The evidence

Every figure in Just Enough is traceable. This page is organised by claim rather than as a reading list, so you can check the particular thing you came to check.

The two numbers

The recommended intake and the upper limit for calcium and vitamin D, by age and sex. These are the published Dietary Reference Intakes — not our interpretation of them, and not adjusted.

• Dietary Reference Intakes for Calcium and Vitamin D — Institute of Medicine, National Academies. The source table.
• Calcium fact sheet for health professionals — NIH Office of Dietary Supplements. The same figures, republished and maintained.

Why there’s an upper limit

That intake above roughly 2,000 mg a day is associated with an increased risk of kidney stones, and that going above the recommended intake brings no further benefit to bone.

• Prentice et al. (2012) — the Women’s Health Initiative analysis, which found a modest increase in urinary tract stones among women taking calcium with vitamin D.
• Bargagli et al. (2021) — Calcium and Vitamin D Supplementation and Their Association with Kidney Stone Disease.
• Sorensen (2014) — Calcium intake and urinary stone disease, on why dietary calcium should not simply be cut to prevent stones.
• Calcium— Harvard T.H. Chan School of Public Health, for the overview.

How bone is lost with age

That bone is lost gradually from around the mid-thirties, that the loss accelerates sharply for women at menopause, and the fracture figures quoted in our emails and guides.

• Osteoporosis— NIH National Institute of Arthritis and Musculoskeletal and Skin Diseases, on age-related bone loss and the accelerated loss in the years following menopause.
• Epidemiology of osteoporosis and fragility fractures — International Osteoporosis Foundation, for the figure that one in three women and one in five men over 50 will experience an osteoporotic fracture.

Bone density testing

Who the guidelines recommend a DXA scan for, and why the answer differs depending on which guideline you read. Just Enough does not screen, diagnose or decide whether a scan is right for you — that is a conversation with your doctor. These are the published positions.

• Osteoporosis to prevent fractures: screening — US Preventive Services Task Force. Recommends screening women 65 and older, and postmenopausal women under 65 at increased risk as estimated by clinical risk assessment. For men it concludes the current evidence is insufficient to assess the balance of benefits and harms.
• Kanis, Cooper et al. (2019), European guidance for the diagnosis and management of osteoporosis in postmenopausal women, Osteoporosis International — the European counterpart, developed with the European Society for Clinical and Economic Aspects of Osteoporosis and the International Osteoporosis Foundation.
• Diagnosis— International Osteoporosis Foundation, on DXA at the femoral neck and lumbar spine, and the T-score thresholds used to define osteopenia and osteoporosis.

The two frameworks agree on older women and disagree on men, and neither sets a single universal interval for repeat scans. Where guidance is contested, Just Enough says so rather than picking a side.

The heart question, which is genuinely unsettled

Whether calcium from supplements affects cardiovascular risk is contested. Just Enough says so rather than picking a side, because the evidence does not support one.

• MESA, ten-year follow-up — associated supplement use with increased coronary artery calcium.
• The Framingham analysis — found no such association.
• Myung et al., Anderson et al., Zarzour et al. and Samelson et al. are all in the library for the same reason: they do not agree with each other.
• Kopecky SL, Bauer DC, Gulati M, et al. (2016) — Lack of evidence linking calcium with or without vitamin D supplementation to cardiovascular disease in generally healthy adults, Annals of Internal Medicine — a joint clinical guideline from the National Osteoporosis Foundation (now the Bone Health and Osteoporosis Foundation) and the American Society for Preventive Cardiology. Having reviewed the same disagreeing studies, both societies concluded there is no harmful or beneficial effect on heart disease, stroke or death in generally healthy adults — provided intake stays within the upper limit of 2,000 to 2,500 mg a day. Which is the number Just Enough is built around.
• Chung M, Tang AM, Fu Z, et al. (2016) — Calcium intake and cardiovascular disease risk: an updated systematic review and meta-analysis, Annals of Internal Medicine. The evidence report the guideline above was built on.

