Protein Shake

Is Whey Protein Better for Weight Loss Than Soy?

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It seems like everywhere you turn, someone is either talking about a diet, on a diet, or just finished a diet. And the one thing all of those diets have in common is they attempt to limit the number of calories you are eating.

But cutting calories is only successful if the person sticks to the diet. Enter the idea of satiety.

Satiety means you are full, not hungry, not interested in eating. And studies have shown that one reason high-protein diets are effective for weight loss is that they promote satiety, or a feeling of fullness.[1]

What most don’t tell you, though, is what type of protein is best. And this could be an important missing factor.

For example, one study with rats found that those fed a high-protein diet consisting of whey protein had a 4 percent reduction in weight gain and fat loss as compared to rats fed red meat as their protein source.[2]

But what would this type of protein comparison look like with humans? Would it make a difference if people used whey or soy as their protein source? According to a new study from the Journal of Nutrition, the answer appears to be yes.[3]

The Battle of the Proteins

Researchers wondered if supplementing a person’s diet with whey protein, soy protein or maltodextrin (a type of carbohydrate) would have any effect on their body weight and composition.

They randomly divided 90 participants into three groups: a whey group, a soy group and a carbohydrate group. All three groups received two packets a day that they mixed with water and drank. They had a choice of three flavors. (Study doesn’t indicate what the flavors were.)

They were instructed to consume one packet before, during or after breakfast, and the second packet before, during or after dinner. They did this every day for 23 weeks.

Compliance was determined in two ways. First, researchers counted the number of packets they had distributed and then recounted those that had not been used. Second, each packet contained PABA, which leaves the body within 18 hours. So at random, unannounced times, researchers would take urine samples to detect the presence of PABA. If none was there, they knew the person had not consumed the protein or carbohydrate. When compliance was completed, only 73 participants were included in the study.

All participants in the study were either overweight or obese, with a BMI greater than 28, but less than 38. They were also in fairly good health otherwise. Their blood pressure was less than 160/100 mm Hg, and their total cholesterol was less than 6.2 mmol/L.

Other than the packets twice a day, the participants could eat however they chose. They could also exercise. Researchers assessed dietary intake every 10 days by asking them to recall what they ate in the 24 hours prior.

Satiety and hunger were also assessed, as was physical activity, weight, and waist and hip circumference. Additionally, researchers took biological samples at five different times (at the start of the study, after week 12, week 16, week 20 and week 23). They tested:

  • Glucose
  • Insulin
  • Ghrelin (your hunger signal)
  • IGF-1 (insulin-like growth factor)
  • Thyroid hormones T3 and T4

All blood samples were taken after a 12-hour fast.

Whey By a Nose

After 23 weeks, researchers found that those groups taking the protein packets had no significant differences in weight loss, but those in the whey group enjoyed a 2 percent loss (1.8 kg or approximately 4 pounds) as compared to the carbohydrate group.

Similarly, the whey and soy groups did not differ significantly when it came to body fat mass, fat mass was 2.3 kg (a little over 5 pounds) lower in the whey group as compared to the carbohydrate group. There was not difference in lean body mass across all three groups.

There was no significant difference in hip circumference across the groups, but there was a notable change with waist circumference. Those in the whey group measured 2.4 cm less than those in soy and carbohydrate groups.

On the biological front, there was no difference between the groups when it came to glucose levels and IGF-1 levels were slightly higher in the soy group. However, both whey and soy groups enjoyed a reduction in insulin concentration compared to the carbohydrate group.

Those in the whey group also had lower ghrelin levels than the soy and carbohydrate groups. Ghrelin triggers your hunger button. It tells you to eat and eat more, so reduced levels are a good thing.

On the insulin front, the whey group had lower levels of both T3 and T4 than the other two groups.

In a nutshell, researchers point out that the carbohydrate group was heavier at the end of the 23 weeks than either protein group. And the whey group did enjoy a 1.8 kg difference in body weight and 2.3 kg  difference in fat mass, as well as decreased waist circumference, compared to the carb group. Based on this, they concluded, “Protein supplementation, particularly whey protein, in overweight and obese individuals may assist in long-term maintenance of body weight without energy restriction.”

A Few Problems With the Findings

This study does a good job of showing that by adding in a healthy source of calories (i.e., a protein shake), you can fend off hunger and likely reduce the amount of bad foods and calories you eat. But there are several issues with the study design.

First, the carbohydrate group is not a placebo group, and the researchers acknowledge the need for a placebo group. This begs the question, why include the carb group at all?

Second, the way the authors presented the results felt slanted toward a preference for whey. In laying out their objectives, they state, “We hypothesized that supplementation of overweight and obese free-living individuals with whey protein would decrease body weight and fat compared with individuals supplemented with soy protein or carbohydrate.”

And in their reporting of the results, they repeatedly say that the whey and soy groups did not differ significantly, but that the whey group did differ enough from the carb group to make the results significant. So it would seem that the authors are playing a bit of a statistical significance game here.

We’re not convinced they proved their point on the whey versus soy front. Both seemed to be pretty effective, with whey having a slight edge when it came to waist circumference and not triggering ghrelin.

Finally, they used isoflavone-free soy. Isoflavones are the antioxidant-rich, estrogenic part of the soybean. It would be interesting to see if the results would have differed if they used a soy protein that had the isoflavones intact, as they are naturally.

Where to Go From Here

The best takeaway from this study is the clear benefit of using protein shakes as part of a weight loss plan. And you could take it one step further by using the shakes as a meal replacement or snack.

For example, a protein shake makes an easy and delicious breakfast. Try mixing your protein of choice with water, frozen berries and a handful of spinach. Another great blend is almond milk, frozen cherries, a dropper of liquid vanilla stevia and your protein of choice.

If you go the snack route, skip the additions. Simply mix with water and ice to make it frothy and enjoy.


[1] Westerterp-Plantenga, MS et al. Dietary protein, weight loss, and weight maintenance. Annu Rev Nutr. 2009;29:21-41.

[2] Belobrajdic, DP et al. A high-whey-protein diet reduces body weight gain and alters insulin sensitivity relative to red meat in wistar rats. J Nutr. 2004;134:1454-8.

[3] Baer, DJ et al. Whey protein but not soy protein supplementation alters body weight and composition in free-living overweight and obese adults. J Nutr. 2011;141:1489-94.

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