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T. Colin Campbell Center for Nutrition Studies
Is Your Plant-Based Diet the Reason You're Underweight

Weight loss is touted as one of the biggest benefits of switching to a plant-based diet and can be a positive side effect for people who need to lose weight for health reasons. But what if you’re already at a healthy weight and find the pounds dropping off even though you’re eating a balanced diet?

You Can, In Fact, Be Too Thin

Being underweight is defined as having a body mass index (BMI) of 18.5 or lower. This ratio of weight to height[1] has its limitations but is accurate enough to indicate when someone is straying into a danger zone. For example, a 5-foot 4-inch woman weighing less than 108 pounds would be considered too thin.

As of 2010, an estimated 1.7 percent of U.S. adults over age 20 were underweight.[2] The number dropped to 1.4 percent in 2014[3], an amount small enough to be easily overlooked in the face of the current obesity epidemic. However, being underweight can be just as bad for you as carrying around too many extra pounds.

Women are more likely to be underweight than men, putting them at risk for:

  • Amenorrhea[4]
  • Cessation of ovulation
  • Permanent infertility

Portion Perception

How can you accidentally undereat when living a healthy lifestyle? The difference between portion sizes on a standard American diet, or even a plant-heavy diet in which animal foods are still included, and a whole food, plant-based diet can be significant. The New York Times has an eye-opening article visualizing 2,000 calories of various types of food[5], and Eat This, Not That shows variations in snack-size portions of 100 calories.[6]

Oil is another significant contributor of calories in most modern diets, and it’s easy to forget about these when switching to an oil-free lifestyle. One tablespoon of oil typically has 120 to 130 calories. Compare this to:

  • One medium to large apple
  • One large banana
  • Two medium oranges
  • 3 ½ cups of kale

If you don’t need to lose weight, it’s important replace oil calories with these types of whole foods throughout the day.

Also keep in mind some parts of plant foods aren’t digested at all, such as the resistant starch in beans, and evidence suggests up to 70 percent of the calories from nuts have little or no impact on weight due to dietary compensation, indigestibility and increased fat burning.[7] In addition, eating and digesting whole foods instead of processed foods takes more energy, burning a small number of additional calories between meals.[8]

This can amount to a significant reduction in calories upon adopting a plant-based lifestyle. Simply cutting out oil can remove hundreds of calories from your daily intake, and the lower caloric density of plant foods means you’re likely to feel full much sooner than you’re used to. For some people, this can lead to a potentially dangerous drop in weight.

Preventing Excessive Weight Loss

What can you do to avoid losing too much weight on a plant-based diet? The best place to start is to work out an approximate caloric intake to aim for each day based on your activity level and whether or not you have weight to lose.[9]

Another helpful tactic is to invest in a food scale to help you visualize portions and calories. Because whole plant foods makes you feel full sooner, you may not initially be able to follow the common advice to eat until you’re full and not eat again until you’re hungry. Weigh out your food for a few weeks to reorient your perceptions and train yourself to eat enough to nourish your body.

Once you know what your daily caloric intake looks like, you can plan meals and snacks to ensure your hit your goals every day. You may find it necessary to eat smaller amounts more frequently until your body adjusts to the increased bulk and higher fiber levels in plant-based foods.

Readjusting the way you look at portions and teaching your body to handle a larger volume of food can help you maintain your weight on a plant-based diet. If you continue to lose weight even though you’re eating enough calories to support your activity level, talk with your doctor about other potential causes. Unexplained weight loss can indicate underlying conditions, including Celiac disease and thyroid complications.[10] In most cases, however, eating a varied diet of whole plant foods will provide adequate calories and nutrition to support a healthy weight.



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