The USDA settled this decades ago — peanuts sit in the "Legumes and Legume Products" category, right alongside beans, lentils, and soybeans. Most people know that much. Almost none of them have pulled up the actual nutrition data to see whether the legume label carries through to the numbers.
Are Peanuts Nuts or Legumes — and Does the Answer Matter?
Peanuts are legumes — classified by the USDA under "Legumes and Legume Products," not "Nut and Seed Products." Their nutritional profile reflects that biology: more protein than every tree nut (23.2g vs almonds' 21g), nearly four times the niacin of the highest tree nut, less total fat than all nine, and isoflavones characteristic of legumes.
— USDA FoodData Central · 2022–2023 · Foundation data, n=8 samples per nutrient
It does. For every hundred grams, raw peanuts deliver 23.2 grams of protein. The highest-protein tree nuts — almonds and pistachios — top out at 21. Most of the rest fall between 14 and 15. Macadamias, at the bottom, manage 8. A legume just topped every nut on the shelf on the metric the fitness world cares about most.
The protein gap is not isolated. Peanut butter contains 17.2 milligrams of niacin per 100 grams — nearly four times the amount in pine nuts, the highest-niacin tree nut at 4.39 milligrams. Most tree nuts fall below 2. That kind of margin does not show up between foods that share a nutritional identity.
And on fat — the metric most people assume would be identical — peanuts carry 43.3 grams of total fat per 100 grams. All nine tree nuts carry more. Pistachios, the leanest, sit at 45. Macadamias reach 76. The legume that ended up shelved next to almonds is the leanest option in the aisle.
PEANUTS (LEGUME)
23.2g protein · 17.2mg niacin · 43.3g fat
BEST TREE NUT
21g protein · 4.39mg niacin · 45g fat
One compound makes the distinction biochemical. Peanut butter contains daidzein — an isoflavone — at 11.5 milligrams per 100 grams. Isoflavones are characteristic of legumes. The USDA's tree nut comparison table does not even include an isoflavone column — the compound simply does not belong in that family of foods.
The pattern has limits. On fiber, peanuts land at 8 grams per 100 grams — mid-range. Almonds deliver 12. Pistachios and hazelnuts each reach 10. If fiber matters most for a particular meal, tree nuts win that round — or the peanut butter moves into a bowl of oats.
The next time peanut butter and almond butter share a shelf, one of them carries more protein per gram, nearly four times the niacin, and less total fat. The classification everyone already knew is the explanation nobody checked. Whether that protein advantage changes the math on body composition depends on a number the nutrition label rarely makes obvious.