How Low Can Carbohydrate or Fat Go: A Reference

The question of how low carbohydrate or fat can go is usually answered with a diet name. A more useful answer is arithmetic: what does the body require, what do labels permit, and what did the controlled trials actually find when calories and protein were matched? The short version is that the difference between low carb and low fat is small when those two variables are held constant, and adherence dominates. The floors that matter are set by essential fatty acids, fat-soluble vitamin absorption, and glycogen for training.

What the controlled trials found

A systematic review and meta-analysis by Johnston et al. (2014) in the Journal of the American Medical Association compared low-carbohydrate and low-fat diets in overweight and obese adults. At 6 months, the low-carbohydrate groups had a small advantage in weight loss (about 0.7 kg) and in some metabolic markers. By 12 months, the difference in weight loss was not statistically significant. The authors noted that adherence declined in both groups. The trials that matched calories and protein found differences of a few hundred grams to a kilogram, not the large differences often claimed.

A more recent network meta-analysis by Ge et al. (2020) in BMJ found that most macronutrient-focused diets produced similar weight loss at 6 and 12 months when adherence was accounted for. The differences between low carb and low fat were small relative to the variance between individuals.

Where the figures come from

Nutrition numbers on labels and in tables are derived from a few sources. The Atwater factors assign 4 kcal per gram of protein or carbohydrate, 9 kcal per gram of fat, and 7 kcal per gram of alcohol. These are averages: the actual metabolizable energy of a specific food varies by a few percent. The FDA allows a tolerance of up to 20% for most nutrients on labels, and rounding rules permit a label to print “0 g” for fat if the amount is less than 0.5 g per serving. That does not mean the food contains no fat.

Minimum fat intake

The essential fatty acids are linoleic acid (an omega-6) and alpha-linolenic acid (an omega-3). The Institute of Medicine (now the National Academies) set adequate intakes (AI) in 2005: 17 g per day of linoleic acid for men and 12 g for women; 1.6 g per day of alpha-linolenic acid for men and 1.1 g for women. These are not total fat minimums, but they set a floor for the essential fatty acids. Fat also carries fat-soluble vitamins (A, D, E, K). The absorption of these vitamins is reduced when fat intake is very low. A practical minimum for total fat is often given as 20% of calories, but that is a convention, not a requirement. The Food and Nutrition Board set an acceptable macronutrient distribution range (AMDR) for fat at 20-35% of calories for adults. Below 20%, it becomes difficult to meet essential fatty acid and fat-soluble vitamin needs without careful planning.

Minimum carbohydrate intake

There is no established minimum carbohydrate intake for the general population. The brain and red blood cells use glucose, but the body can produce glucose from protein and glycerol via gluconeogenesis. The Institute of Medicine set a recommended dietary allowance (RDA) for carbohydrate at 130 g per day for adults and children, based on the average minimum amount of glucose used by the brain. This is not a requirement for everyone; it is the amount that covers brain glucose use without relying on gluconeogenesis. For athletes, glycogen depletion affects performance. A common recommendation for training is 3-5 g of carbohydrate per kg of body weight per day for moderate exercise, and 6-10 g/kg for endurance athletes. These are from the American College of Sports Medicine position stand (2016).

A worked example: calculating your own floors

Suppose a person weighs 70 kg and wants to know the minimum fat and carbohydrate intakes for a 2,000 kcal diet. Use the AI for essential fatty acids and the RDA for carbohydrate.

Step 1: Essential fatty acids
Linoleic acid AI (adult male): 17 g/day
Alpha-linolenic acid AI (adult male): 1.6 g/day
Total essential fatty acids: 18.6 g/day

Step 2: Convert to calories
18.6 g * 9 kcal/g = 167.4 kcal from essential fatty acids

Step 3: Minimum fat for vitamin absorption
No specific gram minimum, but 20% of 2,000 kcal = 400 kcal from fat
400 kcal / 9 kcal/g = 44.4 g total fat

Step 4: Carbohydrate RDA
130 g/day * 4 kcal/g = 520 kcal from carbohydrate

Step 5: Remaining calories
2,000 - 167.4 - 520 = 1,312.6 kcal
This can come from any macronutrient mix.

This shows that a 2,000 kcal diet can meet essential fatty acid and carbohydrate needs with about 44 g of total fat and 130 g of carbohydrate, leaving room for protein and additional fat or carbohydrate. The exact numbers depend on body weight and energy expenditure.

What people get wrong

A common mistake is to treat a label’s “0 g fat” as literally zero. The FDA allows rounding to zero for amounts below 0.5 g per serving. If a person eats multiple servings, the fat adds up. Similarly, a food labeled “low fat” can still be high in calories from added sugars. The natural error is to trust the label as a precise measurement rather than a rounded approximation. Another mistake is to assume that a very low-fat diet is automatically healthier. The essential fatty acids and fat-soluble vitamins require some fat. A very low-carbohydrate diet can be safe for some people, but it may not be superior for weight loss when calories and protein are matched. The trials show small differences and high dropout rates.

What to do differently

Weigh your food rather than distrusting it. A digital scale gives you the actual grams, which you can then multiply by the Atwater factors to get calories. This bypasses label rounding and tolerance. For example, if a label says a serving has 0 g fat but you weigh 10 g of the product and the ingredient list shows oil, you can estimate the fat from the oil content. The arithmetic is straightforward: 1 g of fat = 9 kcal. If you are tracking, use the USDA FoodData Central database, which provides values per 100 g. That is more reliable than a label’s rounded numbers. The goal is not to hit a perfect number but to understand the range of uncertainty. A 10% error in calorie intake is common; a 20% error is possible with label rounding.

Table: Conditions that change the figure

The floors that matter

The essential fatty acid AIs set a floor for fat. The carbohydrate RDA sets a reference for brain glucose, but not a strict minimum for everyone. For athletes, glycogen needs set a practical carbohydrate floor. The controlled trials that matched calories and protein found small differences between low carb and low fat. Adherence dominates. No single approach is superior for everyone.

This article is for information only and is not medical advice. If you have an eating disorder, contact the National Eating Disorders Association Helpline at 1-800-931-2237.