Is Soy Milk Good for Weight Loss?

Is soy milk good for weight loss? If you did nothing else but swap soy milk for cow’s milk, would you lose weight? Compared to regular cow’s milk, regular soy milk has about the same number of calories. How then could it have any benefit to people who plan to lose weight? The quick answer is that soy milk is good for weight loss, but not because it is less fattening than cow’s milk.

Soy Milk makes you Gain Weight

The calories in soy milk, as in cow’s milk, come mainly from the fats, carbohydrates, and protein found in both. Both soy milk and cow’s milk have a very similar weight-gain effect, as evidenced by many comparative studies. The reason for this is simple: “soy milk” is an artificial copy of milk, designed to match milk’s nutrition. Of course, manufacturers can and do alter the nutrient mix in different types of soy milk. Dairy producers and marketers do too. Soy milk is not an antinutrient (see our post on the topic to read about antinutrients).

Soy Milk Calories equals Cow's milk Calories

In that case, why wonder whether soy milk is good for weight loss or not? Because soy contains some interesting micronutrients which do have a real effect on healthy weight profiles.

Soy Milk contains Isoflavones

Soy milk is made from soybeans, which are unique in that they contain high levels of chemicals known as isoflavones. These isoflavones are a type of plant estrogen (phytoestrogen). Plant estrogen is like human estrogen, so it affects us in a similar to human estrogen, but less so. Estrogen is a hormone which controls many body functions, particularly in females. One of the effects of estrogen is to increase the Resting Metabolic Rate (RMR) in women. RMR is similar to BMR, it is a basic rate of energy burn, so higher RMR equals more weight loss.

When women experience menopause, their estrogen levels decline. This causes a slowdown in RMR and the reduced energy use makes it easier to build fat deposits.

Does the fact that phytoestrogen works like estrogen, and estrogen can help with weight-loss (at least, in some women), mean that soy milk is good for fat burn? The answer seems to be no, as we pointed out earlier. The reason for this is:

Hormones are Complicated

The effect of hormones on body function is incredibly complex and complicated. The full interplay between “male” hormones like testosterone and “female” hormones like estrogen are in many ways still a big mystery to scientists, even after decades of study. There is simply no certain way to predict what effect a specific amount of a specific hormone will have on a specific person. People are individually so different, and their bodies are always in such a state of flux, that hormone “treatment” through natural food components is impractical. Study after study has come up with one single verdict as to the efficacy of soy for weight loss promotion: inconclusive.

Soy Milk affects Hormonal Balance

What effects does Soy Milk have on Women’s and Men’s bodies?

Since soy milk contains these estrogen-like compounds, it is fair to ask what effect they have on the bodies of men and women. Unfortunately, there are no clear and easy answers to this question, due to the intricacies of the way hormones work in different people. In theory, the hormonal effect of soy milk will be thus: people with lots of estrogen in their bodies will have an anti-response, whilst people with low levels of estrogen will have a positive response. This is because when the body detects “too much” estrogen, it triggers the release of counter-hormones to restore balance. It is possible that the phytoestrogen in soy milk replicates the effect of an estrogen supplement. Thus, a pre-menopausal female, or a male with relatively high estrogen levels, might experience the opposite effect of taking extra estrogen. On the other hand, a woman who has had menopause and a man with relatively low estrogen would experience the expected result of more estrogen in the body.

In this way, soy milk might have the effect of speeding RMR for some, whilst slowing it for others. Bear in mind that estrogen has many other effects on women and men, such as regulating menstrual cycles and determining libido.

That said, there is theory and there is actuality, and in this case the studies with soy milk have proven inconclusive.

Why Soy Milk is Good for Weight Loss

If soy milk has similar calories to cow’s milk, and all the studies show no conclusions as to its fat burning credentials, why do we still say that soy milk is good for weight loss? We say that it is a good idea to swap cow’s for soy, at least for trial and intermittent periods, because of studies like this one. This and other studies like it show that soy milk, when swapped directly for cow’s milk, has the effect of reducing the waist circumference of obese people and improving heart health indicators. How does soy milk do this? Again, the studies are inconclusive, but it may be that soy components including essential fatty acids, phytosterols, inositol, and of course the isoflavones contribute to these beneficial effects. Slimmer waists and healthier hearts lead to faster weight loss over time, because both enable greater body activity and promote BMR increases.


Depending on the brand and type of soy milk, it is likely to have similar calories to a similarly positioned cow’s milk brand. Whilst soy milk is not universally accepted as an effective dieting aid, there is some evidence to support the claim that it is good for weight loss. The plant estrogen in soy milk can have an estrogen-like effect on the body, and like estrogen, it might affect different people in different ways. It is probably a good idea to trial a swap of soy milk for cow’s milk, to see whether you benefit from its possible health benefits.