Although high-protein diets can be an effective way to lose weight, not all protein-rich products are ideal for shedding pounds. In addition to considering the protein content of a food, you should assess the total calorie content, as well as the amount of carbs and fat provided. A combination of these nutritional characteristics makes soy protein a less-than-optimal choice for weight loss. Consult a doctor before using protein powder or any other supplements.

Calories

Soy protein is not an optimal choice for weight loss, as it is relatively high in calories. Each scoop of soy protein provides 170 calories, which is more than other protein-rich foods, such as tuna, which provides 60 calories per 2 oz. serving. If you ate 2 oz. of tuna each day rather than drinking one soy protein shake, you'd consume 770 fewer calories in one week, enough to lose nearly 1/4 of a pound.

Carbohydrates

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Soy protein is high in carbs, as each serving contains 19 g. This amount is much more than many other types of protein, such as whey protein, which contains 2 g per serving. Although carbs can supply energy for your athletic pursuits, reducing your carbohydrate intake can aid in weight loss. Research published in the February 2003 issue of the "Journal of Nutrition" indicates that reducing carb intake can promote improved weight and fat loss.

Sugar

Most of the carbs in soy protein come from sugar. Each scoop of this food supplement contains 17 g of sugar. This can be detrimental for weight loss, as beverages rich in sugar can promote obesity, according to the Harvard School of Public Health. The school suggests that this might be due to the fact that such drinks are high in calories, yet are not filling because they are liquid, rather than solid.

Fiber

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Although rich in carbs, soy protein does not contain any fiber, which is an important type of carb that helps you manage your cholesterol health, ensures digestive health and can provide feelings of fullness. Eating fiber-rich foods might help you tame your appetite and eat fewer calories when dieting.

Fat Oxidation

Different protein sources exert different physiological effects, such as energy expenditure and fat oxidation. According to research published in the March 2011 issue of "The American Journal of Clinical Nutrition", soy promotes less energy expenditure and a lower rate of fat oxidation than dairy-based proteins, so it might not be an optimal weight loss choice.