A low-fat diet can work for weight loss, but it doesn’t work better than other approaches. In the largest head-to-head trial comparing low-fat and low-carb diets, participants on the low-fat plan lost an average of 5.3 kg (about 11.7 pounds) over 12 months, which was statistically indistinguishable from the low-carb group. The pattern that emerges across decades of research is consistent: what matters most is eating fewer calories than you burn, not which macronutrient you cut.
What Counts as Low Fat
The average American gets about 36% of daily calories from fat. Federal dietary guidelines recommend keeping total fat between 20% and 35% of calories, so a “low-fat diet” typically means staying at or below that lower end. Very-low-fat diets, like those promoted by Dean Ornish, push fat below 10% of calories. Most clinical trials testing low-fat eating fall somewhere in the 20% to 25% range.
The American Heart Association adds a more specific target for saturated fat: less than 6% of total calories, which works out to roughly 13 grams per day on a 2,000-calorie diet. That’s a meaningful distinction, because swapping saturated fat for unsaturated fat produces different health effects than simply eating less fat overall.
Weight Loss: Similar Results Across Diets
The DIETFITS trial, published in JAMA, randomly assigned over 600 overweight adults to either a healthy low-fat or healthy low-carb diet for a full year. The low-fat group lost 5.3 kg on average. The low-carb group lost 6.0 kg. The difference was not statistically significant, and neither genetic markers nor insulin levels predicted who would do better on which plan.
A two-year trial published in the New England Journal of Medicine compared low-fat, low-carb, and Mediterranean diets and found the same general pattern. Adherence rates were 95% at one year and 85% at two years, which is remarkably high for a diet study. Most dietary trials lose 15% to 50% of participants within the first year alone. The takeaway: people can stick with a low-fat diet when given proper support, and they lose a clinically meaningful amount of weight doing so.
The real driver behind weight loss on any of these plans is the calorie deficit, not the ratio of fat to carbohydrates. Fat is the most calorie-dense macronutrient at 9 calories per gram (compared to 4 for protein and carbs), so cutting fat is one straightforward way to reduce total calories. But it’s not the only way, and it’s not inherently superior.
Effects on Heart Health
Where low-fat diets show a clearer advantage is cholesterol. A meta-analysis of dietary intervention studies found that following a structured low-fat eating plan reduced LDL cholesterol (the harmful kind) by 12%, total cholesterol by 10%, and triglycerides by 8%. These are meaningful improvements, especially for people with elevated cholesterol levels.
The benefits become more pronounced when you pay attention to the type of fat you’re eating, not just the amount. Replacing saturated fats with a mix of unsaturated fats (from sources like olive oil, nuts, and fish) can lower total cholesterol by 10% to 15%, with most of that reduction coming from LDL. Simply cutting all fat indiscriminately produces a smaller benefit and can backfire if you replace fat with refined carbohydrates and sugar.
Blood Sugar and Insulin
Low-fat diets do reduce fasting insulin levels, a marker associated with type 2 diabetes risk. In a randomized trial comparing low-fat and low-carb weight-loss diets, both groups saw significant drops in fasting insulin. However, the two diets performed identically on every metabolic measure tested: fasting blood glucose, hemoglobin A1C, and insulin sensitivity all improved by the same amount regardless of whether participants cut fat or carbs.
This finding reinforces a consistent theme in nutrition research. Losing weight improves metabolic health. The specific dietary composition matters far less than the weight loss itself when it comes to reducing diabetes risk.
The Hunger Problem
One genuine challenge with low-fat diets is satiety. Fat triggers fullness signals in your gut. When fat reaches the lower part of your small intestine, it activates a mechanism called the ileal brake, which slows digestion and releases hormones that suppress appetite. Unsaturated fats (found in olive oil, avocados, and nuts) are particularly effective at boosting the release of cholecystokinin, a hormone that signals fullness. Saturated fats don’t trigger the same response.
This means a very-low-fat diet can leave you hungrier than a moderate-fat diet, which may explain why some people find it harder to sustain. If you’re eating a low-fat diet and struggling with persistent hunger, including small amounts of unsaturated fat at meals can help without undermining the overall approach.
Vitamin Absorption Gets Affected
Vitamins A, D, E, and K are fat-soluble, meaning your body absorbs them alongside dietary fat in the small intestine. On a moderately low-fat diet (around 20% of calories), this is rarely a problem because you’re still eating enough fat for normal absorption. But very-low-fat diets, those pushing below 10% to 15% of calories, can impair the absorption of these nutrients over time.
This is worth keeping in mind if you take a vitamin D supplement or rely on dietary sources of vitamin A. Taking these with a meal that includes at least a small amount of fat improves how much your body actually absorbs.
Your Metabolism Adjusts Either Way
A common concern is that low-fat diets slow your metabolism more than other approaches. A study tracking resting energy expenditure across different diet compositions found that all groups, both low-fat and high-fat, experienced similar declines in calories burned at rest: roughly 65 to 78 fewer calories per day after six months. The differences between diet groups were not statistically significant.
Interestingly, the low-fat groups did show signs of slightly greater “adaptive thermogenesis,” a metabolic slowdown beyond what weight loss alone would predict. But by 24 months, this effect had disappeared entirely across all diet groups. Your metabolism adjusts to weight loss regardless of how you achieve it, and it recovers over time.
Why Low-Fat Foods Can Backfire
The biggest practical pitfall of a low-fat diet isn’t the macronutrient ratio itself. It’s what people eat instead. When food manufacturers remove fat from products, they typically add sugar, refined starch, or other fillers to maintain flavor and texture. A low-fat yogurt or salad dressing can contain significantly more sugar and nearly the same number of calories as the full-fat version.
The low-fat diet craze of the 1980s and 1990s coincided with a surge in processed “low-fat” products and a simultaneous rise in obesity rates. This wasn’t because reducing fat is inherently harmful. It was because people replaced whole food sources of fat with processed carbohydrates and ate more total calories in the process. A low-fat diet built around vegetables, whole grains, legumes, fruits, and lean proteins works. A low-fat diet built around fat-free cookies and sugary cereals does not.
Who Benefits Most
A low-fat approach tends to work well for people who naturally prefer carbohydrate-rich foods like grains, beans, fruits, and starchy vegetables. It also has the strongest evidence base for people with high LDL cholesterol, since reducing saturated fat intake produces a direct and measurable drop in LDL levels. For pure weight loss, it performs as well as any other calorie-controlled plan.
Where it may not be the best fit is for people who find high-carb meals unsatisfying, who experience energy crashes from carbohydrate-heavy eating, or who have conditions like high triglycerides that sometimes respond better to carbohydrate reduction. The best diet is the one that creates a sustainable calorie deficit while providing adequate nutrition, and for many people, that’s a moderate approach rather than an extreme cut in any single macronutrient.