Does Eating Fast Make You Fat? (Explained)

Today, it is a common knowledge that eating slowly can help you lose weight. However, what happen with fast eaters? Does eating fast make you fat?

In general, eating fast does make you fat because rapid eating doesn’t allow enough time for the hypothalamus to release satiety hormones. It usually takes 15-20 minutes to facilitate satiety and eating faster than that can result in overeating or getting hungry soon.

In this article I will explain everything there is to know about eating fast, chewing food and compare current research.

What Happens When You Eat Too Fast?

In general, when you eat too fast your body doesn’t have time to release a sufficient amount of gastric acid and digestive enzymes, which leads to digestion problems. It also doesn’t have time to stimulate satiety hormones that signal your fullness.

This means you can end up either eating more food than normal and still feel hungry. Or you can continue eating until you’re more than full. Either way, this is not healthy and can lead to gaining weight and getting fat.

What is gastric acid? Gastric acid is the mix of stomach acids that is part of your digestive system. It contains hydrochloric acid, sodium chloride, and potassium chloride. Those acids break down food, split up the proteins, and start to extract nutrients.

Why gastric acid is important during eating? Having enough gastric acid juice during eating is important because it helps to facilitate nutrient breakdown, digestion, absorption, and further transport. It also helps to protect from microorganisms and bacterial overgrowth.

In other words, to fully digest, absorb and redistribute essential nutrients you need to have enough gastric juices. Slow eating, smelling, and chewing food for longer facilitate better gastric acid activity. On the other hand, fast eating and not chewing the food long enough reduces its activity.

How fast is too fast to eat? To answer the question of how fast is too fast to eat will depend on the volume of food you eat. Larger meals should take 20-30 minutes to fully facilitate digestion and satiety. Smaller meals or snacks should be eaten within 10-15 minutes.

What Do Studies Say About Eating Fast?

A massive study done by Dr. Rei Otsuka from Nagoya University Graduate School of Medicine documented 6651 participants (5179 males and 1472 females) and the effect of eating duration on obesity (Otsuka, et al. 2006).

The results

First, let’s look at the female group. In the graph below you can see a correlation of body weight in pounds with eating speed at 5 different classifications. Very slow, slow, medium, fast, and very fast.

As you can see, participants who reported eating very fast had the highest body weight, compared to participants who reported eating very slowly.

The same results you can see in the next graph. Here you can see the correlation of eating duration and BMI in females. Participants who reported eating slow also had the lowest BMI. And people who reported eating very fast had the highest.

What about males? Is there any difference? Not really. here you can see the results for males and the correlation between eating fast and gaining weight. In the graph below you can see the body weight of very slow eaters and compare that to very fast eaters.

Finally, the last graph shows the difference in BMI in males. As you can see, eating duration had similar effect on males as on females.

In summary, eating speed and duration has a massive impact on body weight and BMI. In this study, Dr. Rei Otsuka concluded that:

Our results among middle-aged men and women suggest that eating fast would lead to obesity

Does eating fast make you lose weight? In general, eating fast does not make you lose weight because it lowers your fullness and increases your appetite signals. Fast eating leads to eating more than usual, where slow eating leads to reduced food intake and weight loss.

What Happens When You Don’t Chew Your Food Enough?

In general, when you don’t chew your food enough it delays the onset of satiety by inhibiting oro-sensory signaling. A short duration of chewing reduces feedback to the brain and limits exposure of food pieces to sensory receptors in the mouth.

Why is chewing important for digestion? Generally, chewing is important for digestion because it provides multiple physiological signals and necessary feedback to the brain and the stomach. This helps to recognize all information about the food’s nutritional value and provide warning signs for spoiled or poisonous food.

Sensory receptors gather all the important information about the food you eat, its taste, and its texture. It also provides information about the macronutrient composition. This information is then used in the stomach to start releasing essential digestive enzymes.

Not chewing your food enough doesn’t provide enough time to fully facilitate the digestion process, which leads to lower satiety and bigger hunger.

What is oro-sensory signaling? Oro-sensory signaling is the process of mastication (chewing) where several sensors in the mouth initiate physiological triggers (signals) about the food that is eaten. The short duration of oro-sensory exposure leads to insufficient cephalic phase response.

Why oro-sensory signaling is important? Oro-sensory signaling is important because it triggers fullness. The longer the chewing process takes, the more “in-mouth” sensors are exposed to food and its texture. This long exposure provides sufficient feedback that leads to quicker satiety.

This basically means that when you chew your food for longer, you not only can digest food better, but you will feel fuller with less amount of food.

