Sucrose, dextrose, glucose and fructose, but never lactose. Why not, what's wrong with it as a source of fuel?

  • 2
    Lactose on its own would be a form of carbohydrate, it requires an enzyme called Lactase to break it down in a simple sugar. Not an answer as I really don’t know enough about the subject. Lactose is also a dairy product and it wouldn’t be beneficial to someone that has difficulty digesting dairy or intolerant to it.
    – Dan K
    Commented Aug 19, 2020 at 17:01
  • 2
    @DanK raises a good point, and I think that the majority of the world’s adult population lack or are low in the associated enzyme (albeit people of European and South Asian descent are more likely to be able to digest lactose as adults). I think the bigger issue is that you really only get lactose with milk. Because of the protein content, this may not be what you want to fuel during long rides. That said, individual preferences and tolerances should reign. I personally know one (White) super long-distance cyclist who guzzles chocolate milk during her adventures.
    – Weiwen Ng
    Commented Aug 19, 2020 at 17:16
  • 3
    @WeiwenNg it's quite common to see people drinking milk or milk-based drinks in long-distance riding in the UK. I've occasionally done it myself, being prone to under-hydrating which then makes me want my food in liquid form. Many of the drinks have added sugar so are a richer source of carbs than pure milk. For long enough rides some low level protein intake is no bad thing anyway even if it's never be a major fuel.
    – Chris H
    Commented Aug 19, 2020 at 17:44

2 Answers 2


Glucose (dextrose is the same thing) has a special role in metabolism - as well as being a nutrient sugar in its own right it's a vital intermediate in breaking down more complex sugars, starch, and glycogen, our bodies' main store of carbs. This all means we're able to get energy from it very efficiently (excepting cases of diabetes). This is why energy drinks (the ones with calories in, not just caffeine and flavouring) use it.

Maltodextrin, used in energy gels and some energy drinks, is pretty much just a string of glucose molecules.

Fructose is metabolised rather differently, but the pathway is connected to glucose metabolism, so it's not exactly parallel.

Sucrose is a glucose and a fructose joined together, so also a very available source of energy.

Lactose is broken down by lactase (in those individuals that produce it) into glucose and galactose. The latter in turn then ends up as glucose by a variety of routes, probably slower release than glucose, but faster than starch. It would appear to offer no advantage as a parallel source of energy to glucose, while not being any cheaper and intolerable to some people.

So if you like it, you can certainly get energy from it, and milk can often be bought easily - on a recent warm ride it would have been the only way for me to get a chilled non-alcoholic drink in one shop (but I don't like it as a drink so I had an ice cream and water instead). For a manufacturer trying to make money, it would make their products more expensive for no sales benefit.

  • 1
    Milk is cheap too, I often buy a pint of milk for easily half the price of a bottle of water at a service station, but the problem there is expensive water lol
    – Swifty
    Commented Aug 19, 2020 at 19:47
  • 2
    @Swifty absolutely, tiny bottles of water for silly prices are all too common. If you have to buy water in a small shop in the UK, the Co-op is normally your best bet (good value/selection of food too). On one recent (no cafes to fill my bottles) ride the difference between the cheapest and most expensive bottled water I bought was 12x.
    – Chris H
    Commented Aug 19, 2020 at 20:11
  • Yup in the UK individual bottles of refrigerated water, are sold as an alternative to and priced as a to-go drink (alongside fruit juice, coca-cola etc). Milk OTOH is priced as a staple food and yet it's refridgerated and comes in bottles small enough to drink in one go. In supermarkets and larger convenience stores you can get water in the regular food section but then it won't be refridgerated or come in individual bottles (if your lucky it will come in a multipack of small bottles). The result is when you want a cold drink on the go milk is often literally half the price of any other option Commented Aug 20, 2020 at 3:22
  • 1
    @Peter I prefer one big bottle - less plastic waste when I decant it and cheaper than a multipack (but then I'm carrying at least 2 bike bottles, 2 of them litre bottles) In the chilled section of a small supermarket, if you're lucky, you may find a litre of own brand for a much better price than a petrol station
    – Chris H
    Commented Aug 20, 2020 at 6:35

Lactose requires the enzyme lactase to be digested. According to the Wikipedia article on lactose, "in most mammals, the production of lactase gradually decreases with maturity due to a lack of continuing consumption." In other words, most adult mammals, including humans, no longer produce lactase and are therefore unable to digest lactose. In some parts of the world where milk from cattle, goats, and sheep is an important source of food, notably Europe, parts of Asia, and parts of Asia, people have evolved to continue producing lactase into adulthood. Again quoting Wikipedia, "By descent, more than 70% of western Europeans can drink milk as adults, compared with less than 30% of people from areas of Africa, eastern and south-eastern Asia and Oceania."

The problem with lactose is that at least 30% of adults can't digest it. Makers of ice cream have this problem, but generally people who are lactose-intolerant know what ice cream does to them, and ice cream is popular so plenty of people who aren't lactose-intolerant will buy it.

Now consider a company making energy bars marketed to bicyclists. If they put lactose in their energy bars, many unsuspecting lactose-intolerant customers would eat the bars and have undesirable consequences. This would surely hurt the reputation of the product and the company.

Gluten is similar to lactose, being a common food ingredient that some people have problems digesting. In the US, gluten currently has a bad reputation, and food product companies are avoiding it wherever possible. I bought some honey recently that was labeled as "a naturally gluten-free food". Makers of energy food products probably want to avoid including an ingredient that could easily be "the next gluten", an ingredient that people love to hate.

  • Great answer, thank you. For you or anyone else, in principle does lactose act similarly to glucose and fructose in supplying energy, or is it e.g. slower/more complicated?
    – DonnyG
    Commented Aug 19, 2020 at 17:38
  • If you like the answer, please up-vote it. Sorry, I'm no biologist, I'm just good at quoting Wikipedia, so I don't know if lactose works any faster or slower. Please also consider accepting the answer (with the green check mark) if you feel that your question has been answered, but wait a few hours in case better answers come in.
    – rclocher3
    Commented Aug 19, 2020 at 17:45
  • A few of my random selection of energy bars have milk as an allergen (not necessarily lactose but close enough). I was surprised at how few. I'm in the UK and bulk buy when I get a good deal, so I'm not selective. And quite a lot make a point of their lack of gluten.
    – Chris H
    Commented Aug 19, 2020 at 17:48
  • It sounds like you're asking if Lactose would kind of "count-as" what people call a "complex-carb" that requires a lot of metabolic action to turn into cellular energy? As per same wikipedia, Lactase converts Lactose to simple glucose and galactose by enzymatic action in the gut, it's not a metabolic pathway in the cells/liver. Further complicated that even if you're not symptomatically intolerant the actual amount of Lactase you have personally retained into adulthood influences how much actual energy is extracted compared to passed on or consumed by gut bacteria.
    – Affe
    Commented Aug 19, 2020 at 17:58
  • 1
    Curiously, lactose is used in some snack foods that don't otherwise contain milk derivatives. It's the first ingredient in the "salt and vinegar flavouring" on some crisps I had this morning. So it must have some value to manufacturers (perhaps it binds the flavouring while not being a sweet as other sugars)
    – Chris H
    Commented Aug 20, 2020 at 10:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.