Think of malnutrition, and obesity probably isn’t the first health problem that springs to mind. But even though obesity obviously represents overnutrition in the caloric sense, people with obesity can still be malnourished when it comes to micronutrients. Just for example:
- In this study on obese women, 13% had an iron deficiency, and 10% had a Vitamin B12 deficiency.
- In another study, 9.5% were deficient in Vitamin B12, 25% in folic acid, 68% in copper, and 74% in zinc.
- Here’s another study, finding that 35% of pre-bariatric surgery patients were deficient in magnesium, 19% in iron, and 17% in Vitamin A.
This review looked at several different studies, and concluded that Vitamins D, B1, B12, and B9 (aka Folate) deserve serious investigation.
The most obvious question here is whether these deficiencies are a cause or a consequence of obesity, or both. And if they're the cause, will treating the deficiency help with weight loss?
There is a theory, the nutrient deficiency theory of obesity, that nutrient deficiencies contribute to obesity, because your body will tell you to keep eating until all your micronutrient needs are met, even if you’re eating too many calories. According to this theory, becoming better-nourished would aid in weight loss, but there’s room for a lot of complexity (for example, a deficiency might be due to malabsorption and not dietary deficiency, in which case the cure for the deficiency isn’t necessarily eating more of the nutrient in question).
There’s also another theory – call it the Magical Supplement Theory of Obesity – that if obesity is associated with deficiency in some nutrient, then megadose supplements of that nutrient must cause weight loss with no further effort required. This is the one you'll see promoted on Dr. Oz, and as you can probably guess, it's a lot less plausible but unfortunately very widespread.
To give you some basis for judging the two theories, here’s a look at some evidence about obesity and nutrient deficiency – it’s pretty damning for the Magical Supplement Theory, but very interesting from the perspective of absorption, nutrient density, and how different diet and lifestyle factors all affect each other.
According to a recent meta-analysis, the prevalence of Vitamin D deficiency is 35% higher in people with obesity than in people at a normal weight. This is dangerous, because Vitamin D deficiency is a risk factor for heart disease, bone loss, impaired glucose metabolism, metabolic syndrome and diabetes, and all other kinds of chronic health problems associated with obesity. It’s also possible that Vitamin D deficiency contributes to accumulating body fat, making obesity <-> deficiency a vicious cycle.
The researchers considered various reasons why obese people might be Vitamin D-deficient;
- Less sun exposure. Most people get the majority of their Vitamin D from sunlight on their skin. People with obesity are more likely to get cruel comments if they go outside in revealing clothing, like to the pool in a bathing suit. So they’re probably more likely to stay indoors or wear clothing that covers them up, which would reduce Vitamin D absorption. But even if you control for time spent outside, people with obesity are still more commonly Vitamin D deficient than other people.
- Body fat “stealing” Vitamin D. Through a complicated chemical process, it’s possible that excess body fat requires a lot of extra Vitamin D, which diminishes the amount available for other processes.
But that doesn’t answer the million-dollar question: in people who already have obesity, will Vitamin D supplements produce or assist in weight loss?
- This study tested it out, and concluded that there was “no significant influence of vitamin D supplementation on weight, fat mass or waist circumference in type 2 diabetic obese vitamin D deficient participants of Arab ethnicity after one year.”
- This study found that in otherwise-healthy overweight and obese women, Vitamin D supplements reduced body fat mass, but not body weight.
- In this study, a Vitamin D supplement helped improve blood sugar control, but didn’t produce any weight loss.
But does it work in conjunction with a focused weight-loss effort? In this study, women were randomly assigned to a weight loss intervention + Vitamin D or a weight loss intervention + placebo. The Vitamin D group had no significant advantage and lost about the same amount of weight. However, among women who completely corrected their Vitamin D deficiency, there was an improvement compared to placebo.
In other words, Vitamin D is very unlikely to be a weight loss wonder drug. It may improve your health, but it probably won’t make you thin.
B Vitamins and Iron
(Note that none of this applies to patients after weight-loss surgery, who have special nutritional needs that they should discuss with a doctor)
Moving on from Vitamin D, obesity also often comes along with deficiency of B vitamins and iron. Many obese patients are deficient in thiamin (Vitamin B9) – this study suggested a prevalence of 16-29%. Other studies have found deficiencies in Vitamin B12; for example, this one found that serum levels of B12 were significantly lower in obese and overweight patients compared to normal-weight controls.
As for iron, one study found that 20% of pre-bariatric-surgery patients were deficient in iron, and iron deficiency is also more common in overweight children. This study also explored the role of iron deficiency in obesity, which the authors suggested may be caused by obesity-associated inflammation reducing iron absorption. It’s probably not that obese people eat too little iron, but rather that they have increased iron needs and trouble absorbing what they do get.
No studies have shown any direct mechanism for B vitamin and iron deficiency to cause fat gain, but they could contribute indirectly by causing anemia, a problem where your blood can’t carry enough oxygen. Symptoms of that include exhaustion, generally feeling “off,” and physical weakness. This could make it harder to lose weight just by making it harder to get up and do anything at all (who wants to get up and go for a run when they’re feeling exhausted all the time?).
Unfortunately, adding more iron and B12 isn’t necessarily the answer. In fact, in the case of iron, the problem is two-faced. To quote this study:
On the one hand, obesity may promote iron deficiency by inhibition of dietary iron uptake…On the other hand, a condition termed “dysmetabolic iron overload syndrome (DIOS)” has become the most frequent differential diagnosis for elevated ferritin concentrations, affecting approximately one-third of subjects with nonalcoholic fatty liver disease (NAFLD) or metabolic syndrome (MetS).
Because of the specific way that they’re malabsorbing iron, people with obesity get both the problems of iron deficiency and the problems of iron excess, which may be a contributing factor to several obesity-related diseases. As the authors of the study note, just adding more iron pills won’t solve the underlying malabsorption problem.
B vitamins don’t fare much better. In this study, obese women got a multivitamin with Vitamins C, B6, B12, and folate (B9). Their weight and inflammatory markers didn’t change.
Summing it Up
It’s pretty clear that people with obesity are more likely to some important nutrient deficiencies than people at a normal weight. But with apologies to the Magical Supplement Theory, there’s not a lot of evidence that supplementing with those nutrients does much for weight loss. Obesity is a complicated and multifaceted disease; if we could cure it with Vitamin D supplements, everyone would be thin.
This doesn’t actually disprove the nutrient deficiency theory of obesity – for example, if the problem is malabsorption of a nutrient, then taking a supplement which gets absorbed just as poorly won’t address the actual nutrient deficiency driving the continued hunger. But it does suggest that addressing issues of inflammation, gut health/malabsorption, and other root causes of deficiency is likely to be more useful than just supplementing.