The 2-Hour Rule: Is a bottle safe when it’s been left out too long?

On a message board I frequent, a mother recently asked how many of us obey the “don’t feed formula left out over 2 hours” rule. While most people responded that they did abide by this recommendation, a conversation ensued about why we we’d all been told to do this. The answers varied, from vague concepts about bacteria spreading in room temperature bottles, to warnings that the nutrient content of the formula would decrease as time went on.

This all got me wondering: what is the reason we’re told to be so careful about not letting our babies consume “old” formula? For a snacker like Fearlette is – especially one on $30-a-can Alimentum – this rule is a real drag. My sick little theory was that this rule is all b.s., a combined ploy of the formula manufacturers and breastfeeding pushers who want to make it more costly and aggravating to formula feed. So I started researching… and I found plenty of studies on how formula feeding moms don’t follow safe practices, and articles about how unsafe formula handling can kill babies… but no concrete facts, statistics, or studies about just how room-temperature formula becomes an instrument of morbidity.

Go figure, right?

Anyway, let me share with you the information I could find. An Infant Formula Council document warns that the longer formula is left at room temperature, the more likely it is to become contaminated with a bacteria called Enterobacter sakazakii, which can indeed be lethal for premature infants or those with compromised immune systems for other reasons:


A microorganism of particular concern, and which can survive in powdered infant formula, is Enterobacter sakazakii (E. sakazakii). E. sakazakii is an opportunistic pathogen that poses little risk to healthy, term infants. However, in certain highly vulnerable infants this microorganism can cause serious infection that can present severe and life­threatening conditions, including meningitis. This most commonly occurs in low birth weight and immuno­compromised infants, in whom isolated outbreaks of E. sakazakii infections have been reported in a few hospital settings. Some of these cases have been linked to the improper preparation and storage of reconstituted powdered infant formula. For example, blenders used to reconstitute formula have been shown to harbor bacteria. Because reconstituted powdered infant formula is rich in nutrients and is not sterile, it provides a good medium for microbial growth under certain conditions. For example, prolonged periods of storage or administration of prepared powdered infant formula at room temperature will increase the bacterial load. Thus, proper handling and use of powdered infant formula in the health care setting is an important patient safety issue.

The AAP also warns about the dangers of improper formula handling:


In the United States, infant formulas are available in 3 types: ready-to-feed, liquid concentrate, and powdered. Liquid concentrates and powders require appropriate dilution with clean water. Adding too much water may lead to inadequate intake of calories and nutrients, whereas adding too little water may lead to dehydration, diarrhea, and excessive intake of calories. The safety of liquid concentrate and powdered formulas can be compromised if they are diluted with water of poor quality.1 Unlike ready-to-feed and liquid concentrates, powdered formulas are not sterile when purchased and could contain bacteria. To reduce the risk of infant illness caused by bacteria in powdered formula,3 the World Health Organization urges caregivers to use water no less than 158°F when reconstituting powdered formula.4

I checked the references of both these articles – which were coming from two different organizations with dramatically different biases (the AAP one was written, in part, by an IBCLC) – and it was insanely frustrating, They used other articles as backup for these theories that also offered no stark data. I couldn’t find any documentation of cases where children got sick from formula left out of the fridge too long, or suffered long-term health effects because their parents let them drink a bottle they’d been snacking on for 2 hours and 15 minutes.

In terms of preventing contamination Enterobacter sakazakii (ES), a rather stubborn bacteria which does indeed confer risks to infant formula, I did find a long chain of good studies which examined how this bug can survive even under refrigeration, and can cling to surfaces which makes it a real threat in NICU units. Apparently, the longer formula is left out, the more likely it is to become contaminated. But again… no concrete evidence.

