What diet changes can achieve
Average weight loss from sustained dietary changes in clinical trials — compared to prescription medication.
Averages from clinical trials. Adherence matters more than which diet you follow — the best diet is the one you can maintain.
The bottom line on diet and weight loss
All dietary approaches produce similar weight loss when calories are matched and adherence is sustained. The critical question is not "which diet works best?" but "which approach can you maintain long-term?" High protein intake is the one consistent modifier — it preserves lean mass regardless of which eating pattern you follow. If you are on GLP-1 medication, diet still matters: what you eat determines body composition even when the drug handles appetite suppression.
Caloric deficit
Creating an energy deficit — consuming fewer calories than the body uses — is the fundamental mechanism behind all dietary weight loss. This is not contested in nutrition science. A deficit of approximately 500 kcal/day produces about 0.5 kg of fat loss per week in controlled conditions.
A 2024 meta-analysis of 47 randomised trials (3,363 participants) compared various forms of caloric restriction. All approaches — continuous restriction, alternate day fasting, and time-restricted eating — produced clinically meaningful weight loss with sustained adherence.
As weight is lost, metabolic rate decreases and hunger hormones increase — the deficit needed to continue losing weight becomes harder to maintain over time. Very large deficits (over 1,000 kcal/day) increase lean mass loss and are harder to sustain.
High protein diet
Among macro-focused dietary strategies, higher protein intake has the most consistent evidence for preserving lean mass during weight loss and producing marginally greater fat loss compared to lower-protein diets at matched calorie levels.
A meta-analysis of 24 RCTs found that high-protein energy-restricted diets produced greater weight loss (−0.79 kg), greater fat loss (−0.87 kg), more lean mass preservation (+0.43 kg), and higher resting metabolic rate (+142 kcal/day) compared to standard-protein diets. The recommended target is 1.2–1.6 g protein per kg body weight per day during active weight loss.
This is especially important for people on GLP-1 medications, which can cause rapid weight loss. Higher protein helps ensure more of the weight lost is fat rather than muscle. Good sources include chicken, fish, Greek yoghurt, eggs, lentils, and tofu.
Low-carb vs low-fat
When the two most popular dietary approaches are compared directly in randomised trials, the differences are smaller than most people expect.
The most rigorous head-to-head comparison found no significant difference between healthy low-carb and healthy low-fat diets. Both produced approximately 5–6 kg of weight loss at 12 months. Neither genetic markers nor insulin levels predicted which diet would work better for a given individual.
A broader meta-analysis of 33 RCTs (3,939 participants) found a modest 1.33 kg advantage for low-carb diets. Low-carb shows better triglyceride and HDL improvements; low-fat shows better LDL reduction. The practical takeaway: both work, and the best choice depends on which approach you find easier to sustain.
Intermittent fasting
Intermittent fasting and time-restricted eating produce similar weight loss to continuous caloric restriction in clinical trials — the main advantage is that some people find it easier to adhere to than daily calorie counting.
A 2024 meta-analysis of 47 RCTs ranked alternate day fasting as producing the greatest weight loss, ahead of continuous caloric restriction and time-restricted eating (16:8). However, the differences are modest. Most of the weight loss comes from reduced total caloric intake rather than any special metabolic effect.
If you are on GLP-1 medication, check with your healthcare provider before combining with fasting — the appetite suppression from the drug already creates a significant caloric deficit, and adding fasting on top can make it harder to get adequate protein and nutrients.
Ultra-processed food
A landmark 2019 NIH inpatient trial provided the first direct experimental evidence that ultra-processed food causes overconsumption — independent of calories, macros, sugar, sodium, and fibre.
Participants ate ad libitum (as much as they wanted) from either an ultra-processed or unprocessed diet for two weeks each. Despite meals being matched for available calories and macronutrients, participants consumed 508 more calories per day on the ultra-processed diet. The mechanism appears related to eating speed — ultra-processed food is consumed faster, outpacing satiety signals.
Not all processed food is problematic. The NOVA classification system distinguishes between minimally processed (fresh, frozen), processed (canned, preserved), and ultra-processed (industrially manufactured with additives). The evidence specifically concerns ultra-processed products. Reducing UPF and replacing with whole foods naturally reduces calorie intake in most people.
If you're also taking medication
Diet doesn't stop mattering when you start a weight loss drug — it shifts. GLP-1 medications reduce appetite, which makes it easier to eat less. But what you eat still matters for muscle preservation, energy, and long-term health. Prioritise protein (1.2–1.6 g/kg/day), eat whole foods where possible, and focus on nutrient density over calorie counting — the drug handles the deficit.
A note on diet trials
Diet research is hard to conduct — participants can't be blinded to what they eat. Long-term results depend more on adherence than which diet people follow. Take all "best diet" claims with scepticism; the evidence consistently points to sustainability as the critical variable.
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Last reviewed: April 2026