Does it matter for your weight whether your calories come from fat or carbohydrates?
In the long term, the ratio of fat versus carbohydrates matters little for your weight, as long as your total calorie intake stays the same. How well you stick to a diet counts for more than which macronutrients it contains.
When calorie intake is equal, it makes very little difference in the long run whether those calories come mainly from fat or carbohydrates. In a study of 811 overweight adults, all diet groups lost an average of 3 to 3.6 kg after two years, regardless of whether they followed a low-fat or a low-carbohydrate diet. The difference between the groups was not statistically demonstrable.
In the short term, the picture is somewhat more nuanced. An analysis of multiple meta-analyses finds a small advantage for low-carbohydrate diets in the first six months. After twelve months, that difference disappears. The strongest predictor of success turns out to be not the fat-to-carbohydrate ratio, but how well a person sticks to the diet.
An interesting short-term difference was found in tightly controlled hospital research: with equal calories, 19 participants on a fat-restricted diet lost more body fat per day than on a carbohydrate-restricted diet. Computer simulations suggest, however, that this difference diminishes over the longer term. Another hospital study of 17 men showed that a ketogenic diet (virtually no carbohydrates) slightly increased energy expenditure, but actually slowed fat loss, and greater muscle mass was lost.
What does matter is how much you spontaneously eat. In an NIH study of 20 participants, people on a plant-based low-fat diet ate almost 689 kcal per day less than on a ketogenic diet, without consciously restricting portions. This has little to do with the macronutrient ratio itself, but with how satiating and calorie-dense the food is. On the keto diet, one participant had to stop due to blood sugar that was too low.
A meta-analysis of 13 randomised studies found that a strict low-carbohydrate ketogenic diet produced an average of 0.9 kg more weight loss in the long term than a low-fat diet. That is statistically significant, but a small difference in practice. Ketogenic eating also improved certain blood lipids, but raised LDL cholesterol.
Based on a combination of large randomised studies (n=811), metabolic ward studies (n=17-20), one meta-analysis of 13 RCTs (n=1415), and a review article of meta-analyses. The overall body of evidence is broad but variable in design and duration.
Is a low-carbohydrate or ketogenic diet good for longevity?
A ketogenic diet can help with weight problems and metabolic disorders, but for people who simply want to live longer and healthier, no proven advantages over a varied diet have been demonstrated. Those who exercise intensively may even be at a disadvantage.
Does the time of day you eat affect how quickly you age?
The time at which you eat does appear to matter, although the evidence in humans is still associative. A nightly fasting window of 10 to 12 hours and concentrating your meals during the day are the most concrete guidelines that current research offers.
Do you actually live longer if you structurally eat fewer calories?
In laboratory animals, eating less convincingly extends lifespan, but this has not yet been demonstrated for healthy people without excess weight. If you want to benefit from what has been shown, eating moderately less or following an interrupted eating pattern are the safest, most reasonably supported options.
Does the order in which you eat food and vegetables during a meal matter?
Eating vegetables and/or protein first and carbohydrates last demonstrably blunts the post-meal blood sugar spike, even in healthy people. It is a small adjustment with no downsides that you can simply try out.
Do you live longer on a vegetarian or plant-based diet?
A high-quality plant-based diet is associated with less premature death. However, eating strictly vegetarian or vegan has not been proven to be better than a varied diet that occasionally includes meat or fish.
Does extra protein help with losing weight without losing muscle?
Extra protein helps preserve muscle mass while losing weight, but works best in combination with resistance training. Aim for 1.6-2.4 g per kilogram of body weight per day, and make sure to train alongside it.