Know Your Numbers | Vitamin D

If you’ve ever sent me your blood report, you already know this — I almost always scan for your Vitamin D levels first. In fact, for most of my weight loss clients, I specifically ask for a Vitamin D test. Because when progress feels unusually slow, energy is low, mood is off, or recovery isn’t great… this quiet little number often explains a lot. But then you open your report and see two different Vitamin D tests — and it instantly feels confusing.

Let me simplify it for you.

A) 25-Hydroxy Vitamin D (25(OH)D)

This is the main test. The one that actually tells us how much Vitamin D is circulating in your body. It measures both Vitamin D2 and D3 together. When I say “Check your Vitamin D,” this is the number I’m talking about.

How to interpret it:

• Below 20 ng/mL → Deficient

• 21–29 ng/mL → Insufficient

• 30–50 ng/mL → Sufficient

• 50–60 ng/mL → Often considered optimal

• Above 150 ng/mL → Potential toxicity

In my practice, I usually like to see clients closer to that optimal range — especially if we’re working on fat loss, strength, hormones, or overall vitality.

B) 1,25-Dihydroxy Vitamin D (1,25(OH)₂D)

This one sounds more complicated — and honestly, it kind of is. It measures the active form of Vitamin D. But here’s the important part: it does not reflect your Vitamin D stores. So it’s not typically the test we use to diagnose deficiency. Doctors may order this when they’re evaluating specific medical conditions related to calcium balance or kidney function.

What abnormal levels may indicate:

• High levels → Conditions like primary hyperparathyroidism, sarcoidosis, or other disorders

• Low levels → Severe deficiency or impaired kidney function

For general health, fat loss, immunity, and hormone support, the 25(OH)D test is the one that matters most.

In an ideal world, everyone would simply test 25(OH)D and get a clear answer.

But let’s be practical. In some countries — and even in many cities here — getting these Vitamin D tests isn’t always straightforward. You may need a doctor’s approval, insurance may not cover it, or the out-of-pocket cost may feel unnecessary. If that’s the case, there are other markers that can give us clues. They don’t replace a proper Vitamin D test — but they can help us read between the lines.

Here’s what I look at:

1. Parathyroid Hormone (PTH)

Think of PTH as Vitamin D’s balancing partner. When Vitamin D levels drop, your body tries to maintain normal calcium levels by increasing PTH. So the relationship is usually inverse — low Vitamin D, high PTH. If PTH is elevated without another clear cause, it can be a subtle signal that Vitamin D may be insufficient.

2. Serum Calcium

Vitamin D helps your body absorb calcium from the gut. If Vitamin D is low, calcium absorption drops. In some cases, this can show up as low blood calcium (hypocalcemia). Even if calcium is still within range, trends can be informative when viewed alongside symptoms.

3. Phosphorus

Vitamin D also plays a role in phosphorus absorption. Low Vitamin D can impair phosphorus metabolism, which may lead to low blood phosphorus levels (hypophosphatemia). It’s another small piece of the larger puzzle.

4. Alkaline Phosphatase (ALP)

ALP is an enzyme involved in bone turnover. When Vitamin D is low, the body may increase bone turnover in an attempt to maintain calcium balance — and this can sometimes show up as elevated ALP levels. If I see high ALP along with symptoms like fatigue, body aches, or poor recovery, I start asking more questions.

5. Bone Mineral Density (DEXA Scan)

A DEXA scan measures bone mineral density. Chronic Vitamin D deficiency can contribute to reduced bone density over time, increasing the risk of osteoporosis and fractures. While this is more of a long-term indicator, it’s important — especially for women over 35–40.

6. Muscle Strength & Function

This one often gets overlooked. Vitamin D receptors exist in muscle tissue. Deficiency can show up as muscle weakness, poor balance, slower recovery, or that vague “heavy body” feeling many people describe. Sometimes your body gives you functional clues even before your lab report does.

The key is this: no single marker (other than 25(OH)D) gives you the full picture. But when you combine symptoms, lifestyle, and these indirect markers, you can make informed decisions — even when testing isn’t easily accessible. As always, context matters. And numbers should never be interpreted in isolation.

