Why doesn't a smoothie fill me up like food?

The apple test: 58 adults, lunch plus what they had first

1,024 Nothing first

1,015 Apple juice

989 Juice with fiber added

928 Applesauce

837 A whole apple

A drink leaves your stomach fast, nothing gets chewed, and most fruit smoothies have almost no protein, so you’re hungry again soon. A glass that lasts until lunch has a meal’s calories, protein at your line, enough thickness to need a spoon or a slow straw, and you sip it as the meal.

It’s one of the most common complaints about smoothies. Someone drinks a 400 calorie glass of fruit at 7:30, which on paper is a proper breakfast, and by ten they’re raiding the office snacks. It feels like a personal failing, or like the smoothie was a lie. It’s neither, because a drink works differently in your body from the same food on a plate, and once you know how, you can build around it.

The apple test

In 2009 Barbara Rolls’s lab ran one of the neatest experiments on this. They gave 58 adults a snack of apple 15 minutes before lunch, once a week for five weeks. Each time it was 266 g and about 125 calories, eaten over 10 minutes. One week it was a whole apple, then applesauce, then apple juice, then apple juice with the apple’s own fiber stirred back in, and one week nothing at all. Then lunch, as much as they liked.

With nothing first, people ate 1,024 calories at lunch. After the whole apple they ate 709, so lunch plus the apple came to 837, about 187 fewer calories in all. After the applesauce, the total was 928. After the juice, 1,015, almost as if they’d had nothing. They felt fullest after the whole apple, then the applesauce, then the two juices.

The surprise was the fiber. Putting it back into the juice didn’t help at all. It was the same apple and the same calories, and the less chewing it took, the less it filled people up.

A meal with the chewing done for you

A smoothie sits between the applesauce and the juice. The blender has done the chewing, and the fiber is all still there, cut small. That’s why it’s fast on a busy morning, and it’s also why it holds you less than the same food on a plate.

Three things happen with a drink:

1. It goes down fast. You can finish 20 oz in a few minutes, far faster than you’d eat the same fruit.
2. It leaves the stomach fast. Liquid empties quicker than food you chewed, unless it’s thick or carries a lot of calories.
3. It feels less like food. In a 2018 study, the more food-like a drink seemed and the more slowly people had it, the fuller they felt.

A fruit smoothie with no protein has all three working against it. A 20 oz Caribbean Way has 390 calories and 2 g of protein. A glass with 30 g of protein, chia and frozen fruit thick enough to need a slow straw is a different drink.

Thick shakes and an MRI scanner

In 2016 researchers at Wageningen University in the Netherlands put 15 healthy young men in an MRI scanner after four different dairy shakes, all 500 ml. Two were thin and two were thick, and each came at 100 or 500 calories. Every 10 minutes the scanner measured what was left in the stomach, and the men rated how full they felt.

Calories did the most to keep the stomach full. But thickness did the most for how full people felt: at 40 minutes the thick 100 calorie shake was rated fuller (58 on a 100 point scale) than the thin 500 calorie one (48). The authors called it “phantom fullness”, because the stomach of the thick low calorie shake was emptier than it felt.

A good glass uses both. A meal’s calories keep your stomach working, and thickness makes it feel like breakfast. Only the calories decide how full a shake keeps you goes through this study in more detail.

The soda and the jelly beans

In a study published in 2000, 15 adults (7 men and 8 women) got an extra 450 calories a day for four weeks, once as soda and once as jelly beans. With the jelly beans, they ate less of everything else, and the extra calories were fully made up. With the soda, they didn’t eat any less, so the 450 calories were all extra and their weight went up.

It was soda, not a smoothie, and a small study. But it’s why we’re strict that a smoothie replaces a meal. Added on top of breakfast, its calories are likely to go uncounted by your appetite. How to use a smoothie to lose weight builds on it.

When the drink is the meal

A 2018 study by Peter Rogers and Roya Shahrokni at the University of Bristol looks, at first, like it says the opposite. They gave 47 adults about 136 calories of water, blackcurrant squash, milk, a fruit smoothie, or the smoothie’s fruit eaten whole, then a meal 2 minutes later (23 people) or 2 hours later (24 people). The whole fruit felt the most filling. But at the meal, every snack with calories in it, the drinks and the fruit alike, cut what people ate by the same amount compared with water. They made up about 82% of those calories when the meal came 2 minutes later, and only about 12% when it came 2 hours later.

A drink’s calories do count, then, as long as the meal is close behind. The fullness from any small snack fades within a couple of hours, which is why a smoothie works best as the meal itself, big enough to be one, and not as a snack before one.

The glass that lasts until lunch

• Protein at your line. 30 g for a 165 lb adult. A cup of milk and 7 oz of Greek yogurt have 28 g together.
• A meal’s calories. At 2,000 a day over three meals, about 500 to 830. A 250 calorie fruit glass is a snack.
• Thick. Frozen fruit instead of ice, Greek yogurt, a quarter cup of oats or half an ounce of chia, and less liquid.
• Sipped. Sit down with it, or eat it with a spoon as a smoothie bowl.

Build your glass puts all of this on its Hunger meter, protein, fiber and how thick the glass is, beside the other five jobs a meal does, and the quiz tells you whether yours is a meal or a dessert. meals.guide’s Hunger page has the same meter from the plate’s side, with the boiled potato that beat the croissant.

What we don’t know

These are small studies: 58 adults in the apple test, 47 in the Bristol study, 15 in each of the others. Each tested one sitting or a few weeks, and none tested a protein smoothie built the way we build ours. They all point the same way, though. meals.guide covers why some plates hold you for hours.