people eating the same extra calories to intentionally gain weight can end up with very different results, for reasons that have nothing to do with how carefully either of them followed the plan.
What this actually means for you
If you and someone else follow what looks like the exact same plan and get different results, part of that gap is genuinely how closely each of you stuck to it. But the studies that actually measured intake directly, not just self-reported it, still found real, unexplained differences between people facing an identical deficit or surplus. So a gap in results isn’t proof that one of you is secretly cheating or simply trying harder either. Both things can be true at once.
This is part of why Ori’s target adjusts to your own data rather than assuming a formula that worked for someone else will work the same way for you. It’s less about calculating the “right” number once, and more about watching what’s actually happening for you specifically, and adjusting from there. Making your experience with Ori how we think any app should be, personal to you!
Sources
- Discrepancy between self-reported and actual caloric intake and exercise in obese subjects — New England Journal of Medicine (Lichtman, Pisarska, Berman et al., 1992)
- Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial — JAMA (Gardner, Trepanowski, Del Gobbo et al., 2018)
- The Response to Exercise with Constant Energy Intake in Identical Twins — Obesity Research (Bouchard, Tremblay et al., 1994)
- Individual Variability Following 12 Weeks of Supervised Exercise: Identification and Characterization of Compensation for Exercise-Induced Weight Loss — International Journal of Obesity (King, Hopkins, Caudwell, Stubbs & Blundell, 2008)
- Role of Nonexercise Activity Thermogenesis in Resistance to Fat Gain in Humans — Science (Levine, Eberhardt & Jensen, 1999)
A gentle safety note
Ori supports general wellness. It is not medical care. If you have a history of disordered eating, are pregnant or breastfeeding, take medicines that affect weight or appetite, or have a clinical nutrition need, speak with a qualified healthcare professional before changing your intake or weight goal.
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