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3 Reasons Why Undereating is a Bad Idea (And Won't Actually Help You Lose Weight)

3 Reasons Why Undereating is a Bad Idea (And Won't Actually Help You Lose Weight)

Dorothy Krueger - Certified Personal Trainer

As a Personal Trainer, I see a lot of people skipping meals thinking this can help them lose weight.  We all know that calories matter a lot when it comes to weight loss. As long as you eat fewer calories than you burn, you should lose weight, right?  The logic is sound for most healthy adults, but we can also take that logic too far. In the case of healthy, sustainable weight loss, more restriction doesn’t always lead to better weight loss. In fact, regularly eating too few calories can put you at risk of malnutrition, resulting in unhealthy weight loss and nutrient deficiencies.

What really happens when you chronically undereat?

Going without food for several days or eating less than the calorie minimum for weeks to months puts you at risk of malnutrition. As well-nourished individuals, we do carry enough stored fuel to meet our needs for one to three months in the form of muscle tissue and fat. However, our body can only store 1–2 days’ worth of glycogen (the body’s carbohydrate stores), which, if not replenished, is quickly used up to maintain blood sugar.

After several days of undereating, the body switches to energy-conservation mode, meaning your metabolism slows way down, making you feel tired and edgy and causing you to burn fewer calories! As carbohydrate stores run low, protein and fat become the dominant sources of fuel. After 48 hours without food, your body runs out of glycogen to power the two organs that need it the most: red blood cells and the brain. While glucose is the only fuel blood cells can run on, the brain will begin to adapt to power itself with ketones the body made from fat. To meet basic energy needs, your body ramps up breakdown of muscles and organs in addition to fat.

3 Reasons Why Undereating Is a Bad Idea (and Won’t Actually Help You Lose Weight)
For most of us, consistently eating less than the calorie minimum is a bad idea because it leads to a:

  1. Slower metabolism and lousy side effects. With too few calories on board to power you through your daily activities, your body learns to live on less by significantly slowing your metabolism. Short term, you may feel sluggish, irritable and apathetic. Once you stop undereating, it takes a while before your body to recover and your metabolism to rev back up.
  2. Loss of valuable muscles and organs. Just because you’ve adapted to using ketone bodies doesn’t mean your body won’t need glucose at all. A minimum blood glucose level must be maintained to keep you alive so your body continues to break down muscles and organs. It’s a major problem in the long run, because your body doesn’t distinguish between essential tissues  (heart, kidney, blood cells) and less essential tissues (skeletal muscle). Over time, this breakdown weakens and damages your vital organs.
  3. Higher risk for nutrient deficiencies. Eating very few calories will also decrease the variety of foods you can eat, increasing your risk for nutrient deficiency. The type of nutrient deficiency that can occur depends on the foods that are being restricted. While not everyone who chronically undereats can automatically be diagnosed with an eating disorder, anorexia gives insight on the types of nutrient deficiencies that are likely to occur from prolonged starvation. This includes but isn’t limited to deficiencies in calcium, iron, zinc, vitamin D, and B Vitamins in addition to dangerous electrolyte imbalances and protein malnutrition.

Every body is different, so it’s important to experiment and see what works best for you. Just know that undereating is not a good long-term solution for living a healthy life or achieving or maintaining weight loss.  Exercise and proper nutrition works every time!