Keto vs Low Fat: What the Head-to-Head Trials Found
Summary: In randomized trials, low-carb and ketogenic diets usually lose a little more weight than low-fat diets at first, but by one to two years the difference is small, so the diet you can keep matters most.
On this page
- The answer up front
- The trials at a glance
- Short trials: an early lead
- Long trials: the gap narrows
- What changes besides weight
- What the trials cannot tell you
- How to use this
- Related
The answer up front
Keto is not a dramatically better weight-loss diet than low fat. It is also not worse.
Across the best-known trials, low-carb groups often lead at three to six months. By one to two years, the difference shrinks to a kilogram or two, or to nothing. A pooled analysis of 13 long trials put the edge for very-low-carb ketogenic diets at 0.91 kg (Bueno 2013).
The bigger driver is adherence: whether people keep eating the way the diet asks.
KetoDick on this: His explainer calls keto one kind of low-carb diet and cites studies showing better adherence on low carb than low fat (video s_sNd1v8Low). His recurring point is that most diets work if people stay on them (video UygKGoP_BtA).
The trials at a glance
| Trial | Who | Length | Low-carb result | Low-fat result | Difference |
|---|---|---|---|---|---|
| Foster 2003 | 63 adults | 12 months | More loss at 3 and 6 months | Less loss early | Not significant at 12 months |
| Samaha 2003 | 132 adults with severe obesity | 6 months | 5.8 kg | 1.9 kg | Low-carb ahead |
| DIRECT, Shai 2008 | 322 adults | 2 years | 4.7 kg | 2.9 kg | Low-carb ahead; Mediterranean 4.4 kg |
| DIETFITS, Gardner 2018 | 609 adults | 12 months | 6.0 kg | 5.3 kg | Not significant |
| Bueno 2013 | 13 trials pooled | 12 months or more | — | — | 0.91 kg more on very-low-carb ketogenic diets (95% CI 0.17 to 1.65 kg) |
"Not significant" means the gap was small enough that chance could explain it. See how to read a study.
Short trials: an early lead
Short trials tend to favor low carb.
In Samaha 2003, adults with severe obesity lost 5.8 kg on low carb vs 1.9 kg on low fat over six months (Samaha 2003). In Foster 2003, the low-carb group lost more at 3 and 6 months (Foster 2003).
Some of any early drop on low carb may be water rather than fat. KetoDick attributes most of his own early loss to water released with stored glycogen (video 9Hk0jRb_9Es); the trials above report weight, not water and fat separately. See water weight.
Long trials: the gap narrows
Longer trials show a smaller difference.
- Foster 2003: by 12 months, the low-carb lead was no longer significant. Many people dropped out of both arms.
- DIRECT: over two years, low carb lost 4.7 kg, Mediterranean 4.4 kg and low fat 2.9 kg. All three groups lost weight.
- DIETFITS: after a year, healthy low carb lost 6.0 kg and healthy low fat 5.3 kg, not a significant difference. Neither genotype nor insulin secretion changed which diet worked better.
- Bueno 2013: pooling 13 trials of a year or more, very-low-carb ketogenic diets came out 0.91 kg ahead.
The National Lipid Association summed it up: low- and very-low-carb diets are not superior for long-term weight loss and may give more short-term loss (Kirkpatrick 2019). Diabetes UK says low-carb diets can be effective short-term and that there is no single ideal mix of carbs, fat and protein (Diabetes UK 2018).
KetoDick on this: He questions the year-long JAMA low-fat vs low-carb study (DIETFITS) cited in a Men's Health article, pointing to self-reported intake and wide individual variation (video 1JbK1_ClDmY). The trial's published result is the one in the table above: 6.0 vs 5.3 kg, not significantly different. Both are stated so you can weigh them.
What changes besides weight
Weight is not the only thing that moved in these trials.
| Marker | What low-carb or ketogenic diets showed |
|---|---|
| Triglycerides | Lower (Bueno 2013; Samaha 2003) |
| HDL ("good") cholesterol | Higher (Bueno 2013) |
| LDL cholesterol | Higher in the pooled data (Bueno 2013); variable across people (Kirkpatrick 2019) |
| Total cholesterol to HDL ratio | Largest improvement on low carb in DIRECT (Shai 2008) |
| Diastolic blood pressure | Lower (Bueno 2013) |
| Insulin sensitivity | Larger gains on low carb (Samaha 2003) |
The LDL result is the one to watch. If you have a history of high cholesterol or heart disease, ask your doctor for a lipid panel before you start and a few months in. See cholesterol.
What the trials cannot tell you
These trials are strong evidence, but they have limits.
- Most were not strict keto. DIETFITS used a moderate low-carb arm, not a ketogenic one. Bueno 2013 is the analysis most specific to ketogenic diets.
- Most participants had overweight or obesity. Results may differ for someone with less to lose.
- Few were specific to women aged 30 to 60. The samples were mostly mixed adults.
- People drifted. Free-living trials measure what people were told to eat and what they actually kept doing, which are not the same.
- Averages hide individuals. A small average gap can include people who did very well on each diet.
Metabolic-ward studies answer a different question, what happens at equal calories, and are covered in Energy Balance on Keto and Insulin and Fat Storage.
How to use this
- Pick the pattern you can keep. If keto lowers your hunger and you enjoy the food, that is a real advantage. If it does not, a different pattern can work too.
- Plan for month 12, not week 2. The early lead fades. What you do at maintenance decides the result. See habits.
- Watch your own markers. Get lipids and blood sugar checked with your doctor, especially if you take diabetes or blood pressure medication.
KetoDick on this: He frames keto's advantage as adherence, not a metabolic trick: restricting carbs lowers hunger so people can eat less; the diet "doesn't rely on willpower" (from the video) (video 9AGpKAfs3Yk).
Related
Guide
- Chapter 2: How Fat Loss Actually Works
- Energy Balance on Keto
- Insulin and Fat Storage
- Hunger and Appetite on Keto
- Observational studies vs RCTs
Blog
- The DIETFITS study explained
- Kevin Hall's metabolic ward studies
- Keto vs low carb
- Why do most diets fail?
- Is keto sustainable long term?
- What the ADA says about low-carb diets
Videos
- What is the ketogenic diet and how does it work?
- Men's Health hack job on Keto
- Keto advantage in diet adherence through insulin reduction
Part of Chapter 2: How Fat Loss Actually Works · Previous in chapter: Hunger and Appetite on Keto
Last verified: 2026-10-31