Keto vs Intermittent Fasting vs Vegan: Which Diet Fits Which Person?

Keto, intermittent fasting, and a vegan diet all can support weight loss — and the strongest predictor of which one will work for you isn't biology, it's whether you can keep doing it. The 12-month DIETFITS trial of 609 adults found a healthy low-fat and a healthy low-carb diet produced almost identical average weight loss — 5.3 kg vs 6.0 kg, no statistically significant difference. The honest framing isn't "which diet is best." It's "which framework fits which person." Below, what each one actually does, who it tends to suit, and how to pick.

What keto, intermittent fasting, and vegan diets actually do

The three are often lumped together as "diets," but they work via different levers — that's why fit matters more than ranking.

Keto is a macronutrient pattern: very low carbohydrate (typically under 50 g/day), high fat, moderate protein. The body shifts from glucose to ketones as its primary fuel after a few days to a few weeks. Initial weight drop is partly real fat loss and partly water — every gram of stored glycogen carries roughly 3 g of water with it, so depleting glycogen alone can shed 1–3 kg in week one.

Intermittent fasting (IF) is a timing pattern, not a food rulebook. Common formats: 16:8 (eat in an 8-hour window), 5:2 (two low-calorie days a week), or alternate-day fasting. The mechanism is largely simple — most people end up eating less when their eating window is shorter — though there are also documented metabolic effects on insulin sensitivity and a "metabolic switch" between glucose and ketone use, summarised by the National Institute on Aging's review of the 2019 NEJM paper by de Cabo and Mattson.

Vegan is a food-source rule: no animal products. The weight-management effect comes from the food matrix, not the label. Plant-forward eating tends to be higher in fibre and lower in calorie density, which keeps people fuller for fewer calories. The Adventist Health Study-2, with over 60,000 participants, found average BMI of 23.6 in vegans versus 28.8 in non-vegetarians — a substantial gap, though some of that reflects lifestyle factors beyond diet alone.

If you want a deep head-to-head specifically on the first two, our detailed comparison of keto and intermittent fasting for sustainable weight loss goes further on the keto-vs-IF trade-offs. This article stays at the higher-level fit question.

Not sure which framework suits you? Take the free 60-second NutriGuide quiz and you'll get a personalised meal plan built around your schedule, food preferences, and goals — without picking a diet camp first.

Who each approach tends to fit

The research consistently points to adherence — not macros — as the main driver of long-term results. Pick the framework you can sustain, not the one that sounds most disciplined.

Keto tends to suit people who feel best on protein and fat, dislike grazing, and don't mind a structured shopping list. The strict carb cap removes a lot of decision-making — for some, that's freeing. Harvard Health notes that keto can produce faster early weight loss than low-fat approaches but the difference tends to narrow over months, and the diet is "really hard to follow over the long run." It's a poor fit for endurance athletes, anyone with a history of disordered eating around restriction, or households where shared meals matter.

Intermittent fasting tends to suit people whose mornings are easier without breakfast, who already snack lightly during the day, or who travel and want fewer meal decisions. It's flexible — no banned foods. It tends not to suit people with high training loads, irregular shift work, pregnancy or breastfeeding, or a history of binge-restrict cycles. If a long fasting window leaves you eating frantically at noon, the format is fighting you.

A vegan or mostly-plant diet tends to suit people who already enjoy beans, lentils, grains, and vegetables, and who are willing to plan a little for protein and B12. Calorie density is naturally lower, fibre is naturally higher — both helpful for satiety. It tends not to suit people who travel constantly through low-veg food environments or who find the cooking learning curve overwhelming on top of everything else. Strong starter recipes go a long way: try spinach and red lentil curry, quinoa and vegetable stir fry, or a tropical smoothie bowl for an easy plant-based breakfast.

A pragmatic option that often outperforms "pick a camp": eat mostly plants, time your meals in a way that fits your day, and watch portion sizes on energy-dense foods — without committing to any of the three labels at all.

What the evidence actually says about long-term results

Here's the part the diet industry tends to skip. Long-term randomised trials repeatedly show that named diets converge. The 12-month DIETFITS trial — 609 adults, healthy low-fat versus healthy low-carb — found mean losses of 5.3 kg and 6.0 kg respectively, with no statistically significant difference between groups. Genetics didn't predict who did better on which arm. Insulin secretion didn't either. The factor that did predict success: how much each person stuck to the assigned plan.

The NHS frames safe, sustainable weight loss as 0.5 to 1 kg per week, achieved by reducing daily intake by around 600 kcal below maintenance. The CDC echoes 1 to 2 pounds per week as the rate associated with people who keep the weight off long-term. Neither body recommends keto, IF, or vegan specifically — both recommend a sustained, modest deficit you can live with.

Translation: pick the vehicle (keto, IF, vegan, or none of the above) that helps you maintain that deficit comfortably. The deficit is the mechanism. The diet name is the wrapper.

For more on building meals around protein, fibre, and a comfortable calorie ceiling without picking a diet camp, see our beginner's guide to balancing macros for weight management.

How to pick — a five-question filter

Before committing to any of the three, run the request past these:

1. Could I eat this way in twelve months, on holiday, at a wedding, on a tired Tuesday? If no, the framework will fail you no matter how clean the science.
2. Does it fit my training, sleep, and stress levels right now? A long fasting window during perimenopause or a heavy lifting block is a different ask from the same window during a calm desk-job stretch.
3. Can I hit my protein target inside it? Roughly 1.2–1.6 g per kg of bodyweight protects lean mass during weight loss. Keto and most vegan plans both can — they just need different shopping lists.
4. Does it leave room for the food I actually love? A plan that bans your Friday-night go-to is a plan you'll quietly resent.
5. Have I checked it against any medical conditions or medication? Particularly relevant for diabetes, blood pressure medication, thyroid issues, or a history of disordered eating. Talk to your GP or a registered dietitian first.

If three or more of those answers are uncertain, the framework probably isn't the one — and that's useful information, not a failure.

The honest answer

There isn't a "best" diet for weight management. There's the framework that fits your week, your kitchen, your training, your social life, and your tastes — and that one will outperform the theoretically optimal plan you abandon in six weeks. Pick adherence first. The mechanism (a sustained, modest calorie deficit, with enough protein, with food you enjoy) is the same underneath whichever wrapper you choose.

Want a plan built around your schedule and preferences instead of a diet label? Start the free NutriGuide quiz — 60 seconds, no app download, no credit card. You'll get a personalised meal plan with recipes you'll actually want to cook, calibrated to your goals, your routine, and the foods you already enjoy.

This article is general information, not medical advice. Speak to your GP or a registered dietitian before starting any restrictive eating pattern, especially if you take regular medication, are pregnant or breastfeeding, are managing a chronic condition (including diabetes, thyroid disease, or disordered eating history), or are under 18.