Why am I asked for a Vitamin D test in a weight loss program?

This is a very fair question. Is Vitamin D deficiency the direct cause of weight gain? We don’t have definitive proof of that. The science shows an association, but causality hasn’t been firmly established. Even I won’t claim that we fully understand the “why.” But in practice, I rarely see obesity without low Vitamin D walking alongside it.

Over the years, I’ve noticed that when clients struggle with stubborn fat loss, low energy, slow recovery, poor mood, or constant cravings, their Vitamin D levels are often suboptimal. I don’t see Vitamin D as a magic fat-loss switch. I see it as a variable that can quietly make everything harder when it’s low.

There are several plausible mechanisms behind this association.

• Vitamin D receptors are present in parts of the brain involved in appetite regulation and energy balance. Some research suggests Vitamin D may influence satiety signaling — essentially how effectively your body registers fullness. If these pathways are disrupted, hunger cues and appetite regulation may become less efficient. It’s not the only factor influencing overeating, but it could be one piece of the puzzle.

• Vitamin D also appears to play a role in insulin sensitivity and glucose metabolism. Insulin resistance — where your cells don’t respond properly to insulin — is strongly linked to weight gain, especially around the abdomen. If Vitamin D status affects how efficiently your body handles blood sugar, deficiency may indirectly worsen metabolic efficiency.

• Then there is inflammation. Obesity is not just excess weight; it is often associated with chronic low-grade inflammation. Vitamin D has anti-inflammatory properties, and when levels are low, inflammatory pathways may become more active. This can influence how fat tissue behaves, how it stores energy, and how it communicates with the rest of the body.

• Muscle function is another important piece. Vitamin D is crucial for muscle strength and physical performance. If you feel weak, fatigued, or heavy, your natural activity levels tend to drop. Lower movement over time means lower energy expenditure. Sometimes what looks like poor motivation is actually a physiological limitation.

• Vitamin D may also influence hormones involved in metabolism, such as leptin and adiponectin. When these hormonal signals are disrupted, hunger regulation, fat storage, and metabolic flexibility can become less efficient.

So no, Vitamin D is not the sole cause of weight gain. But when it’s low, it can quietly create resistance in multiple systems at once. As a coach, I prefer removing as many hidden obstacles as possible.

Why are my Vitamin D levels low?

Many people are surprised to discover they are deficient despite living in a sunny country.

The primary source of Vitamin D is synthesis in the skin through exposure to UVB radiation from sunlight. However, modern lifestyles significantly limit this. Indoor work, stepping out only early morning or late evening, consistent sunscreen use, air pollution, and even skin pigmentation can all reduce how much Vitamin D your body produces. So sunlight availability does not automatically translate to adequate Vitamin D levels.

• Dietary intake plays a role, but usually a small one. Vitamin D is found in fatty fish such as salmon, mackerel, and tuna, in egg yolks, liver, and fortified foods like milk. Realistically, diet alone rarely provides optimal levels, especially in largely vegetarian populations.

• Age is another factor. As we grow older, our skin becomes less efficient at synthesizing Vitamin D. Absorption and overall metabolic efficiency may also decline, increasing the risk of deficiency after the mid-30s and beyond.

• Body fat percentage also matters. Vitamin D is fat-soluble, which means it can be stored in adipose tissue. In individuals with higher body fat, more Vitamin D may be sequestered in fat stores, reducing its availability in circulation. This can create a frustrating cycle where deficiency and obesity coexist.

• Certain medical conditions can impair absorption. Disorders that affect fat absorption, such as celiac disease, Crohn’s disease, or cystic fibrosis, can reduce the body’s ability to absorb Vitamin D from food.

• Liver and kidney function are also crucial because Vitamin D requires activation in these organs. If either is not functioning optimally, active Vitamin D levels may decline. In such cases, medical supervision is essential.

• Finally, some medications — including glucocorticoids, anticonvulsants, and certain antiretroviral drugs — can interfere with Vitamin D metabolism.

Low Vitamin D is rarely caused by a single factor. It is usually the result of lifestyle, environment, body composition, and physiology interacting together. When we are working on fat loss, strength, hormones, and long-term health, it is simply not a variable I am willing to ignore.