Why 500 mg at a time

That absorption efficiency falls as a single dose rises, so splitting the same amount across the day increases absorption compared with one large tablet. Also that calcium carbonate is best taken with food while calcium citrate is not, and that citrate suits people whose stomach acid is reduced.

• LeBoff MS, Greenspan SL, Insogna KL, et al. (2022) — The clinician’s guide to prevention and treatment of osteoporosis, Osteoporosis International 33:2049–2102. States that calcium of all types is best absorbed in doses of about 500 mg or less; that carbonate needs stomach acid and so is best taken with food while citrate is absorbed equally well on an empty stomach; and that citrate suits people with reduced gastric acid, including those taking proton-pump inhibitors.
• Calcium fact sheet — NIH Office of Dietary Supplements.

Calcium and medicines

That calcium can reduce the absorption of some medicines when taken at the same time, which is why Just Enough says to space them apart rather than to stop either.

• Calcium fact sheet — NIH Office of Dietary Supplements, which names quinolone antibiotics, levothyroxine and bisphosphonates.
• Wiesner A, Gajewska D, Paśko P (2021) — Levothyroxine interactions with food and dietary supplements: a systematic review, Pharmaceuticals 14(3):206. Sixty-three studies. It describes the evidence on calcium specifically aslimited, while noting that every study of it found levothyroxine absorption reduced — which is the honest way to put it, and why the advice is to separate the doses rather than to alarm anybody.

Movement

Every figure the app shows for movement, and where each one comes from.

• Bone responds to mechanical loading, and resistance and impact load it in two different ways — resistance through muscle pulling on bone, impact through landing and stepping. Brooke-Wavell et al. 2022; Santos et al. 2017.
• Impact exercise on most days, including at least 50 impacts in a session. Brooke-Wavell et al. 2022.
• Resistance exercise on two or three days a week, working every major muscle group including the back. Brooke-Wavell et al. 2022; WHO 2020, which states two or more days.
• Balance work at least twice a week. Brooke-Wavell et al. 2022. WHO 2020 recommends three or more days from age 65; the app uses the lower, un-age-gated figure, because it applies to everybody who uses it.
• 150 to 300 minutes of moderate activity a week, or 75 to 150 vigorous, or a combination — and that a vigorous minute counts as two moderate ones, which is that guideline’s own arithmetic.WHO 2020; Bull et al. 2020.
• Walking is not enough on its own to build bone. It is weight-bearing and worth doing, but it is not an osteogenic stimulus in the way impact and resistance are. Brooke-Wavell et al. 2022.
• Swimming and cycling do not load bone at all, being non-weight-bearing, however good they are for the heart.Brooke-Wavell et al. 2022; 2024 Adult Compendium.
• Aerobic activity has no documented benefit for bone. The app says so rather than letting a daily walk be mistaken for bone work.Brooke-Wavell et al. 2022.
• Moderate means 3.0 to 5.9 METs and vigorous 6.0 or more — the basis for how every activity in the app is sorted, and why some of them ask how hard it felt. 2024 Adult Compendium.
• Falls are what most broken bones follow. A fragility fracture is defined as one caused by low-energy trauma, such as a fall from standing height or less — so balance work is part of not breaking a bone.WHO, fragility fractures.
• Yoga and Pilates are credited for balance, not for bone. Both can build strength, but whether they load the skeleton enough to change bone density is not established, and current guidelines reserve their strongest recommendations for progressive resistance and impact work.Brooke-Wavell et al. 2022.

The sources behind those claims:

