You may not be able to add table sugar to your coffee on a low-carb diet, but sugar substitutes are allowed as long as you count the carbs. Not all popular low-carb diets recommend aspartame, however, though that may have more to do with the safety concerns with the sugar substitute than how it affects your ability to lose weight. If you want to use a sugar substitute on your low-carb plan, consult a registered dietitian to help you decide which one might work best.
Low-Carb Diet and Artificial Sweeteners
While there's no set definition for a low-carb diet, limiting carbs to 50 to 150 grams a day is considered low-carb by most professionals, according to a 2007 article published in the the American Journal of Clinical Nutrition. Most plans restrict you to 20 to 50 grams to start, however, and add carbs after you've lost the weight.
Every carb counts when you're restricted, especially during those early stages. Sugar has 4 grams of carbs per teaspoon and offers no nutritional value. Most low-carb plans recommend the bulk of your carbs come from low-carb, nutrient-rich veggies such as endive, asparagus, spinach and broccoli, so you get the vitamins and minerals your body needs without all the carbs. Artificial sweeteners, such as aspartame, can be part of your plan, but should have 1 gram of net carbs or less per serving. The term "net carbs" refers to the carbs that affect blood sugar. You estimate net carbs by subtracting grams of fiber or sugar alcohol from grams of total carbs. Most low-carb diet plans count net carbs, not total carbs.
You may know aspartame better by its brand names Equal or Nutrasweet. Made from the amino acids phenylalanine and aspartic acid, aspartame is 200 times sweeter than sugar and has only 1 gram of net carbs per packet. In addition to using the packets to sweeten your coffee and tea, you may also find the artificial sweetener in diet drinks, sugar-free gum and low-cal sweet treats.
While aspartame is considered safe, there are concerns that it may cause cancer. The research available doesn't seem to indicate a direct link between aspartame and cancer, however, according to the American Cancer Society. Some people also complain that it causes various symptoms, including headaches, digestive issues and mood changes, but there's no definitive evidence of this either, according to the cancer society. Avoid aspartame if it seems to cause ill effects.
Anyone with phenylketonuria, or PKU -- a rare genetic disorder that prevents the body from breaking down phenylalanine, an amino acid -- should avoid aspartame and any foods that contain the sweetener. When phenylalanine builds up in the blood, it blocks essential chemicals from getting to the brain, which affects brain health and development, especially in children.
Aspartame, Appetite and Weight Gain
While aspartame is safe and low in carbs, it may increase your appetite, according to a 2003 study published in Physiology and Behavior, which may be another reason why you might want to avoid it on your low-carb diet. This study found that the phenylanine in aspartame boosts hormones that affect appetite.
Using foods sweetened with aspartame may also lead to weight gain. An animal study from 2013 published in Appetite found that rats given yogurt sweetened with aspartame gained more weight than rats given yogurt sweetened with sugar. But while these studies seem to indicate that aspartame may not make the best choice if you're trying to lose weight, studies on humans are necessary to determine if there's an association between the use of aspartame and an increase in appetite and weight gain.
Alternate Artificial Sweeteners
If you want to avoid aspartame, consider using another low-carb artificial sweetener such as sucralose or saccharine. Like aspartame, each of these also has only 1 gram of net carbs per packet. If you prefer a more natural sugar substitute, you might consider stevia, which also has 1 gram of net carbs per packet. Made from a plant grown in South America, stevia is 200 to 300 times sweeter than sugar. Like aspartame, these sweeteners can be added to foods and beverages, or you can find them in some of the low-carb diet foods you include in your diet.