Why a Low-Calorie or Low-Carb Diet May Cause Blurry Vision

A sudden change in vision can be a scary thing. Though there are many potential causes, one factor to consider is your diet — yes, under certain circumstances dieting and blurred vision can be linked.

Carrots aside, it may seem strange that what you eat could affect how well you see, but there's a scientific reason behind the diet-vision connection. Vision changes from eating too little have to do with the level of glucose, or sugar, in your blood.

"Glucose is the main source of energy to the brain. When blood sugar drops too low, the brain isn't getting steady energy and therefore cannot function as normal and may lead to an impairment in vision," says Erin Palinski-Wade, RD, CDCDS, a registered dietitian and certified diabetes care and education specialist in Sparta, New Jersey.

Fortunately, blurry vision from low blood sugar isn't likely to cause permanent damage. "This change is temporary and resolves as soon as blood sugar is back in the normal range," Palinski-Wade says.

Another potential cause of blurry vision on a low-calorie diet is dehydration. If you're not eating a lot, you may not be getting enough fluid.

"Dehydration can trigger eye strain, which in turn can lead to tired eyes and blurry vision," Palinski-Wade says. "Since the brain is made up mostly of water, dehydration can also impair mental function, which in turn can impair vision in a similar way low blood sugar can. This is temporary and restored once hydration is back to a normal level."

If you're dieting, keep tabs on your fluid intake as you scale back on calories. Drinking 8 to 10 glasses of water a day will also help avoid dry eye, another possible consequence of dehydration, according to the American Optometric Association.

A popular eating plan for weight loss, this extremely low-carb, high-fat diet plan alters the balance of macronutrients to force the body into a metabolic state called ketosis, in which it uses fat for fuel, Palinski-Wade says.

Dialing back on carbohydrates can have a major impact on the amount of sugar in your blood. "A keto diet provides very few carbohydrates, the macronutrient easiest to convert into glucose in the body," Palinski-Wade notes. "When you eat few carbs, your blood sugar can lower as well."

Still, Palinski-Wade says blurry vision on the keto diet is unlikely for most people. However, you'll need to be more careful if you have diabetes. Research published in May 2018 in the journal ‌Diabetic Medicine‌ showed that although the keto diet often improves blood sugar control in people with type 1 diabetes, it can result in a high number of hypoglycemic (low blood sugar) episodes.

According to Mayo Clinic, blurred vision can be a sign of worsening hypoglycemia, which is a fasting blood sugar of 70 milligrams per deciliter or below.

Like the keto diet, intermittent fasting is another trendy means of shedding pounds. Most intermittent fasting plans involve abstaining from food for many hours or restricting food to just one meal a day. But like any diet that restricts calories, intermittent fasting can take your blood sugar on a rollercoaster of ups and downs, Palinski-Wade says.

"When you go long periods of time without eating, such as during a fast, the body can drain energy stores and see a drop in blood sugar levels," she says. "For most individuals, blood sugar will not drop dangerously low, yet a blood sugar on the low end of normal can make you feel fatigued or shaky or cause mental fogginess." Again, if blood sugar dips low enough, you may experience blurry vision.

If you've noticed vision changes after starting a new eating plan, it may not mean you have to drop the diet. Still, it's best to get the problem checked out. "Any time you experience blurry vision, you should speak to your physician to determine the cause," Palinski-Wade says. "If the underlying cause is dehydration or low blood sugar, diet should be adjusted to prevent further drops in blood sugar and/or hydration status."