The short version
A new study found that adults with overweight who replaced animal protein with plant protein — as part of a low-fat vegan diet — lost weight, even though their total protein intake went down. When the same people followed a Mediterranean diet instead, their weight did not change significantly.
The takeaway is not "eat less protein." It is that where your protein comes from may matter as much as how much you eat.
What the study did
This was a randomized clinical crossover trial — the strongest type of nutrition study, because each person tries both diets and acts as their own comparison.
- 62 adults with overweight took part; 52 completed the full 36-week study.
- Each person followed a low-fat vegan diet for 16 weeks and a Mediterranean diet for 16 weeks, with a 4-week break in between, then switched.
- The vegan diet centred on fruits, vegetables, grains and legumes, targeting 75% carbohydrate, 15% protein and 10% fat. The Mediterranean diet included vegetables, fruits, fish, nuts and olive oil.
- Food intake was tracked with three-day food records checked by registered dietitians.
What happened
On the low-fat vegan diet (16 weeks):
- Average weight loss of around 13 pounds (6 kg).
- Total protein intake fell from about 80 g per day to about 46 g per day — a drop of 33.3 g per day.
- Animal protein fell by 47.7 g per day, while plant protein rose by 14.4 g per day.
On the Mediterranean diet (16 weeks):
- No significant change in body weight or protein intake.
Senior study author Hana Kahleova, MD, PhD, put it plainly:
"The [protein] source matters. Shifting the balance toward beans, lentils, tofu, and whole grains — and away from meat, dairy, and eggs — gave our participants a metabolic and a weight benefit they didn't get by simply eating more vegetables on a Mediterranean-style diet."
The researchers also found that a reduction in four amino acids — isoleucine, lysine, methionine and tyrosine — which are found in higher amounts in animal protein, was linked to greater weight loss.
The honest caveat (read this before changing your diet)
The researchers are clear: they cannot confirm that the protein change alone caused the weight loss. When people switched to the vegan diet, their fat, carbohydrate and calorie intake changed at the same time — so this is not a clean animal-vs-plant protein comparison. More studies are needed.
Experts also note the Mediterranean diet may be the better choice for some people — for example, those with high triglycerides, metabolic syndrome, or older adults at risk of muscle loss.
So this study is a strong hint, not a final verdict.
What this means for your macros
If you are tracking macros for weight loss, here is the practical side:
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Hit your protein target first
Most adults aiming for fat loss do best around 1.6–2.2 g of protein per kg of body weight per day. Use our protein calculator → to find your number.
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Shift some of it toward plants
You do not have to go vegan. Swapping one or two animal-protein meals a day for beans, lentils, tofu, chickpeas or whole grains moves your protein mix in the direction this study tested.
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Watch total calories, not just protein
The vegan group also ate fewer calories overall — that is part of why they lost weight. Protein source helps, but a calorie deficit is still what drives fat loss.
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Keep enough protein if you are older or building muscle
Very low protein (the vegan group averaged only 46 g/day) is not a target to copy — it is just what happened in this trial.
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Frequently Asked Questions
Does plant protein help weight loss more than animal protein? This study suggests it may — people who replaced animal protein with plant protein lost about 6 kg in 16 weeks. But the researchers cannot yet prove the protein swap alone caused it, because calories and other nutrients changed too.
How much protein should I eat for weight loss? Most guidance is 1.6–2.2 g per kg of body weight per day. That is higher than the 46 g/day the vegan group in this study averaged — so use a calculator, not this study's intake, as your target.
Do I have to go vegan to get the benefit? No. The study tested a full swap, but the practical lesson is to shift some of your protein toward beans, lentils, tofu and whole grains — even one plant-based meal a day is a step in that direction.
Is the Mediterranean diet bad for weight loss? No. In this trial it simply did not produce significant weight change, while the vegan diet did. For people with high triglycerides, metabolic syndrome, or muscle-loss risk, experts say the Mediterranean diet may still be the better fit.
Is this medical advice? No. This is general information only, not medical or diet advice. Talk to a doctor or registered dietitian before changing your diet.