Here is what I would like to know (and gold stars for anyone who can find a study which answers these questions):

1. Many of the studies on ES recommend using breast milk or ready-t0-feed formula, since ES has been found in cans of powdered formula…but then they also discuss other ways the bacteria can spread, like through feeding tubes, etc. Couldn’t this bacteria theoretically infect a baby in the NICU fed through one of these tubes with human milk or RTF formula? If so, shouldn’t this be added to the warning? According to this study, ES can thrive in breastmilk; also, although we are innundated with the message that “formula is not sterile” and therefore unsafe…. neither is expressed breastmilk, for the exact same reasons. I don’t see much information on dangerous bacteria being passed to the millions of women who pump their breastmilk daily, and I think it’s worth mentioning.

2. Have there been any documented cases of an infant becoming sick after consuming formula left out more than 2 hours? Why is the 2 hours the limit? I’m looking specifically for a study which examined harmful bacteria growth on a sample of formula over time…

Now, none of this is to say that formula-related dangers don’t exist. I personally know a few parents who’ve experienced health crises due to improper formula usage, which is why I think formula education is incredibly important. It makes complete sense that we should wash our hands prior to making a bottle, or use safe water (there are PLENTY of documented cases in developing nations of babies dying from formula made with contaminated water). But the no-bottles-after-2-hours rule, in areas with clean water or in cases where purified water is being used, just seems to have a startling absence of evidence to back it up.

It seems that this recommendation is based on pure hypotheticals – bacteria could feasibly grow in formula, and two hours seems like a decent cut-off point. But where is the evidence? If we’re seriously concerned about our babies’ safety, shouldn’t we be demanding some real statistics on this potential threat?



Interestingly, while researching this post, I came across a study done by the company which makes Dr. Brown’s bottles. The study looked at how the type of bottle can effect the nutrient decomposition of both breastmilk and formula:


The study measured the level of loss of vitamins A, C and E during baby bottle feedings. The results suggest that the amount of air within a baby bottle, the bottle’s design, and the impact on vitamin levels warrant closer examination….The bottle study was designed to investigate changes in nutrient levels that might occur during a typical bottle feeding time of 20 minutes, using both expressed human milk and infant formula, in both vented bottles and unvented bottles…

As milk is removed from the bottle by the infant, the milk is replaced by ambient air. Nutrient loss is likely caused by the oxidation of nutrients that takes place as air is introduced into the liquid. The amount of air moving through the milk and into the bottle depends on the bottle type, bottle shape, and bottle size.

The lead researcher on this study also mentioned that storage of both human and formula milk could effect the nutrient levels. She offered the following tips for bottle-feeding moms:

Select bottles that minimize air traveling through the bottle.

• Look for little to no bubbles forming in the milk as the baby feeds.

• Use breastmilk that is as fresh as possible.

• Use small bottles that minimize the amount of air at the top of the milk.

• When using infant formula, make it fresh for every feeding.

• Feed babies with small, frequent feedings.

Although this study was funded by the makers of the same bottle which fared the best in the study (Dr. Brown’s), I still think it’s a superb example of the type of research we should be focusing on. It tells us a specific effect of a specific behavior, and offers suggestions to counteract the negative results of the study. Good, useful, empowering information.

I’m certainly not in the business of giving recommendations, and as I’ve said a million times before, I am not a doctor, nor do I play one on tv. But here’s my personal take on it: I’d be pretty careful about the 2-hour rule if your baby is under 6 months old or immuno-compromised in any way. But after that… I don’t know. The fact is, once your kid is on solids, they are going to be at risk for foodborne illness from all sorts of products, not just infant formula. I haven’t found any research that makes me believe Fearlette is at risk because I let her snack on the same bottle for 3 hours rather than 2.

Not that I’d do that. We’re talking hypotheticals here, remember?

About the Author:

Suzanne Barston is a blogger and author of BOTTLED UP. Fearless Formula Feeder is a blog – and community – dedicated to infant feeding choice, and committed to providing non-judgmental support for all new parents. It exists to protect women from misleading or misrepresented “facts”; essentialist ideals about what mothers should think, feel, or do; government and health authorities who form policy statements based on ambivalent research; and the insidious beast known as Internetus Trolliamus, Mommy Blog Varietal.

Suzanne Barston – who has written posts on Fearless Formula Feeder.


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