What is a cephalic phase response? The cephalic phase response is the process of initiating digestion by releasing gastric acid from tasting, smelling, or even thinking about the food. Around 20% of gastric acids start to be released before the food enters the stomach.

Why cephalic phase response is important in eating? The cephalic phase response is important because it promotes salivation and provides necessary feedback to the brain about the food. This helps to facilitate swallowing and digestion more efficiently.

As you can see, the chewing starts whole cascade of processed that plays a role in proper digestion and satiety. How is all of that looks in the practice?

What Do Studies Says About Chewing Fast?

Take a look at this fascinating study done by Dr. Bridget Cassady, a registered dietitian, nutritionist, and research scientist. She compared a group of people eating 5-g almond portions for s specified number of chews per portion (Cassady et al. 2009).

  • 10 chews
  • 25 chews
  • 40 chews

After participants ate almonds, the blood samples were collected at various periods of time. This way, the researchers could easily measure satiety hormone concentration in the blood after each eating session. This is crucial part of this research because fullness or hunger are very subjective in nature.

Which makes it very difficult to assess the difference just from the sensory perspective. Feeling hungry or feeling full can have different meaning to different people.

Each of the participants can actually have no idea how to compare their fullness or appetite after chewing almonds 10 times versus 40 times. However, with the blood samples, there is no need for subjectivity because everything is in the blood.

The results

First, let’s start with the calories. The results showed that when people chew almonds longer, the more energy (calories) and fat they were able to absorb. However, people who chewed their almonds 10 times, didn’t absorb as many calories and fat as the group who chewed almonds 40 times.

This means the longer you chew your meal, the more calories and nutrients you’re able to absorb.

The next in line is appetite and fullness. Here the results showed a significant difference in fullness and satiety from the group who chewed 40 times, compared to the group who chewed only 10 times. Participants who chew the almonds 40 times had the strongest reduction in hunger and increase of fullness.

As you can see from the graph above, the number of chews was directly related to the satiety effects and to the duration of fullness. This means that chewing almonds 40 times not only makes you fuller quicker but also this fullness lasts for longer.

Now watch this. The most fascinating is that satiety hormones went below the baseline after chewing almonds 10 times. This means that chewing your food for a short period of time can actually make you more hungry than before.

Does Not Chewing Food Make You Fat?

In general, not chewing food does make you fat because it inhibits the satiety and fullness response. Not chewing your food properly leads to overeating, without feeling full after. It also down-regulate digestion and limits the nutrient extraction.

This means that people who spend minimum time on chewing or even swallow big chunks of food may have hard to to absorb nutrients from this food.

Is It Bad To Swallow Big Chunks Of Food?

Generally, it is bad to swallow big chunks of food because it can interfere with gastric acid secretion and limits the accessibility of nutrients. Chewing food helps to reduce particle size before swallowing and ensures sufficient food digestion.

Can you lose weight by chewing your food more? As a general rule, you can lose weight by chewing your food more because it helps to elicit satiety and fullness quicker. Spending the time and chewing food for lower helps to reduce appetite and lower overall food intake without even realizing it.

Why is chewing food important? Chewing your food is important because it helps with digestion, lowers the appetite and kick starts the physiological processes responsible for satiety and fullness. It also helps to swallow and emulsify food easier by stimulating salivation.

It is a combination of sensory and motor activity of the mouth. The sensory activity allows you to register what type of food you eat and motor activity

What will happen if food is not properly chewed? When food is not properly chewed it will be inefficiently digested. Chewing facilitates digestion, moistens the food pieces, makes a bolus, and assists with swallowing. It also stimulates digestive enzymes to help break down food and extract micro and macronutrients.

Does Eating Slower Make You Fuller?

Generally, eating slower does make you fuller because the longer you chew, the more it provides sensory output with all information about the food. It helps to moisture, lubricate, break down, and in the end, it initiates fullness.

Multiple studies confirm that eating slowly does lead to lower appetite, higher satiety, and reducing overall food intake (Andrade, et al. 2008).


Eating fast does contribute to positive energy balance by a variety of mechanisms, mainly my delay of satiety and fullness. With fast eating, we tend to eat more and feel hungry much faster.

Spending more time chewing the food can not only help with digestion but also reduce food intake without even trying.

Michal Sieroslawski

Michal is an exercise physiologist (MSc), nutrition coach, Ashtanga teacher, and fitness blogger. He shares his successes and failures to help busy men and women squash down 20, 50, or even 100 pounds of fat without leaving their home.

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