• Brooke-Wavell K, Skelton DA, Barker KL, et al. (2022) — Strong, steady and straight: UK consensus statement on physical activity and exercise for osteoporosis, British Journal of Sports Medicine 56:837–846. The source for impact on most days, resistance on two or three, balance at least twice a week — and for the finding that walking alone is not enough to build bone.
• WHO guidelines on physical activity and sedentary behaviour (2020), with Bull et al. (2020), British Journal of Sports Medicine, as the guidelines paper. The source for 150–300 minutes of moderate activity a week, or 75–150 vigorous, and for muscle-strengthening on two or more days.
• Herrmann SD, Willis EA, Ainsworth BE, et al. (2024) — 2024 Adult Compendium of Physical Activities, Journal of Sport and Health Science 13(1):6–12. How every activity in the app is classified: moderate is 3.0–5.9 METs and vigorous is 6.0 or more. Note that it is built mainly on adults aged 19–59, and that age, sex and body composition all affect how hard a given activity feels — which is why the app asks.
• Fragility fractures — World Health Organization. Defines a fragility fracture as one caused by low-energy trauma, such as a fall from standing height or less. This is what the balance row rests on.
• Bae S, Lee S, Park H, et al. (2023) — Exercise guidelines for osteoporosis management and fall prevention in osteoporosis patients, Journal of Bone Metabolism 30(2):149–165. A position statement from the Korean Society for Bone and Mineral Research and the Korean Society of Exercise Physiology, built on a systematic review. It is the most specific source for what counts as resistance and as impact — naming squats, leg press and knee extension for the first, and jump rope and drop landings for the second. Written for people who already have osteoporosis or osteopenia, so Just Enough uses it to classify activities rather than to set targets.
• Santos et al. (2017) — Exercise and bone health across the lifespan.
• Fischer et al. (2023) — systematic review and meta-analysis of exercise with vitamin D supplementation and bone mineral density.

The other things bones depend on

Just Enough says that bones also depend on enough vitamin D, protein and magnesium, alongside weight-bearing exercise. Vitamin D and exercise are covered above. These are the other two.

• Rizzoli et al. (2024) — Nutrition and Osteoporosis Prevention, covering calcium, vitamin D, protein and dietary patterns.
• Regulation (EU) No 432/2012 — the European Food Safety Authority assessed magnesium and authorised the claim that it contributes to the maintenance of normal bones, concluding a cause-and-effect relationship is established.
• Rondanelli et al. (2021), Biometals — about 60% of the body’s magnesium is held in bone, and magnesium is a cofactor for activating vitamin D.
• Groenendijk et al. (2022), Bone — a meta-analysis in older adults, which found higher magnesium intake associated with higher hip and femoral-neck density, and not enough evidence for other outcomes.

Magnesium and protein are named because bone needs them, not because Just Enough tracks them. Their evidence is thinner than calcium’s, and there is no agreed bone-specific target or upper limit to count against — which is why the app counts calcium and vitamin D, and says the rest in words.

The UV reading

This one is ours rather than cited, so it deserves more detail. The app calculates the clear-sky UV index on your device from solar position — your latitude, the date and the time — with an ozone term that varies by latitude and season. Nothing is fetched and nothing is sent.

It was checked against measured UV in eleven cities from Singapore to Reykjavík. The daily peak lands within about 0.2 of an index point, and the time of that peak within roughly half an hour of local solar noon.

It is a clear-sky estimate. It does not know about cloud, and it will usually read higher than reality.

Food composition

Every calcium figure for a food in the app, and the serving it is measured against. These are typical published values for a normal serving, not measurements of the food in front of you — two glasses of milk are not identical. The app is precise about arithmetic and approximate about portions, which is the honest way round.

• USDA FoodData Central — the US Department of Agriculture’s food composition database, and the source of most of the figures in the app. Generic reference values for a named food, not a brand.
• McCance and Widdowson’s The Composition of Foods (CoFID, 2021) — Public Health England’s dataset, used where a food is eaten in Britain and Ireland rather than the United States.
• Calcium fact sheet — NIH Office of Dietary Supplements, for which foods are worth counting at all and how well the calcium in them is absorbed.

Where a figure is less certain, the app says so on the food itself.Fortified drinks and cereals vary by brand and country, so those rows carrycheck the label. Tofu depends on the coagulant. Spinach carries a high total that is poorly absorbed. Roughly three dozen of the foods carry a note of this kind; the rest do not need one.

Serving sizes come from the same sources and are shown in grams and millilitres, or in cups and ounces if you choose that in Set up. The milligrams are the same either way.

Just Enough does not

Prevent osteoporosis or fractures, because no tracker can. It compares what you logged against published medical guidelines.

Read the guides →